Diodes: A Comprehensive Educational Resource

Diodes, Semiconductor Diodes, Thermionic Diodes, Rectification, Demodulation, Logic Circuits, Temperature Sensing, Light Emission, Threshold Voltage, Reverse Breakdown, Zener Diodes, Avalanche Diodes, Varactor Diodes, Tunnel Diodes, Gunn Diodes, IMPATT Diodes, Light-Emitting Diodes, LEDs, Shot-Noise Generators, Vacuum Tube Diodes, Thermionic Valves, Semiconductor Diodes, Point-Contact Diodes, Junction Diodes, p-n Junction Diodes, Schottky Diodes, Current-Voltage Characteristic, Shockley Diode Equation, Small-Signal Behavior

Explore the definition, types, functions, history, and applications of diodes, fundamental two-terminal electronic components.

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This resource provides a detailed exploration of diodes, fundamental two-terminal electronic components. We will delve into their definition, types, functions, history, and applications, aiming to build a comprehensive understanding of these essential devices.

Introduction to Diodes

A diode is a fundamental electronic component characterized by its two terminals and its primary function of conducting electric current in only one direction. This unidirectional conductivity is known as asymmetric conductance.

Asymmetric Conductance: The property of a device to conduct electric current more easily in one direction than the other. In an ideal diode, conductance is high (ideally infinite) in one direction and low (ideally zero) in the opposite direction.

In simpler terms, a diode acts like a one-way valve for electrical current. It presents very low resistance to current flow in one direction (the forward direction) and very high resistance in the opposite direction (the reverse direction).

The most common type of diode today is the semiconductor diode.

Semiconductor Diode: A type of diode constructed from semiconductor material, typically silicon, germanium, or gallium arsenide. It utilizes a p–n junction to achieve unidirectional current flow and exhibits an exponential current-voltage characteristic.

Semiconductor diodes were groundbreaking as the first semiconductor electronic devices. The principle of asymmetric electrical conduction, the foundation of diode operation, was first observed in 1874 by German physicist Ferdinand Braun using a crystalline mineral and metal contact. While silicon is the dominant material for modern diodes, other semiconductors like gallium arsenide and germanium are also utilized for specific applications.

Historically, another significant type of diode was the thermionic diode.

Thermionic Diode: An obsolete type of diode that utilizes a vacuum tube structure. It consists of two electrodes: a heated cathode and a plate, enclosed in a vacuum. Electron flow is possible only from the cathode to the plate due to thermionic emission.

While largely replaced by semiconductor diodes, thermionic diodes played a crucial role in early electronics.

Diodes have a wide range of applications, including:

• Rectification: Converting alternating current (AC) to direct current (DC).
• Demodulation: Extracting information signals from modulated radio waves in radio receivers.
• Logic Circuits: Implementing basic logic functions.
• Temperature Sensing: Utilizing the temperature-dependent voltage characteristics of diodes.
• Light Emission: In the form of Light-Emitting Diodes (LEDs) for lighting and indicators.

Main Functions of Diodes

Unidirectional Current Flow

The most fundamental function of a diode is to allow current to flow easily in one direction, known as the forward direction, and to block current flow in the opposite direction, the reverse direction. This behavior is analogous to a check valve in hydraulics, which allows fluid to flow in only one direction.

Forward Direction: The direction of current flow through a diode when it is easily conducting. This is typically from the anode (P-side in semiconductor diodes, plate in vacuum diodes) to the cathode (N-side in semiconductor diodes, cathode in vacuum diodes).

Reverse Direction: The direction opposite to the forward direction. In the reverse direction, a diode ideally blocks current flow.

Check Valve: A mechanical valve that allows fluid (liquid or gas) to flow in only one direction, preventing backflow. This is a helpful analogy for understanding the unidirectional current flow in a diode.

This unidirectional property is critical for rectification, the process of converting alternating current (AC) to direct current (DC).

Rectification: The process of converting alternating current (AC), which periodically reverses direction, into direct current (DC), which flows in only one direction. Diodes are essential components in rectifier circuits.

Example: Rectification in a Simple Circuit

Consider a simple circuit with an AC voltage source and a diode connected in series with a resistor. During the positive half-cycle of the AC voltage, the diode is forward-biased and allows current to flow, resulting in a voltage across the resistor. During the negative half-cycle, the diode is reverse-biased and blocks current flow, resulting in minimal voltage across the resistor. The output across the resistor is now pulsating DC, having been rectified from the original AC input.

Rectification is crucial in many electronic devices, such as power supplies that convert AC mains electricity to the DC voltage required by electronic components. Diodes are also used in radio receivers for demodulation, where they extract the audio signal from the amplitude-modulated radio waves.

Threshold Voltage

Semiconductor diodes exhibit a characteristic known as threshold voltage, also referred to as turn-on voltage or cut-in voltage.

Threshold Voltage (Turn-on Voltage, Cut-in Voltage): The minimum forward voltage required across a diode for significant current to begin flowing. Below this voltage, the current is very small.

The threshold voltage is dependent on the diode’s semiconductor material:

• Silicon diodes: Typically around 0.6 - 0.7 V
• Germanium diodes: Typically around 0.3 V
• Schottky diodes: Typically lower, around 0.2 - 0.3 V

This voltage is often loosely termed forward voltage drop or simply voltage drop. Due to the steep exponential current-voltage characteristic, the voltage drop across a forward-biased diode under normal operating conditions will not significantly exceed the threshold voltage.

Example: LED Forward Voltage

Light-Emitting Diodes (LEDs) are a type of diode. Their datasheets typically specify a forward voltage (V_F) at a particular current (e.g., 20 mA). For a typical red LED, the forward voltage might be around 1.8V. This means that approximately 1.8V needs to be applied across the LED in the forward direction for it to light up at 20mA.

It’s important to note that the exponential current-voltage relationship is not a sharp on-off switch but rather a gradual curve. While a linear scale might suggest a distinct “knee” at the threshold voltage, this knee is an illusion of the scale. On a semi-log plot (logarithmic current scale, linear voltage scale), the curve appears more linear, demonstrating the gradual nature of the exponential relationship.

The forward voltage drop of a diode is relatively stable with current changes but is more sensitive to temperature variations. This temperature dependence can be utilized in temperature sensor applications. Additionally, the relatively stable forward voltage drop can be used as an approximate voltage reference.

Reverse Breakdown

While diodes are designed to block current in the reverse direction, this blocking capability has a limit. When the reverse voltage across a diode reaches a critical value known as the breakdown voltage, the diode’s reverse resistance suddenly drops, and a large reverse current can flow.

Breakdown Voltage: The reverse voltage across a diode at which the reverse current increases dramatically. This phenomenon is due to either the Zener effect or the avalanche effect.

This phenomenon, called reverse breakdown, can be used in specialized diodes for specific purposes:

• Zener Diodes: Designed to operate in the reverse breakdown region at a specific voltage, known as the Zener voltage. They are used for voltage regulation, maintaining a stable voltage in circuits.

  Zener Diode: A special type of diode designed to operate in the reverse breakdown region. It exhibits a sharp breakdown voltage (Zener voltage) and is used for voltage regulation and reference applications.

• Avalanche Diodes: Also designed for reverse breakdown but utilize the avalanche effect. They are primarily used for over-voltage protection, diverting high voltage surges to protect sensitive circuits.

  Avalanche Diode: A type of diode designed to break down in reverse bias due to the avalanche effect. It is used for surge protection and high-voltage transient suppression.

Example: Zener Diode Voltage Regulation

In a voltage regulator circuit, a Zener diode is connected in reverse bias in parallel with the load. If the input voltage increases, the Zener diode will start conducting in reverse breakdown, clamping the voltage across the load to its Zener voltage. This ensures a stable output voltage even with input voltage fluctuations.

It’s crucial to note that exceeding the maximum current and power ratings in the reverse breakdown region can damage or destroy the diode.

Other Functions and Specialized Diodes

The current-voltage characteristics of semiconductor diodes can be precisely tailored during manufacturing by selecting specific semiconductor materials and controlling the doping impurities. This customization allows for the creation of special-purpose diodes designed for a wide range of functions beyond simple rectification:

• Varactor Diodes (Varicap Diodes): Used as voltage-controlled capacitors. Their capacitance varies with the applied reverse voltage, making them useful for electronic tuning in radio and TV receivers.

  Varactor Diode (Varicap Diode): A type of diode whose junction capacitance varies with the applied reverse voltage. It is used as a voltage-controlled capacitor in tuning circuits and frequency multipliers.

• Tunnel Diodes, Gunn Diodes, IMPATT Diodes: These diodes exhibit negative resistance, a property where the current decreases as the voltage increases over a certain range. This unique characteristic makes them valuable for generating radio-frequency oscillations in microwave circuits and high-speed switching applications.

  Negative Resistance: A property of certain electronic devices where the current through the device decreases as the voltage across it increases over a specific voltage range. This is in contrast to typical positive resistance where current increases with voltage.

• Light-Emitting Diodes (LEDs): Designed to emit light when forward-biased. They are made from specific semiconductor materials that release photons (light particles) when charge carriers recombine at the p-n junction. LEDs are widely used for lighting, displays, and indicators.

• Shot-Noise Generators: Both vacuum and semiconductor diodes can be used as shot-noise generators. Shot noise is a type of electronic noise inherent in electrical currents, and diodes can be configured to produce this noise for testing and research purposes.

  Shot Noise: A type of electronic noise caused by the discrete nature of electric charge carriers (electrons and holes). It is present in all electronic devices and can be utilized as a random noise source.

History of Diodes

The development of diodes has a rich history, branching into two main paths: thermionic (vacuum-tube) diodes and solid-state (semiconductor) diodes. Remarkably, both types emerged around the same time in the early 1900s, initially as detectors for radio receivers.

Until the 1950s, vacuum diodes were more prevalent in radios. Early point-contact semiconductor diodes suffered from stability issues. Furthermore, vacuum tubes were already common in radios for amplification, and integrating thermionic diodes within the same tube structure was convenient. Vacuum tube rectifiers and gas-filled rectifiers also excelled in high-voltage/high-current rectification tasks compared to early semiconductor diodes like selenium rectifiers.

Key Historical Milestones:

• 1873: Frederick Guthrie’s Observation: Guthrie noticed that a grounded, white-hot metal ball could discharge a positively charged electroscope but not a negatively charged one, hinting at unidirectional conduction.

• 1880: Thomas Edison’s Edison Effect: Edison observed unidirectional current between heated and unheated elements in a light bulb, later termed the Edison effect. He patented this phenomenon for use in a DC voltmeter.

• Early 1900s: John Ambrose Fleming and the Fleming Valve: John Ambrose Fleming, a former Edison employee and scientific advisor to Marconi Company, recognized the potential of the Edison effect for radio detection. He invented and patented the first true thermionic diode, the Fleming valve, in Britain in 1904 and the US in 1905. This marked the beginning of the vacuum tube diode era.

  Fleming Valve: The first practical thermionic diode, invented by John Ambrose Fleming in 1904. It utilized the Edison effect for radio detection.

• 1874: Karl Ferdinand Braun’s Crystal Rectifier Discovery: German scientist Karl Ferdinand Braun discovered unilateral conduction across a contact between a metal and a mineral, laying the groundwork for crystal detectors.

• 1894: Jagadish Chandra Bose’s Crystal Radio Detection: Indian scientist Jagadish Chandra Bose pioneered the use of crystals for detecting radio waves.

• 1903-1906: Greenleaf Whittier Pickard’s Silicon Crystal Detector: Greenleaf Whittier Pickard developed the crystal detector into a practical device for wireless telegraphy. He invented a silicon crystal detector in 1903 and patented it in 1906.

  Crystal Detector: An early type of semiconductor diode that utilized a point contact between a metal wire and a semiconductor crystal (like silicon or germanium) for radio detection.

• 1930s-1940s: Point-Contact Diode Development for Radar: During the 1930s, advancements in physics led researchers at Bell Telephone Laboratories and others to recognize the potential of crystal detectors for microwave technology, particularly for radar applications during World War II. Intensive development of point-contact diodes (also called crystal rectifiers or crystal diodes) ensued.

• Post-WWII: Junction Diode Development: After World War II, AT&T utilized point-contact diodes in microwave towers. In 1946, Sylvania introduced the 1N34 crystal diode. The early 1950s saw the development of junction diodes, a more stable and reliable type of semiconductor diode.

Vacuum tube diodes dominated electronics for decades, used in radios, televisions, sound systems, and instrumentation. However, starting in the late 1940s with selenium rectifier technology and accelerating with semiconductor diodes in the 1960s, they gradually lost market share. Today, vacuum diodes are niche components, still found in some high-power applications where their robustness and ability to withstand transient voltages are advantageous, and in specialized audio equipment for musical instruments and audiophiles.

Etymology of “Diode”

The term “diode” emerged after the invention of asymmetrical conduction devices, which were initially known as rectifiers. In 1919, the same year tetrodes were invented, William Henry Eccles coined the term “diode”.

William Henry Eccles: A British physicist and pioneer in radio communication who coined the term “diode”.

The word “diode” is derived from Greek roots:

• “di” (δί): Meaning ‘two’
• “ode” (οδός): Meaning ‘path’

Therefore, “diode” literally signifies “two-path,” referring to the two terminals of the device. The term “diode” was already in use in multiplex telegraphy, along with “triode,” “tetrode,” “pentode,” and “hexode.”

While all diodes perform rectification, the term “rectifier” is typically reserved for diodes specifically used in power supplies for AC-to-DC conversion, distinguishing them from diodes intended for small signal circuits.

Vacuum Tube Diodes: Thermionic Valves

A vacuum tube diode, also known as a thermionic diode or valve diode, is a thermionic-valve device enclosed in a sealed, evacuated glass or metal envelope. It comprises two key electrodes: a cathode and a plate (also called an anode in some contexts, though plate is the more common term for vacuum tubes).

Cathode (Vacuum Tube): The negatively charged electrode in a vacuum tube diode. It is heated to emit electrons through thermionic emission.

Plate (Vacuum Tube): The positively charged electrode in a vacuum tube diode. It collects electrons emitted by the cathode when positively biased relative to the cathode.

The cathode is the electron emitter and can be heated in two ways:

• Directly Heated Cathode: Made of tungsten wire, heated directly by passing a current through it from an external voltage source.

• Indirectly Heated Cathode: Heated by infrared radiation from a nearby heater. The heater is typically a Nichrome wire coil supplied with current from an external voltage source. Indirect heating allows for more stable cathode temperature and electrical isolation of the heater circuit.

Heater (Vacuum Tube): A resistive element (typically Nichrome wire) in an indirectly heated vacuum tube. It heats the cathode through infrared radiation, causing thermionic emission.

During operation, the cathode is heated to a high temperature, typically 800–1,000 °C (1,470–1,830 °F), causing it to release electrons into the vacuum through thermionic emission.

Thermionic Emission: The emission of electrons from a heated surface, such as the cathode of a vacuum tube. The heat provides electrons with enough energy to overcome the work function of the material and escape into the vacuum.

To enhance electron emission, cathodes are often coated with oxides of alkaline earth metals like barium and strontium oxides. These materials have a low work function, meaning they require less energy to emit electrons compared to uncoated materials like tungsten.

Work Function: The minimum energy required for an electron to escape from the surface of a material into a vacuum. Materials with lower work functions are more efficient electron emitters.

The plate, being unheated, does not emit electrons. However, it is capable of absorbing electrons.

When an alternating voltage intended for rectification is applied between the cathode and the plate:

• Forward Bias (Plate Positive): When the plate voltage is positive relative to the cathode, the plate electrostatically attracts the electrons emitted by the cathode. This allows a current of electrons to flow through the vacuum tube from the cathode to the plate.

• Reverse Bias (Plate Negative): When the plate voltage is negative relative to the cathode, the plate does not emit electrons. Since the cathode is the only electron source, no current can flow from the plate to the cathode.

This unidirectional electron flow explains the rectifying action of vacuum tube diodes.

Semiconductor Diodes

Semiconductor diodes are the dominant type of diode in modern electronics due to their small size, efficiency, and reliability. They are broadly categorized into point-contact diodes and junction diodes.

Point-Contact Diodes

Point-contact diodes evolved from early crystal detector technology, starting in the 1930s. They are primarily used in high-frequency applications, typically in the 3 to 30 gigahertz range (microwave frequencies).

Point-Contact Diode: A type of semiconductor diode constructed with a sharp metal wire in contact with a semiconductor crystal. It utilizes either the Schottky barrier principle (non-welded) or a small p-n junction formed around the point contact (welded).

Point-contact diodes are constructed using a small diameter metal wire in contact with a semiconductor crystal. They come in two main types:

• Non-Welded Contact Type: These diodes operate based on the Schottky barrier principle. The sharp metal point creates a Schottky barrier junction with the semiconductor crystal. The metal side is the pointed end of the small diameter wire, pressed against the semiconductor crystal.

  Schottky Barrier: A potential barrier formed at the interface between a metal and a semiconductor. It is responsible for the rectifying behavior of Schottky diodes and non-welded point-contact diodes.

• Welded Contact Type: In this type, a small P region is intentionally formed within the N-type semiconductor crystal around the metal point during manufacturing. This is achieved by briefly passing a relatively large current through the device. The heat generated during this process causes some of the metal atoms to diffuse into the semiconductor, creating a localized p-type region.

Compared to junction diodes, point-contact diodes generally exhibit:

• Lower Capacitance: Beneficial for high-frequency operation.
• Higher Forward Resistance: Can lead to higher voltage drop at the same current.
• Greater Reverse Leakage: They tend to have more current flow in the reverse direction when reverse biased.

Junction Diodes

Junction diodes are the most common type of semiconductor diode. They are further categorized into p-n junction diodes and Schottky diodes.

p–n Junction Diode

The p–n junction diode is the fundamental type of semiconductor diode. It is fabricated from a semiconductor crystal, most commonly silicon, but also germanium and gallium arsenide.

p–n Junction Diode: A type of semiconductor diode formed by joining a p-type semiconductor material with an n-type semiconductor material. The junction between these materials creates a depletion region and exhibits rectifying behavior.

The manufacturing process involves adding impurities to the semiconductor crystal to create two distinct regions:

• n-type Semiconductor: A region doped with impurities that contribute free electrons as charge carriers. “n” stands for negative, referring to the negative charge of electrons.

  n-type Semiconductor: A semiconductor material that has been doped with donor impurities, increasing the concentration of free electrons (negative charge carriers).

• p-type Semiconductor: A region doped with impurities that create holes (vacancies where electrons are missing) as charge carriers. “p” stands for positive, referring to the effective positive charge of holes.

  p-type Semiconductor: A semiconductor material that has been doped with acceptor impurities, increasing the concentration of holes (positive charge carriers).

When the n-type and p-type materials are brought together, a p–n junction is formed at their interface. Initially, diffusion occurs:

1. Electron Diffusion: Free electrons from the n-type region diffuse across the junction into the p-type region, where they recombine with holes.
2. Hole Diffusion: Simultaneously, holes from the p-type region diffuse into the n-type region, where they recombine with electrons.

This diffusion process leads to the formation of a depletion region near the junction.

Depletion Region (Depletion Layer): A region near the p-n junction that is depleted of mobile charge carriers (electrons and holes). It forms due to diffusion and recombination of charge carriers and acts as an insulator.

In the depletion region:

• Mobile charge carriers (electrons and holes) are depleted due to recombination.
• Fixed, immobile ions are left behind: positively charged donor ions in the n-type region and negatively charged acceptor ions in the p-type region.
• An electric field is established across the depletion region, directed from the n-type to the p-type region. This electric field opposes further diffusion of charge carriers.

The terminals of the diode are connected to the n-type and p-type regions:

• Anode: The terminal connected to the p-type region (positive terminal in circuit diagrams).
• Cathode: The terminal connected to the n-type region (negative terminal in circuit diagrams, often marked with a band on the diode package).

The p–n junction is the active region where the diode’s rectifying behavior originates.

Diode Operation based on Bias:

• Forward Bias: When a sufficiently higher electrical potential is applied to the P side (anode) compared to the N side (cathode), the external voltage opposes the built-in electric field of the depletion region. This reduces the width of the depletion region and allows electrons to flow easily from the N-type side to the P-type side, resulting in significant forward current. The diode is said to be conducting.

• Reverse Bias: When the potential is applied in the opposite direction (N side more positive than P side), the external voltage reinforces the built-in electric field, widening the depletion region. This significantly increases the resistance and blocks the flow of electrons in the reverse direction, resulting in very small reverse current (ideally zero). The diode is said to be blocking or non-conducting.

This behavior effectively creates an electrical check valve, allowing current flow in only one direction.

Schottky Diode

The Schottky diode is another type of junction diode, but it differs from the p-n junction diode in its construction. Instead of a p-n junction, a Schottky diode is formed from a metal-semiconductor junction.

Schottky Diode: A type of semiconductor diode formed by a metal-semiconductor junction instead of a p-n junction. It has a lower forward voltage drop and faster switching speed compared to p-n junction diodes.

Typically, a Schottky diode uses a metal like platinum, chromium, tungsten, or gold in contact with an n-type semiconductor (silicon or gallium arsenide). The metal-semiconductor junction creates a Schottky barrier, which exhibits rectifying behavior.

Advantages of Schottky Diodes over p-n Junction Diodes:

• Lower Forward Voltage Drop: Schottky diodes have a significantly lower forward voltage drop (typically 0.2-0.3 V) compared to silicon p-n junction diodes (0.6-0.7 V). This makes them more efficient in low-voltage circuits and power rectification applications.
• Faster Switching Speed: Schottky diodes are majority carrier devices, meaning current conduction primarily involves majority carriers (electrons in n-type material). They do not rely on minority carrier injection and recombination, unlike p-n junction diodes. This absence of minority carrier storage leads to much faster switching speeds and faster reverse recovery times, making them suitable for high-frequency circuits and fast switching applications.
• Lower Junction Capacitance: Schottky diodes generally have lower junction capacitance compared to p-n junction diodes, further contributing to their faster switching speed.

Disadvantages of Schottky Diodes:

• Higher Reverse Leakage Current: Schottky diodes typically have higher reverse leakage current compared to p-n junction diodes, especially at higher temperatures.
• Lower Reverse Breakdown Voltage: Schottky diodes generally have lower reverse breakdown voltages compared to p-n junction diodes.

Current–Voltage Characteristic

The current-voltage (I-V) characteristic of a semiconductor diode describes its electrical behavior in a circuit. It graphically represents the relationship between the current flowing through the diode and the voltage applied across it.

Current-Voltage (I-V) Characteristic: A graphical representation of the relationship between the current flowing through a device and the voltage applied across it. For a diode, it shows the exponential increase in forward current and the very small reverse current until breakdown.

The shape of the I-V curve is determined by the transport of charge carriers through the depletion region at the p-n junction.

Depletion Region Formation and Built-in Potential:

As described earlier, when a p-n junction is formed, diffusion of electrons and holes and subsequent recombination create the depletion region. This process results in a built-in potential across the depletion zone.

Built-in Potential: The potential difference that exists across the depletion region of a p-n junction in equilibrium (no external voltage applied). It is caused by the diffusion of charge carriers and the resulting electric field.

The built-in potential opposes further diffusion and establishes equilibrium.

Reverse Bias Operation

When a reverse bias voltage is applied across the diode (positive terminal connected to n-type, negative terminal to p-type), it adds to the built-in potential and widens the depletion region.

Reverse Bias: Applying a voltage across a diode such that the positive terminal is connected to the n-type region (cathode) and the negative terminal to the p-type region (anode). This increases the depletion region width and reduces current flow.

In reverse bias, the depletion region acts as an insulator, preventing significant current flow. Ideally, no current should flow, but in reality, a small reverse saturation current (I_s) exists. This current is primarily due to:

• Thermally generated electron-hole pairs: Even in the depletion region, thermal energy can create electron-hole pairs. These minority carriers can be swept across the junction by the electric field, contributing to a small reverse current.
• Surface leakage current: Imperfections and surface effects can create leakage paths around the junction, allowing a small current to flow.

The reverse saturation current is very small (typically in the micro-ampere range for silicon diodes) and is highly temperature-dependent. It increases significantly with temperature.

Forward Bias Operation

When a forward bias voltage is applied (positive terminal to p-type, negative terminal to n-type), it opposes the built-in potential and narrows the depletion region.

Forward Bias: Applying a voltage across a diode such that the positive terminal is connected to the p-type region (anode) and the negative terminal to the n-type region (cathode). This reduces the depletion region width and allows significant current flow.

As the forward voltage increases, the depletion region narrows further, and the potential barrier to current flow decreases exponentially. This leads to a rapid exponential increase in forward current. Substantial current starts flowing when the forward voltage exceeds the threshold voltage.

Operating Regions of a Diode

The current-voltage characteristic of a diode can be approximated by four distinct operating regions, categorized by the applied bias voltage:

1. Breakdown Region: At very large reverse bias voltages, exceeding the peak inverse voltage (PIV), reverse breakdown occurs.

   Peak Inverse Voltage (PIV): The maximum reverse voltage that a diode can withstand without breaking down.

   Reverse Breakdown: A phenomenon where the reverse current through a diode increases dramatically when the reverse voltage exceeds the breakdown voltage. This can be due to the avalanche effect or Zener effect.

   In breakdown, a large reverse current flows, potentially damaging the diode permanently (unless it’s a specifically designed breakdown diode like Zener or avalanche diodes).

   • Avalanche Breakdown: Occurs in diodes with lightly doped junctions at higher reverse voltages (typically above 6.2V). High electric fields accelerate minority carriers, which collide with atoms, creating more electron-hole pairs through ionization in an avalanche-like process.

   • Zener Breakdown: Occurs in heavily doped junctions at lower reverse voltages (typically below 5V). The electric field becomes so strong that electrons can tunnel directly from the valence band of the p-type material to the conduction band of the n-type material, resulting in reverse current.

2. Reverse Biased Region: For reverse bias voltages between breakdown and 0V, the reverse current is very small and asymptotically approaches the negative of the reverse saturation current (-I_s). As mentioned earlier, this current is temperature-dependent and may increase significantly at high temperatures. Surface leakage also contributes to reverse current.

3. Forward Biased Region: In this region, the current-voltage curve is exponential, closely following the Shockley diode equation. On a linear current scale, the curve exhibits a smooth “knee” around the threshold voltage. However, on a semi-log graph, the exponential curve appears more linear, without a distinct threshold.

4. Leveling Off Region (High Forward Current Region): At very high forward currents, the I-V curve deviates from the exponential behavior. It starts to be dominated by the ohmic resistance of the bulk semiconductor material (the resistance of the p-type and n-type regions themselves). The curve becomes asymptotic to a straight line with a slope equal to the bulk resistance. This region is particularly important for power diodes handling large currents. It can be modeled by an ideal diode in series with a fixed resistor representing the bulk resistance.

Shockley Diode Equation

The Shockley diode equation, also known as the diode law, mathematically describes the exponential current-voltage (I-V) relationship of diodes in moderate forward and reverse bias regions.

Shockley Diode Equation (Diode Law): A mathematical equation that models the current-voltage characteristic of an ideal diode. It describes the exponential relationship between forward current and voltage and the small reverse saturation current.

The equation is:

I = I_S (e^{(V_D / (n V_T))} - 1)

Where:

• I is the diode current.
• I_S is the reverse saturation current (also called leakage current).
• V_D is the voltage across the diode.
• n is the ideality factor (typically between 1 and 2, depending on the diode type and current level).
• V_T is the thermal voltage, given by V_T = kT/q, where:
  • k is the Boltzmann constant.
  • T is the absolute temperature in Kelvin.
  • q is the elementary charge.

At room temperature (approximately 25°C or 298K), V_T is approximately 26 mV.

Simplified form for forward bias (V_D >> V_T):

I ≈ I_S e^{(V_D / (n V_T))}

Simplified form for reverse bias (V_D << -V_T):

I ≈ -I_S

The Shockley diode equation is an idealized model and does not account for all real-world diode behaviors, particularly at very high currents or in breakdown regions. However, it provides a good approximation for many applications.

Small-Signal Behavior

For small voltage variations around a DC bias point in the forward biased region, the diode’s behavior can be approximated as linear. The small-signal resistance (r_d), also known as dynamic resistance, of the diode can be derived from the Shockley diode equation.

Small-Signal Resistance (Dynamic Resistance): The resistance of a diode to small AC signal variations around a DC bias point. It is the reciprocal of the slope of the I-V curve at the bias point.

The small-signal resistance is given by:

r_d = dV_D / dI ≈ n V_T / I_D

Where I_D is the DC bias current through the diode.

This small-signal resistance is inversely proportional to the DC bias current. At higher bias currents, the diode behaves more like a small resistance to AC signals.

In applications like detectors and mixers, where small signals are applied to a biased diode, the non-linear I-V characteristic is exploited. The diode current can be approximated using a Taylor series expansion. For detector and mixer applications, the square-law region (where current is approximately proportional to the square of the input voltage) is often utilized.

Reverse-Recovery Effect

The reverse-recovery effect is a non-ideal behavior of p-n junction diodes that becomes significant in high-speed switching applications.

Reverse-Recovery Effect: A phenomenon in p-n junction diodes where a reverse current flows for a short time after the forward current is switched off. This is due to the removal of stored charge carriers from the depletion region and the semiconductor regions adjacent to the junction.

When a p-n junction diode is forward-biased and conducting, minority carriers (holes in the n-region, electrons in the p-region) are injected across the junction and stored in the regions adjacent to the depletion region. When the forward bias is suddenly removed or reversed, these stored minority carriers need to be removed before the diode can fully block reverse current.

Reverse Recovery Process:

1. Storage Phase: Initially, when the voltage is reversed, the stored minority carriers are swept back across the junction, resulting in a reverse current. The diode is said to be in the storage phase. The magnitude of this reverse current depends on the circuit impedance and the amount of stored charge.

2. Transition Phase: As the stored charge is depleted, the reverse current gradually decreases.

3. Blocking Phase: Once the stored charge is fully removed, the diode returns to its blocking state, and the reverse current becomes very small (reverse saturation current).

The reverse recovery time (t_rr) is the time taken to remove the reverse recovery charge (Q_rr) and for the diode to regain its blocking capability.

Reverse Recovery Time (t_rr): The time it takes for a diode to switch from the conducting state to the blocking state when the voltage polarity is reversed. It is the time required to remove the stored charge carriers from the junction region.

Reverse recovery time can range from tens of nanoseconds to a few microseconds, depending on the diode type.

Impact of Reverse Recovery:

• Switching Losses: The reverse current and the associated voltage across the diode during reverse recovery lead to power losses in switching circuits.
• Noise and EMI: The abrupt changes in current during reverse recovery can generate noise and electromagnetic interference (EMI).

Mitigation of Reverse Recovery Effects:

• Schottky Diodes: Schottky diodes, being majority carrier devices, do not exhibit significant minority carrier storage and have very fast reverse recovery times. They are preferred in high-speed switching applications where reverse recovery is a concern.
• Fast Recovery Diodes: Special p-n junction diodes are designed with reduced minority carrier lifetime to minimize reverse recovery time.
• Soft Recovery Diodes: These diodes are designed to have a gradual decrease in reverse current during recovery, reducing noise and EMI compared to abrupt recovery diodes.

Step Recovery Diodes (Snap-off Diodes):

Step recovery diodes (SRDs), also known as snap-off diodes, are specifically designed to exploit the abrupt cessation of reverse current during reverse recovery.

Step Recovery Diode (SRD, Snap-off Diode): A type of diode designed to have an abrupt transition from reverse conduction to blocking during reverse recovery. This rapid transition is used to generate very short pulses and high-frequency harmonics.

In SRDs, the reverse recovery process is engineered to be very abrupt. The reverse current ceases very suddenly, creating a fast voltage transition. This rapid voltage step is used to generate extremely short pulses and high-frequency harmonics, making SRDs valuable in frequency multipliers, pulse generators, and sampling circuits.

Types of Semiconductor Diodes

Semiconductor diodes come in a wide variety of types, each tailored for specific applications by modifying their materials, doping profiles, and construction. Here is a breakdown of common types:

• Normal (p–n) Diodes: These are the standard diodes made from doped silicon or germanium, operating as described previously. They are the most common type, found in general-purpose rectification, signal diodes, and integrated circuits.

• Avalanche Diodes: Designed to operate in reverse breakdown mode due to the avalanche effect. They are used for voltage regulation (similar to Zener diodes, but with a different breakdown mechanism at higher voltages) and surge protection. They have a well-defined breakdown voltage and are designed to withstand repeated breakdown events without damage (within their power ratings). They are often mistakenly called Zener diodes, especially for breakdown voltages above approximately 6.2V.

  • Distinction from Zener Diodes: While both are breakdown diodes, avalanche diodes break down due to the avalanche effect, while Zener diodes break down due to the Zener effect. The primary practical difference is their temperature coefficient of breakdown voltage has opposite polarities. Avalanche diodes have a positive temperature coefficient, while Zener diodes have a negative temperature coefficient.

• Constant-Current Diodes (CLDs, Current-Regulating Diodes, Diode-Connected Transistors): These are not actually diodes in the traditional p-n junction sense. They are effectively Junction Field-Effect Transistors (JFETs) with the gate terminal shorted to the source terminal. They behave like a two-terminal current limiter, analogous to Zener diodes for voltage limiting. The current through them rises to a certain value and then levels off at a specific constant current value, regardless of voltage changes over a certain range. They are used for current regulation and current source applications.

• Crystal Rectifiers or Crystal Diodes: These are point-contact diodes, historically significant as early radio detectors. Examples include the 1N21 series and 1N34A. They are still used in some specialized mixer and detector applications, particularly in radar and microwave receivers.

• Gunn Diodes: These are specialized diodes made from materials like gallium arsenide (GaAs) or indium phosphide (InP) that exhibit negative differential resistance. With appropriate biasing, dipole domains form and travel across the diode, leading to high-frequency current oscillations. Gunn diodes are used to build high-frequency microwave oscillators.

• Light-Emitting Diodes (LEDs): Diodes made from direct band-gap semiconductors like gallium arsenide (GaAs), gallium phosphide (GaP), and gallium nitride (GaN). When charge carriers recombine at the p-n junction, they emit photons (light). The wavelength (color) of the emitted light depends on the semiconductor material. LEDs are used for lighting, displays, indicators, and optical communication.

  • White LEDs: “White” LEDs are typically not true white light emitters. They are usually blue LEDs coated with a yellow phosphor (scintillator) that converts some of the blue light to yellow, creating a broader spectrum that appears white. Alternatively, some white LEDs use a combination of red, green, and blue LEDs in a single package.

  • LEDs as Photodiodes: LEDs can also function as low-efficiency photodiodes in signal applications, detecting light. They can be combined with photodiodes or phototransistors in opto-isolators.

• Laser Diodes: Structurally similar to LEDs but with a resonant cavity formed by polished parallel end faces. When forward-biased, they emit coherent light (laser light) through stimulated emission. Laser diodes are used in optical storage (CD/DVD/Blu-ray players), optical communication, laser pointers, and barcode scanners.

• Thermal Diodes: This term has two meanings:

  • Conventional p–n diodes used as temperature sensors: Utilize the temperature dependence of the forward voltage drop to measure temperature.

  • Peltier Heat Pumps (Thermoelectric Coolers): Devices used for thermoelectric heating and cooling based on the Peltier effect. While often referred to as “thermal diodes,” Peltier devices are not diodes in the rectifying sense. They are typically made from semiconductor materials (often bismuth telluride) but do not have rectifying junctions. They use the differing behavior of charge carriers in n-type and p-type semiconductors to transfer heat when current flows.

• Photodiodes: Diodes specifically designed to detect light. They are packaged in light-transparent materials and are typically PIN diodes (p-type/intrinsic/n-type structure) for enhanced light sensitivity. When light falls on the depletion region, it generates electron-hole pairs, increasing reverse current. Photodiodes are used in solar cells, light meters (photometry), optical communication receivers, and light sensors.

  • Photodiode Arrays: Multiple photodiodes can be packaged in linear arrays or two-dimensional arrays for imaging and sensing applications. These arrays should not be confused with Charge-Coupled Devices (CCDs), which are a different type of image sensor.

• PIN Diodes: Diodes with a central intrinsic (undoped) semiconductor layer sandwiched between p-type and n-type layers, forming a p-type/intrinsic/n-type (PIN) structure.

  Intrinsic Semiconductor: A semiconductor material that is in its pure, undoped form. It has a very low concentration of charge carriers at room temperature.

  PIN diodes are used as:

  • Radio Frequency (RF) Switches and Attenuators: The intrinsic layer increases the diode’s resistance in reverse bias and reduces capacitance, making them suitable for RF switching and signal control.
  • Ionizing Radiation Detectors: The large depletion region in the intrinsic layer makes them effective for detecting ionizing radiation.
  • Photodetectors: PIN structure enhances light sensitivity compared to standard p-n photodiodes.
  • Power Electronics: The intrinsic layer can withstand high voltages, making PIN structures useful in power semiconductor devices like IGBTs, power MOSFETs, and thyristors.

• Schottky Diodes: Formed from a metal-semiconductor junction. As discussed earlier, they have lower forward voltage drop, faster switching speed, and lower junction capacitance compared to p-n junction diodes. They are used in:

  • Voltage Clamping: Due to their low forward voltage drop, they are effective for clamping voltages to prevent transistor saturation and protect circuits from overvoltage.
  • Low-Loss Rectifiers: Efficient rectifiers, especially at low voltages.
  • High-Speed Circuitry and RF Devices: Used in switched-mode power supplies, mixers, detectors, and other high-frequency applications due to their fast switching capabilities.

• Super Barrier Diodes: A type of rectifier diode that combines the advantages of Schottky diodes (low forward voltage drop) and p-n junction diodes (surge handling capability and low reverse leakage current). They offer improved performance compared to standard Schottky diodes in certain applications.

• Gold-Doped Diodes: p-n junction diodes where gold (or platinum) is intentionally introduced as a dopant. Gold acts as recombination centers, promoting faster recombination of minority carriers. This results in:

  • Faster Switching Speed: Faster than standard p-n diodes (but not as fast as Schottky diodes).
  • Higher Forward Voltage Drop: A trade-off for faster speed.
  • Lower Reverse Leakage Current: Less reverse leakage compared to Schottky diodes (but more than standard p-n diodes).

  Gold-doped diodes are used in applications requiring faster switching than standard p-n diodes but with lower reverse leakage than Schottky diodes. A common example is the 1N914 signal diode.

• Snap-Off or Step Recovery Diodes (SRDs): As discussed earlier, these diodes exploit the abrupt reverse recovery characteristic to generate fast voltage transitions and short pulses.

• Stabistors or Forward Reference Diodes: Special diodes designed for voltage stabilization. They exhibit extremely stable forward voltage characteristics over a wide current range and temperature variations. They are used in low-voltage stabilization applications requiring high voltage stability.

• Transient Voltage Suppression Diodes (TVS Diodes): Specialized avalanche diodes designed specifically for overvoltage protection. Their p-n junctions have a much larger cross-sectional area than normal diodes, allowing them to handle and conduct large surge currents to ground without damage, protecting sensitive components from voltage transients like electrostatic discharge (ESD) and lightning strikes.

• Tunnel Diodes or Esaki Diodes: Highly doped p-n junction diodes that exhibit negative resistance due to quantum tunneling.

  Quantum Tunneling: A quantum mechanical phenomenon where particles can pass through a potential barrier even if they do not have enough energy to overcome it classically. In tunnel diodes, electrons can tunnel through the depletion region under certain bias conditions.

  The negative resistance region in their I-V characteristic makes them useful for:

  • Amplification of Signals: In oscillator and amplifier circuits.
  • Bistable Circuits: For simple switching and memory applications. Tunnel diodes are very fast, can operate at low temperatures (mK), high magnetic fields, and in high radiation environments, making them suitable for specialized applications like spacecraft electronics.

• Varicap or Varactor Diodes: Used as voltage-controlled capacitors. Their junction capacitance varies with the applied reverse voltage. They are crucial in:

  • PLL (Phase-Locked Loop) and FLL (Frequency-Locked Loop) Circuits: For frequency tuning and synchronization.
  • Tunable Oscillators: Used in early radios and television receivers for electronic tuning.

• Zener Diodes: Designed to operate in reverse breakdown at a specific Zener voltage. They are used as precision voltage references and in voltage regulation circuits.

  • Colloquial Use: The term “Zener diode” is often used loosely to refer to any type of breakdown diode, but strictly speaking, true Zener diodes operate based on the Zener effect at breakdown voltages below approximately 5V. Diodes with breakdown voltages above this are typically avalanche diodes.
  • Temperature Coefficient Compensation: For precise voltage reference circuits, Zener diodes are often connected in series with forward-biased diodes (like switching diodes) in opposite directions to compensate for temperature coefficient variations.
  • Transient Absorbers (Transorbs): Two equivalent Zener diodes connected in series and reverse order in the same package form a transient voltage suppressor (TVS) device, often marketed under the trademark “Transorb.”

Graphic Symbols

Circuit diagrams use standardized graphic symbols to represent diodes, visually conveying their function. The triangle in the symbol always points in the forward direction, indicating the direction of conventional current flow (from positive to negative).

[Insert Gallery of Diode Symbols from Wikipedia Article Here]

Numbering and Coding Schemes

Diodes are identified using various numbering and coding schemes, standardized or manufacturer-specific. The two most prevalent are:

EIA/JEDEC Standard

The EIA/JEDEC (Joint Electron Device Engineering Council) standard, introduced in the US around 1960, uses the 1N-series numbering system (EIA370). Most diodes in this system have a “1N” prefix followed by a number (e.g., 1N4007).

Popular 1N-series Diodes:

• 1N34A / 1N270: Germanium signal diodes (older types).
• 1N914 / 1N4148: Silicon signal diodes (common general-purpose signal diodes).
• 1N400x Series (1N4001, 1N4002, …, 1N4007): Silicon 1A power rectifier diodes (widely used for general rectification).
• 1N580x Series (1N5802, 1N5804, …, 1N5809): Silicon 3A power rectifier diodes (for higher current rectification).

JIS Standard

The Japanese Industrial Standard (JIS) for semiconductor devices uses designations starting with “1S” for all semiconductor diodes (e.g., 1SS…).

Pro Electron Standard

The European Pro Electron coding system, introduced in 1966, uses a two-letter prefix followed by a part code.

• First Letter: Indicates the semiconductor material:

  • A: Germanium
  • B: Silicon

• Second Letter: Indicates the general function of the diode:

  • A: Low-power/signal diode
  • B: Variable capacitance diode (Varicap)
  • X: Multiplier diode
  • Y: Rectifier diode
  • Z: Zener diode (voltage reference)

Examples of Pro Electron Codes:

• AA-series: Germanium low-power/signal diodes (e.g., AA119).
• BA-series: Silicon low-power/signal diodes (e.g., BAT18 silicon RF switching diode).
• BY-series: Silicon rectifier diodes (e.g., BY127 1250V, 1A rectifier diode).
• BZ-series: Silicon Zener diodes (e.g., BZY88C4V7 4.7V Zener diode).

Other Numbering/Coding Systems

Other systems, often manufacturer-driven, include:

• GD-series: Germanium diodes (e.g., GD9) - older coding system.
• OA-series: Germanium diodes (e.g., OA47) - developed by Mullard (UK company).

Related Devices

Diodes are fundamental building blocks in electronics and are closely related to other semiconductor devices:

• Rectifier: A circuit or device (often using diodes) designed to convert AC to DC.
• Transistor: A three-terminal semiconductor device that can amplify or switch electronic signals and power. Diodes are often used in transistor circuits for various functions.
• Thyristor or Silicon Controlled Rectifier (SCR): A four-layer semiconductor device that acts as a unidirectional switch, similar to a diode, but can be triggered to turn “on” and conduct even in the forward direction.
• TRIAC: A three-terminal semiconductor device that acts as a bidirectional switch, capable of conducting current in both directions when triggered.
• DIAC: A two-terminal semiconductor device that behaves like a bidirectional trigger diode, used to trigger TRIACs.
• Varistor: A voltage-dependent resistor whose resistance decreases significantly when the voltage across it exceeds a certain threshold. Used for surge protection, similar to TVS diodes but based on a different principle.

In optics, the equivalent of a diode for laser light is the optical isolator (also known as an optical diode). It allows light to pass in only one direction, typically using a Faraday rotator as the key component.

Applications of Diodes

Diodes are ubiquitous in electronics and have a vast array of applications, leveraging their unidirectional current flow and other unique characteristics.

Radio Demodulation

Historically, the first major application of diodes was radio demodulation of amplitude modulated (AM) radio broadcasts.

Amplitude Modulation (AM): A type of modulation where the amplitude of a carrier wave is varied in proportion to the information signal (e.g., audio signal).

AM Demodulation Process using a Diode:

1. AM Signal: An AM radio signal consists of a high-frequency carrier wave whose amplitude is modulated by the audio signal. It has alternating positive and negative peaks.

2. Diode Rectification: The diode is used as a rectifier to pass only the positive peaks of the AM radio frequency signal and block the negative peaks. This process is called envelope detection.

3. Filtering: A simple filter circuit (typically a capacitor and resistor) is used to smooth out the rectified signal, removing the high-frequency carrier wave and leaving behind the audio envelope, which represents the original audio signal.

4. Audio Amplification and Output: The extracted audio signal is then amplified by an audio amplifier and fed to a speaker or other audio transducer to produce sound waves.

Crystal Detectors in Early Radio: The history of this application is deeply rooted in the development of crystal detectors in the early days of radio.

Microwave and Millimeter Wave Demodulation: In microwave and millimeter wave technology, starting in the 1930s, miniaturized point-contact diodes (crystal diodes) and Schottky diodes became essential for detectors in radar, microwave, and millimeter wave receivers.

Power Conversion

Rectifiers, circuits built using diodes, are fundamental for power conversion, specifically converting alternating current (AC) electricity into direct current (DC).

Examples of Power Conversion Applications:

• Automotive Alternators: Modern car alternators use diodes to rectify the AC output of the alternator into DC to charge the car battery and power the vehicle’s electrical system. Diode rectifiers offer improved performance and efficiency compared to older mechanical commutators or dynamos.

• AC-DC Power Supplies: Most electronic devices require DC power. Diodes are the core components in AC-DC power supplies that convert AC mains electricity from wall outlets into the required DC voltages.

• Cockcroft–Walton Voltage Multipliers: These circuits, using diodes and capacitors, are used to generate high DC voltages from AC inputs. They are used in applications like high-voltage power supplies for older television sets and scientific equipment.

Reverse-Voltage Protection

Electronic circuits are often sensitive to reverse polarity in their power supply inputs. Applying voltage with the wrong polarity can damage components. Reverse-voltage protection circuits using diodes are used to prevent damage in such situations.

Reverse-Voltage Protection (Reverse Polarity Protection, Reverse Battery Protection): Circuit techniques to prevent damage to electronic circuits when the power supply polarity is accidentally reversed.

Reverse-Voltage Protection using a Series Diode:

A simple method is to place a diode in series with the power supply input.

• Correct Polarity: When the power supply is connected with the correct polarity, the diode is forward-biased and allows current to flow to the circuit.
• Reverse Polarity: If the polarity is reversed, the diode becomes reverse-biased and blocks current flow, preventing damage to the circuit.

Limitations: A series diode introduces a voltage drop (forward voltage drop of the diode) in normal operation and also dissipates power.

Over-Voltage Protection

Diodes are frequently employed for over-voltage protection, diverting damaging high voltages away from sensitive electronic devices.

Over-Voltage Protection: Circuit techniques and components used to protect electronic circuits from damage caused by excessive voltage surges or transients.

Over-Voltage Protection using Diodes:

• Reverse-Biased Diodes: Diodes used for over-voltage protection are typically connected in reverse bias across the circuit or component they are protecting. Under normal operating voltages, they are non-conducting and have minimal impact on the circuit.

• Forward Conduction under Overvoltage: When the voltage rises above the normal range and exceeds the diode’s breakdown voltage (for breakdown diodes like Zener or TVS diodes) or forward voltage (for standard diodes in some cases), the diode becomes forward-biased (or breaks down in reverse bias) and starts conducting heavily.

• Voltage Clamping and Current Diversion: The diode effectively clamps the voltage to its forward voltage drop (or breakdown voltage) and diverts the excess current away from the sensitive component.

Examples of Over-Voltage Protection Applications:

• Flyback Diodes (Freewheeling Diodes): Used in inductive circuits like motor controllers (stepper motors, H-bridges), relay circuits, and solenoid drivers. When the current through an inductor is suddenly switched off (e.g., when a transistor switches off a motor), the inductor generates a large voltage spike (flyback voltage) due to its stored energy. A flyback diode is connected in parallel with the inductor in reverse bias. When the switch opens, the flyback diode becomes forward-biased and provides a path for the inductor current to circulate, dissipating the energy and preventing damaging voltage spikes that could harm transistors or other components.

• Integrated Circuit (IC) Input Protection: Many ICs incorporate protection diodes on their input pins to prevent external voltages from exceeding the IC’s voltage ratings and damaging sensitive internal transistors. These diodes typically clamp input voltages to within the safe operating range of the IC.

• Specialized Over-Voltage Protection Diodes: TVS diodes (Transient Voltage Suppression diodes), Zener diodes, and avalanche diodes are specifically designed for over-voltage protection in higher power applications, providing robust surge protection.

Logic Gates

Diodes can be used to construct basic logic gates, specifically AND gates and OR gates, using Diode-Resistor Logic (DRL).

Logic Gate: A fundamental building block in digital circuits that performs a Boolean logic operation on one or more binary inputs and produces a single binary output. Examples include AND, OR, NOT, NAND, NOR, XOR, XNOR gates.

Diode-Resistor Logic (DRL): An early form of digital logic circuitry that used diodes to implement logic functions and resistors for signal level restoration. It was simpler than transistor-based logic but had limitations in fan-out and speed.

Diode OR Gate:

• A simple OR gate can be implemented using diodes. Multiple diodes are connected with their anodes as inputs and their cathodes connected together to form the output through a pull-down resistor to ground.
• If any input is HIGH (logic 1, positive voltage), the corresponding diode will be forward-biased, pulling the output HIGH.
• If all inputs are LOW (logic 0, ground or near ground), all diodes are reverse-biased, and the pull-down resistor pulls the output LOW.

Diode AND Gate:

• A simple AND gate can be implemented using diodes. Multiple diodes are connected with their cathodes as inputs and their anodes connected together to form the output through a pull-up resistor to VCC (positive supply voltage).
• If all inputs are HIGH, all diodes are reverse-biased, and the pull-up resistor pulls the output HIGH.
• If any input is LOW, the corresponding diode will be forward-biased, pulling the output LOW.

Limitations of DRL:

• Signal Degradation: DRL circuits suffer from signal degradation (voltage drop across diodes) in cascaded gates.
• Limited Fan-Out: DRL gates have limited fan-out (number of gates an output can drive).
• No Inversion: DRL alone cannot implement a complete set of logic functions (functional completeness) without adding an active device for inversion (like a transistor).

Diode-Transistor Logic (DTL): To overcome the limitations of DRL, Diode-Transistor Logic (DTL) was developed. DTL combines diodes for logic gating with transistors for amplification and inversion, providing functional completeness and improved performance.

Ionizing Radiation Detectors

Semiconductor diodes are sensitive not only to light (photodiodes) but also to more energetic ionizing radiation, such as cosmic rays, alpha particles, beta particles, gamma rays, and X-rays.

Ionizing Radiation: High-energy radiation that can remove electrons from atoms or molecules, creating ions. Examples include alpha particles, beta particles, gamma rays, X-rays, and cosmic rays.

Radiation Detection Mechanism:

When ionizing radiation interacts with the semiconductor material of a diode:

1. Energy Deposition: The radiation deposits its energy in the semiconductor, creating electron-hole pairs. A single particle of radiation with high energy (thousands or millions of electron volts) can generate a large number of electron-hole pairs.

2. Charge Collection in Depletion Region: If the radiation interacts within or near the depletion region, the electric field in the depletion region sweeps the generated electrons and holes in opposite directions.

3. Current Pulse: This movement of charge carriers creates a current pulse that can be detected and measured externally.

Semiconductor Radiation Detectors:

Semiconductor diodes, particularly PIN diodes with their large depletion region, are used as radiation detectors.

Advantages of Semiconductor Radiation Detectors:

• Direct Charge Collection: They directly convert radiation energy into electrical charge, allowing for direct measurement of particle energy.
• Energy Resolution: With careful design and low leakage current, they can provide relatively accurate measurements of particle energy without complex magnetic spectrometers.
• Compact Size: Compared to other types of radiation detectors, semiconductor detectors can be made relatively compact.

Requirements for Semiconductor Radiation Detectors:

• Efficient and Uniform Charge Collection: Ensuring that all generated charge carriers are collected effectively.
• Low Leakage Current: Minimizing background noise and improving signal-to-noise ratio.
• Cooling: Often cooled with liquid nitrogen to reduce thermal noise and leakage current, especially for high-resolution detectors.
• Large Depletion Region: For detecting longer-range particles or stopping heavy particles, a large depletion depth is needed.
• Thin Contacts: For short-range particles, contacts and undepleted semiconductor regions on at least one surface need to be very thin to minimize energy loss before reaching the depletion region.
• High Reverse Bias Voltage: Back-bias voltages near breakdown (around 1000 volts per centimeter) are used to maximize the depletion region width.

Common Materials: Germanium and silicon are commonly used semiconductor materials for radiation detectors. Germanium has better gamma-ray conversion efficiency than silicon.

Types of Semiconductor Radiation Detectors:

• Energy-Dispersive Detectors: Measure the energy of individual radiation particles.
• Position-Sensitive Detectors: Sense both the energy and position of radiation interactions.
• Pixel Detectors: Two-dimensional arrays of detectors for imaging and tracking applications.

Applications: High-energy physics experiments, nuclear medicine, radiation monitoring, space missions, and homeland security.

Radiation Damage: Semiconductor detectors have a finite lifespan, especially when exposed to heavy particles, due to radiation damage that degrades their performance over time.

Temperature Measurements

The forward voltage drop across a diode is temperature-dependent. This property is used to create diode temperature sensors.

Temperature Dependence of Forward Voltage:

According to the Shockley diode equation, the forward voltage appears to have a positive temperature coefficient (at constant current) due to the thermal voltage term (V_T). However, in most diodes, the reverse saturation current (I_S) is much more sensitive to temperature and increases significantly with temperature. The increase in I_S dominates, leading to an overall negative temperature coefficient for the forward voltage drop at a constant current.

Temperature Coefficient of Forward Voltage: The change in forward voltage drop per degree Celsius (or Kelvin) change in temperature, typically at a constant forward current. For silicon diodes, it is typically negative, around -2 mV/°C.

Diode Temperature Sensor Operation:

1. Constant Current Bias: The diode is biased with a constant forward current.

2. Voltage Measurement: The forward voltage drop across the diode is measured.

3. Temperature Calculation: The temperature is calculated based on the measured forward voltage and the known temperature coefficient of the diode.

Advantages of Diode Temperature Sensors:

• Simplicity: Simple circuit implementation.
• Low Cost: Diodes are inexpensive.
• Wide Temperature Range: Can be used over a broad temperature range, especially cryogenic temperatures.

Limitations:

• Non-linearity: The relationship between forward voltage and temperature is not perfectly linear, although it is approximately linear over a limited temperature range.
• Accuracy: Accuracy is limited by variations in diode characteristics and the need for calibration.

Typical Temperature Coefficient: For silicon diodes, the temperature coefficient is typically around -2 mV/°C (negative temperature coefficient). This value is approximately constant for temperatures above about 20 Kelvin.

Example Diodes for Temperature Sensing: 1N400x series diodes, CY7 cryogenic temperature sensor.

Applications:

• Electronic Thermometers: Simple digital thermometers.
• Temperature Compensation Circuits: Compensating for temperature variations in other electronic circuits.
• Cryogenic Temperature Sensing: Measuring very low temperatures.

Current Steering

Diodes are essential for current steering, directing current flow in desired paths and preventing current flow in unintended directions.

Applications of Current Steering:

• Backup Power Systems (UPS): In uninterruptible power supplies (UPS), diodes are used to automatically switch to battery power during a power failure.

  • Normal Operation: During normal operation, the circuit draws power from the main AC power supply.
  • Power Failure: When the AC power fails, diodes automatically allow the circuit to draw current from a battery backup, ensuring continuous operation. Diodes prevent the battery from discharging back into the AC power supply when it is present.

• Dual Battery Systems in Boats: Small boats often have two separate battery circuits: one for engine starting (high-charge battery) and one for domestic use (lower-charge battery).

  • Single Alternator Charging: Both batteries are charged from a single alternator when the engine is running.
  • Split-Charge Diode: A heavy-duty split-charge diode (or diode isolator) is used to isolate the batteries when the alternator is not running. It prevents the higher-charge engine battery from discharging through the lower-charge domestic battery, ensuring that the engine battery retains sufficient charge for starting.

• Keyboard Matrix Circuits in Electronic Musical Keyboards and Pinball Machines: Electronic musical keyboards and solid-state pinball machines use keyboard matrix circuits to reduce wiring complexity.

  • Switch Matrix: Keys are arranged in a matrix of rows and columns. The keyboard controller scans rows and columns to detect key presses.
  • Phantom Keys and Ghost Notes: When multiple keys are pressed simultaneously in a matrix circuit, phantom keys (false key presses) and ghost notes (unintended notes) can be triggered due to current flowing backward through the circuit.
  • Diodes for Key Isolation: To prevent phantom keys and ghost notes, diodes are soldered in series with the switch under each key. The diodes block reverse current flow, ensuring that only the intended keys are registered when multiple keys are pressed. This principle is also used in switch matrices in solid-state pinball machines.

Waveform Clipper (Limiter)

Diodes can be used as waveform clippers (also called limiters) to limit the positive or negative excursion of a signal to a prescribed voltage level.

Waveform Clipper (Limiter): A circuit that limits the voltage of a signal to a certain level, clipping off portions of the waveform that exceed the limit. Diodes are commonly used in clipper circuits.

Clipper Circuit Operation:

• Series Diode Clipper: A diode connected in series with a signal path can clip either the positive or negative portion of the waveform, depending on the diode’s orientation.
• Parallel Diode Clipper: Diodes connected in parallel with the signal path (often with a voltage reference) can clip both positive and negative portions of the waveform.

Types of Clippers:

• Positive Clipper: Limits the positive portion of the waveform.
• Negative Clipper: Limits the negative portion of the waveform.
• Double Clipper (Slicer): Limits both positive and negative portions, creating a voltage “window.”

Applications of Clippers:

• Signal Shaping: Shaping waveforms for specific applications.
• Overvoltage Protection: Limiting signal voltages to protect sensitive circuits.
• Amplitude Modulation (AM) Detection: Clipping can be used in simple AM detectors.
• Square Wave Generation: Clipping a sine wave can approximate a square wave.

Clamper (DC Restorer)

A diode clamper circuit (also called a DC restorer) shifts the DC level of a periodic AC signal without altering its shape or peak-to-peak amplitude.

Clamper Circuit (DC Restorer): A circuit that shifts the DC level of a periodic AC signal so that either the positive or negative peaks of the signal are clamped to a specific DC voltage level. Diodes and capacitors are essential components in clamper circuits.

Clamper Circuit Operation:

Clamper circuits typically use a diode, a capacitor, and a resistor.

1. Capacitor Charging: During one half-cycle of the input AC signal, the capacitor is charged through the diode to the peak value of the input signal (or a voltage close to it, considering the diode’s forward voltage drop).

2. Clamping Action: During the other half-cycle, the diode is reverse-biased. The capacitor holds its charge and acts as a DC voltage source in series with the input signal. This effectively shifts the entire AC waveform up or down in voltage, clamping either the positive or negative peaks to a desired DC level (often ground or a reference voltage).

Types of Clampers:

• Positive Clamper: Shifts the waveform down so that the positive peaks are clamped to a specific level (e.g., ground).
• Negative Clamper: Shifts the waveform up so that the negative peaks are clamped to a specific level (e.g., ground).
• Biased Clamper: Clamps the peaks to a level other than ground using a DC bias voltage in the circuit.

Applications of Clampers:

• Video Signal Processing: Clamping video signals to establish a proper black level.
• Pulse Circuits: Restoring the DC level of pulses that may have shifted due to AC coupling.
• Level Shifting: Shifting the DC level of signals to be compatible with different circuit voltage levels.

Computing Exponentials and Logarithms

The exponential current-voltage relationship of a diode can be exploited in analog circuits to perform exponentiation and logarithm operations using operational amplifiers (op-amps).

Analog Computation using Diodes and Op-Amps:

• Exponential Output Circuit: An op-amp circuit using a diode in its feedback path can create an exponential amplifier. The output voltage is an exponential function of the input voltage.

• Logarithmic Output Circuit: An op-amp circuit with a diode in the input path and feedback resistor can create a logarithmic amplifier. The output voltage is proportional to the logarithm of the input voltage.

Applications:

• Analog Computation: Performing mathematical operations in analog circuits.
• Signal Compression and Expansion: Compressing and expanding signal ranges for signal processing.
• Analog Multipliers and Dividers: Implementing multiplication and division using exponential and logarithmic circuits.

Oscillator

Regular semiconductor diodes, like the 1N4148, can be modified to exhibit negative differential resistance and be used in oscillators.

Diode Oscillator Principle:

1. Negative Differential Resistance: By injecting calibrated current pulses into a standard diode (like 1N4148) when it is reverse-biased near its avalanche breakdown region, the diode can be made to exhibit negative differential resistance over a certain voltage range.

2. Oscillation with L/C Circuit: When this modified diode is connected in parallel with an L/C resonant circuit (inductor and capacitor), the negative resistance can overcome the losses in the resonant circuit and sustain oscillations.

3. Frequency Control: The oscillation frequency is primarily determined by the resonant frequency of the L/C circuit.

Frequency Range: The maximum oscillation frequency depends on the diode type and its characteristics. With a 1N4148 diode, oscillations up to 100 MHz can be achieved.

Applications:

• Simple Oscillators: Building basic oscillators for signal generation.
• Experimental Circuits: Demonstrating negative resistance and oscillation principles.

Limitations: Diode oscillators are generally not as stable or high-performance as oscillators based on transistors or specialized negative resistance devices like tunnel diodes or Gunn diodes.

Abbreviations

In circuit diagrams and printed circuit boards (PCBs), diodes are commonly abbreviated as D. Sometimes, the abbreviation CR, standing for crystal rectifier (a historical term for early diodes), is also used.

See Also

• Active Rectification: Rectification using active components like transistors or op-amps for improved efficiency and lower voltage drop compared to passive diode rectifiers.
• Diode-Connected Transistor: A transistor configured to behave like a diode by connecting its gate to its drain or source.
• Diode Modeling: Mathematical models and equivalent circuits used to represent the behavior of diodes in circuit simulations and analysis.
• Fast/Ultrafast Diode: Diodes designed for fast switching speed and short reverse recovery time, often using Schottky or fast recovery p-n junction technologies.
• Flame Rectification: Rectification using the ionization properties of a flame, a very early form of rectification.
• Lambda Diode: A type of negative resistance device exhibiting an “N”-shaped I-V curve, similar to a tunnel diode but based on a different mechanism.
• Lr-Diode: A hypothetical or specialized diode type, not a standard term in electronics.
• p–n Junction: The fundamental junction between p-type and n-type semiconductor materials that forms the basis of most semiconductor diodes.
• Small-Signal Model: A linear approximation of a diode’s behavior for small signal variations around a DC bias point, used for circuit analysis.

This detailed resource provides a comprehensive overview of diodes, from their fundamental principles to their diverse types and applications. It aims to serve as a valuable educational tool for students, engineers, and anyone seeking a deeper understanding of these essential electronic components.