Transistors: A Detailed Educational Resource

transistor, semiconductor, electronics, amplifier, switch, history, MOSFET, BJT, FET

A detailed educational resource on transistors, their history, types, and applications. Learn about the invention of transistors, their operation as amplifiers and switches, and their importance in modern electronics.

Read the original article here.

Introduction to Transistors

A transistor is a fundamental semiconductor device that acts as an electronic switch and amplifier. It is one of the most crucial building blocks of modern electronics, enabling the functionality of virtually all electronic devices we use today.

Semiconductor: A material that has electrical conductivity between that of a conductor (like copper) and an insulator (like rubber). The conductivity of semiconductors can be controlled by factors like temperature, light, and the introduction of impurities (doping). Common semiconductor materials include silicon and germanium.

Transistors are typically made from semiconductor materials, most commonly silicon, and are designed with at least three terminals to connect to an electronic circuit. By applying a small voltage or current to one pair of terminals, a transistor can control a much larger current flowing through another pair of terminals. This control mechanism allows transistors to perform two primary functions:

• Amplification: A transistor can increase the power of an electrical signal. Because the output power can be significantly greater than the input power, transistors can boost weak signals, making them stronger. This is essential in audio amplifiers, radio receivers, and many other applications.
• Switching: A transistor can act as an electronic switch, turning current flow on or off. This is the basis of digital logic and is crucial for computers, microprocessors, and digital circuits.

Transistors are manufactured in vast quantities, ranging from individual components to billions integrated within integrated circuits (ICs), also known as microchips. Their invention and subsequent development have revolutionized electronics, leading to smaller, more efficient, and more powerful electronic devices. Many consider the transistor to be one of the most significant inventions of the 20th century due to its transformative impact on technology and society.

History of the Transistor

The journey to the transistor was paved by earlier inventions, most notably the vacuum tube, which dominated electronics in the early 20th century.

Vacuum Tube (Thermionic Valve): An electronic device that controls electric current between electrodes in an evacuated glass or ceramic envelope. Vacuum tubes were crucial for early radio, television, and amplification technologies but were bulky, power-hungry, and fragile.

Precursors to the Transistor: The Vacuum Tube and Crystal Diode

The thermionic triode, a type of vacuum tube invented in 1907, was a pivotal invention that enabled amplified radio signals and long-distance telephone communication. The triode could amplify weak signals, making long-range communication feasible. However, vacuum tubes had significant drawbacks:

• Fragility: Vacuum tubes were made of glass and contained delicate internal structures, making them prone to breakage.
• High Power Consumption: They required significant power to heat a filament (cathode heater) to emit electrons, which was essential for their operation.
• Bulkiness: Vacuum tubes were relatively large and heavy, limiting the miniaturization of electronic devices.

In 1909, physicist William Eccles made another important discovery – the crystal diode oscillator. Crystal diodes, using semiconductor crystals, were more efficient than vacuum tubes for certain applications, but they lacked the amplification capabilities of triodes.

Crystal Diode: An early type of semiconductor diode using a crystal of semiconductor material, often germanium or silicon, to rectify electrical signals. These were predecessors to modern semiconductor diodes and transistors.

Early Concepts of Solid-State Amplifiers: Lilienfeld and Heil

Recognizing the limitations of vacuum tubes, scientists sought solid-state alternatives. Julius Edgar Lilienfeld, a physicist, conceptualized the field-effect transistor (FET) as early as 1925. He filed patents in Canada (1925), the United States (1926 and 1928) for a device intended to be a solid-state replacement for the triode. Lilienfeld’s concept involved controlling current flow in a semiconductor using an electric field, similar in principle to modern FETs. Independently, inventor Oskar Heil patented a similar device in Europe in 1934.

Despite these early patents, a practical, working FET could not be constructed at that time. The primary obstacle was the lack of sufficiently pure semiconductor materials and the understanding of semiconductor surface properties. High-quality semiconductor material production was still decades away. Therefore, Lilienfeld’s and Heil’s ideas, though visionary, remained largely theoretical for many years.

The Bipolar Transistor Revolution

The breakthrough came at Bell Labs in 1947. Physicists John Bardeen, Walter Brattain, and William Shockley were researching semiconductors and aiming to build a solid-state amplifier. From November 17 to December 23, 1947, Bardeen and Brattain conducted experiments that led to the invention of the first working transistor – the point-contact transistor.

Point-Contact Transistor: The first type of transistor invented at Bell Labs in 1947. It used two closely spaced gold contacts pressed onto a germanium crystal to achieve amplification.

They observed that when two gold point contacts were applied to a germanium crystal, the output signal power was greater than the input. This was the “transistor effect” – amplification in a solid-state device. The term “transistor” itself was coined by John R. Pierce as a contraction of “transresistance.”

William Shockley, the leader of the Solid State Physics Group at Bell Labs, recognized the immense potential of this discovery. He worked to deepen the understanding of semiconductors and initially proposed a patent for a field-effect transistor, based on Lilienfeld’s earlier work. However, Bell Labs’ lawyers, after discovering Lilienfeld’s obscure patents, advised against this approach. Instead, the patent focused on the point-contact transistor invented by Bardeen and Brattain.

In 1956, Shockley, Bardeen, and Brattain were jointly awarded the Nobel Prize in Physics for “their researches on semiconductors and their discovery of the transistor effect.”

In 1948, independently from Bell Labs, physicists Herbert Mataré and Heinrich Welker at Compagnie des Freins et Signaux Westinghouse in Paris also invented a point-contact transistor, which they called the “transistron.” Mataré’s prior experience with germanium and silicon rectifiers from his work in German radar during World War II proved invaluable.

The Junction Transistor: While the point-contact transistor was a significant first step, it was somewhat unstable and difficult to manufacture reliably. William Shockley continued his work and invented the first bipolar junction transistor (BJT) in 1948.

Bipolar Junction Transistor (BJT): A type of transistor that uses both electrons and holes as charge carriers. It is constructed with three layers of semiconductor material (NPN or PNP) and has terminals called emitter, base, and collector. BJTs are known for their current amplification capabilities.

In 1950, Bell Labs chemists Gordon Teal and Morgan Sparks successfully created a working bipolar NPN junction germanium transistor, a more stable and manufacturable device. Bell Labs announced this “sandwich” transistor in 1951.

High-Frequency Transistors and Early Applications: The first high-frequency transistor, the surface-barrier germanium transistor, was developed by Philco in 1953, capable of operating up to 60 MHz. This opened up new possibilities for radio and high-speed electronics.

AT&T was the first to use transistors in telecommunications equipment in 1953, in their No. 4A Toll Crossbar Switching System. Early transistors were also instrumental in the development of the first pocket transistor radios. INTERMETALL, founded by Herbert Mataré, showcased a prototype pocket transistor radio in 1953. The first production model, the Regency TR-1, was released in 1954, a joint effort by Regency and Texas Instruments.

The first all-transistor car radio was developed by Chrysler and Philco in 1955, demonstrating the growing versatility of transistor technology. However, it was the Sony TR-63, released in 1957, that truly revolutionized the market, becoming the first mass-produced transistor radio and leading to the widespread adoption of transistors over vacuum tubes by the late 1950s.

Silicon Transistors: Early transistors were primarily made from germanium. However, silicon offered advantages in terms of temperature stability and manufacturing. The first working silicon transistor was developed at Bell Labs in 1954 by Morris Tanenbaum. Texas Instruments announced the first commercial silicon transistor in May 1954, thanks to the work of Gordon Teal, who had previously worked at Bell Labs and was an expert in growing high-purity silicon crystals. Silicon rapidly became the dominant material for transistor manufacturing.

The Field-Effect Transistor (FET) Revisited

While bipolar transistors gained early prominence, the field-effect transistor (FET), initially conceived by Lilienfeld, continued to be developed. The FET concept was also explored by Oskar Heil and William Shockley in the 1930s and 1940s.

In 1945, Heinrich Welker patented the junction FET (JFET). Following Shockley’s theoretical work on JFETs in 1952, a practical JFET was created in 1953 by George C. Dacey and Ian M. Ross.

In 1948, Bardeen and Brattain also patented the precursor to the MOSFET at Bell Labs – an insulated-gate FET (IGFET) with an inversion layer. This concept of an inversion layer is fundamental to modern CMOS and DRAM technology.

However, early FETs suffered from problems, particularly the surface state barrier. This barrier prevented the external electric field from effectively controlling the conductivity of the semiconductor material. This issue hindered the practical development and widespread use of FETs for some time.

The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) Breakthrough

A crucial breakthrough in FET technology occurred in 1955 at Bell Labs. Carl Frosch and Lincoln Derick accidentally grew a layer of silicon dioxide (SiO₂) on a silicon wafer. They observed that this silicon dioxide layer had a passivation effect, improving the surface properties of the silicon.

Surface Passivation: The process of making the semiconductor surface inert and stable by reducing the density of surface states, typically by forming a layer of silicon dioxide on silicon. This is crucial for the proper functioning of MOSFETs.

By 1957, Frosch and Derick, using masking and predeposition techniques, were able to manufacture the first planar transistors. Planar transistors had the drain and source terminals adjacent on the same surface, which was a significant step towards integrated circuits. They demonstrated that silicon dioxide not only insulated but also protected the silicon wafer and prevented dopants from unwanted diffusion.

J.R. Ligenza and W.G. Spitzer further studied the mechanism of thermally grown silicon dioxide and published their findings in 1960, detailing how to fabricate high-quality Si/SiO₂ stacks.

Building on this research, Mohamed Atalla and Dawon Kahng at Bell Labs proposed the silicon MOS transistor in 1959 and successfully demonstrated a working MOS device in 1960. The MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) was born.

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET): The most common type of field-effect transistor. It uses an insulated gate (typically silicon dioxide) to control the flow of current between the source and drain terminals. MOSFETs are characterized by high input impedance, low power consumption, and excellent scalability, making them ideal for integrated circuits.

The MOSFET’s advantages were immediately apparent:

• High Scalability: MOSFETs could be made very small, allowing for high-density integration.
• Low Power Consumption: They consumed significantly less power compared to bipolar transistors.
• High Density: MOSFETs enabled the packing of more transistors onto a single chip.

These features made the MOSFET the ideal transistor for integrated circuits. It became possible to integrate tens of thousands of transistors on a single chip, paving the way for the modern microelectronics revolution.

Further MOSFET Developments:

• CMOS (Complementary MOS): Invented by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963, CMOS technology combined both p-channel and n-channel MOSFETs in complementary pairs. CMOS offered even lower power consumption and became the dominant technology for digital logic.
• Floating-Gate MOSFET: First reported by Dawon Kahng and Simon Sze in 1967, this type of MOSFET is used for non-volatile memory, such as flash memory.
• Self-Aligned Gate MOSFET: Developed by Robert Kerwin, Donald Klein, and John Sarace at Bell Labs in 1967, and used by Federico Faggin and Tom Klein at Fairchild Semiconductor to create the first silicon-gate MOS integrated circuit. This innovation improved manufacturing precision and performance.
• Double-Gate MOSFET: Demonstrated in 1984 by Toshihiro Sekigawa and Yutaka Hayashi at Electrotechnical Laboratory.
• FinFET (Fin Field-Effect Transistor): A 3D multi-gate MOSFET, originating from research by Digh Hisamoto and his team at Hitachi Central Research Laboratory in 1989. FinFETs further improved transistor density and performance, especially at smaller technology nodes.

Importance of Transistors

Transistors are undeniably one of the most important inventions of the 20th century and beyond. Their impact on modern life is profound and ubiquitous.

• Foundation of Modern Electronics: Transistors are the key active components in virtually all modern electronic devices, from smartphones and computers to televisions and automobiles.
• Miniaturization and Efficiency: Transistors replaced bulky and inefficient vacuum tubes, enabling the miniaturization of electronic devices and significantly reducing power consumption.
• Integrated Circuits and the Digital Revolution: The MOSFET, in particular, made integrated circuits possible, leading to the exponential growth of computing power and the digital revolution.
• Ubiquitous Computing: Transistors have made computing and electronics accessible and affordable, permeating almost every aspect of modern life.

The invention of the first transistor at Bell Labs was recognized as an IEEE Milestone in 2009. The junction transistor (1948) and the MOSFET (1959) are also IEEE Milestones, highlighting their monumental impact.

The MOSFET is by far the most widely used transistor globally. It is considered the most important transistor, possibly the most important invention in electronics, and the device that truly enabled modern electronics and the digital age. The U.S. Patent and Trademark Office calls it a “groundbreaking invention that transformed life and culture around the world.”

The mass production of MOSFETs through automated semiconductor device fabrication allows for incredibly low per-transistor costs. MOSFETs are the most numerously produced artificial objects in history, with over 13 sextillion (13 x 10²¹) manufactured by 2018 and production continuing to increase.

While billions of discrete transistors are produced annually, the vast majority are integrated into integrated circuits (ICs) or microchips. These ICs combine transistors with other components like diodes, resistors, and capacitors to create complex electronic circuits. A simple logic gate might contain around 20 transistors, while advanced microprocessors in 2023 can contain over 100 billion transistors, with exceptional chips reaching trillions.

Transistors are organized into logic gates within microprocessors to perform computations. Their low cost, flexibility, and reliability have made them indispensable. Transistorized circuits have replaced electromechanical devices in countless applications, from controlling appliances to industrial machinery. In many cases, using a microcontroller programmed with software to perform a control function is simpler and cheaper than designing an equivalent mechanical system.

Simplified Operation of Transistors

At their core, transistors are devices that use a small electrical signal to control a larger electrical signal. This property, known as gain, is what allows transistors to act as amplifiers and switches.

Gain: The ability of a transistor to increase the power or amplitude of a signal. It’s the ratio of output signal strength to input signal strength.

There are two main types of transistors, each operating on slightly different principles:

1. Bipolar Junction Transistor (BJT)
2. Field-Effect Transistor (FET)

Bipolar Junction Transistors (BJTs)

A bipolar junction transistor (BJT) has three terminals:

• Base (B): The control terminal.
• Collector (C): One terminal of the controlled current path.
• Emitter (E): The other terminal of the controlled current path.

In a BJT, a small current applied to the base terminal (flowing between the base and emitter) can control or switch a much larger current flowing between the collector and emitter. The base-emitter junction behaves like a semiconductor diode, resulting in a voltage drop (V_BE) between them when current flows.

Field-Effect Transistors (FETs)

A field-effect transistor (FET) also has three main terminals:

• Gate (G): The control terminal.
• Source (S): The terminal where charge carriers enter the channel.
• Drain (D): The terminal where charge carriers leave the channel.

In an FET, a voltage applied to the gate controls the current flowing between the source and drain. The voltage at the gate creates an electric field that modulates the conductivity of a channel between the source and drain, thereby controlling the current flow.

Transistor as a Switch

Transistors are fundamental components in digital circuits, where they function as electronic switches. They can rapidly switch between an “on” state (conducting current) and an “off” state (blocking current). This switching capability is used in both high-power applications like switched-mode power supplies and low-power applications such as logic gates in digital circuits.

Logic Gate: A basic building block of digital circuits that performs a logical operation on one or more binary inputs to produce a single binary output. Logic gates are implemented using transistors as switches. Examples include AND, OR, NOT, NAND, and NOR gates.

Key parameters for transistors used as switches include:

• Current Switched: The amount of current the transistor can handle in the “on” state.
• Voltage Handled: The voltage the transistor can withstand in the “off” state without breaking down.
• Switching Speed: How quickly the transistor can transition between “on” and “off” states, characterized by rise and fall times.

Ideally, a transistor switch should behave like:

• Open Circuit (Off State): Infinite resistance, no current flow except for minimal leakage current.
• Short Circuit (On State): Zero resistance, allowing maximum current flow.
• Instantaneous Transition: Switching between states should be as fast as possible.

Example: BJT as a Switch in a Grounded-Emitter Circuit

Consider a simple light-switch circuit using a BJT in a grounded-emitter configuration. As the base voltage increases, the base current and consequently the collector current (current through the light bulb) rise exponentially. This reduces the voltage drop between the collector and emitter.

When the base voltage is sufficiently high, the transistor enters saturation.

Saturation (Transistor): A state in a bipolar transistor where increasing the base current no longer significantly increases the collector current. The transistor is essentially “fully on,” acting like a closed switch.

In saturation, the voltage difference between the collector and emitter becomes very small (ideally zero), and the collector current is limited primarily by the load resistance (the light bulb) and the supply voltage. The transistor is effectively “on,” allowing current to flow and light up the bulb.

To use a BJT as a switch, it needs to be biased to operate between the cut-off region (off state) and the saturation region (on state). Sufficient base drive current is required to ensure saturation. Due to the current gain of the BJT, a small base current can control a much larger collector current. The value of the base resistor in the circuit is chosen to provide adequate base current to saturate the transistor. This value is calculated based on the supply voltage, transistor parameters (like V_BE), collector current requirements, and the transistor’s current gain (beta, β).

Transistor as an Amplifier

Transistors are also crucial for amplification, increasing the strength of electrical signals. They can amplify both voltage and current, depending on the circuit configuration.

Amplifier: An electronic circuit that increases the power of a signal. Transistors are the active components in most amplifiers.

Example: Common-Emitter Amplifier

The common-emitter amplifier configuration is a widely used circuit for voltage amplification. In this configuration, a small change in the input voltage (V_in) applied to the base of the transistor causes a small change in the base current. Due to the transistor’s current amplification, this small base current change results in a much larger change in the collector current. This amplified collector current, when passed through a load resistor, produces a large change in the output voltage (V_out). Thus, a small input voltage variation is transformed into a larger output voltage variation – voltage amplification.

Various amplifier configurations exist, each optimized for specific gain characteristics (current gain, voltage gain, or both). Transistor amplifiers are found in a vast array of electronic products, from mobile phones and televisions for sound reproduction and signal processing to radio transmission systems.

Early transistor audio amplifiers provided only a few hundred milliwatts of power. However, as transistor technology advanced, both power output and audio fidelity improved dramatically. Modern transistor audio amplifiers, capable of hundreds of watts, are common, affordable, and deliver high-quality sound reproduction.

Comparison with Vacuum Tubes

Before the advent of transistors, vacuum tubes (also known as electron tubes or thermionic valves) were the dominant active components in electronics. While transistors largely replaced vacuum tubes, understanding their differences is important.

Advantages of Transistors over Vacuum Tubes

Transistors offer numerous advantages that led to their widespread adoption and the eventual obsolescence of vacuum tubes in most applications:

• No Cathode Heater: Unlike vacuum tubes that require a heated cathode to emit electrons (producing the characteristic orange glow), transistors operate without a heater. This results in:

  • Reduced Power Consumption: Transistors are far more energy-efficient.

  • No Warm-up Delay: Transistors operate instantly, eliminating the warm-up time needed for vacuum tubes.

  • Immunity to Cathode Issues: Vacuum tubes are susceptible to cathode poisoning and depletion, which degrade performance over time. Transistors do not suffer from these issues.

  Cathode Poisoning: A phenomenon in vacuum tubes where impurities from the cathode or other tube components contaminate the cathode’s emissive surface, reducing its electron emission and tube performance.

• Small Size and Weight: Transistors are significantly smaller and lighter than vacuum tubes, enabling the miniaturization of electronic devices.

• Integrated Circuits: Vast numbers of extremely small transistors can be manufactured as a single integrated circuit (IC), a feat impossible with vacuum tubes.

• Low Operating Voltages: Transistors operate at low voltages, compatible with batteries, making portable electronic devices practical. Vacuum tubes often require high voltages.

• Energy Efficiency: Transistor circuits are generally more energy-efficient, especially for low-power applications like voltage amplification.

• Complementary Devices: Transistors are available in complementary types (NPN and PNP, N-channel and P-channel), allowing for flexible circuit designs like complementary-symmetry circuits, which are not feasible with vacuum tubes.

• Mechanical Ruggedness: Transistors are highly resistant to mechanical shock and vibration, making them physically robust and less prone to failure or microphonics (noise induced by vibration).

  Microphonics: Undesired noise or distortion produced in vacuum tubes or other electronic components due to mechanical vibrations affecting their internal structures.

• Durability: Transistors are not susceptible to breakage of a glass envelope, leakage, outgassing, or other physical damage that can plague vacuum tubes.

Limitations of Transistors

Despite their numerous advantages, transistors also have some limitations compared to vacuum tubes:

• Lower Electron Mobility at High Frequencies: Transistors, particularly silicon transistors, have lower electron mobility than the vacuum in vacuum tubes. This is a limitation for very high-power, high-frequency applications, such as high-power over-the-air television transmitters and traveling-wave tubes used in some satellites. Vacuum tubes can operate at higher frequencies due to the faster movement of electrons in a vacuum.

  Electron Mobility: A measure of how quickly electrons can move through a material under the influence of an electric field. Higher electron mobility generally leads to faster device operation.

• Sensitivity to Electrical and Thermal Events: Transistors and other solid-state devices are more susceptible to damage from brief electrical and thermal events, including electrostatic discharge (ESD) during handling. Vacuum tubes are electrically more rugged.

• Radiation Sensitivity: Transistors are sensitive to radiation and cosmic rays, which can alter their performance or cause damage. Specialized radiation-hardened chips are required for spacecraft and other radiation-exposed environments.

• Audio Characteristics (Subjective): In audio applications, some audiophiles prefer the “tube sound” of vacuum tube amplifiers, which is characterized by lower-harmonic distortion. Transistor amplifiers, especially early designs, could sometimes produce harsher distortion. This is a subjective preference, and modern transistor amplifiers have achieved excellent audio fidelity.

Types of Transistors

Transistors are classified in various ways based on their structure, materials, electrical characteristics, and applications.

Classification of Transistors

Transistors can be categorized by:

• Structure:

  • MOSFET (IGFET): Metal-Oxide-Semiconductor Field-Effect Transistor (Insulated-Gate Field-Effect Transistor)
  • BJT: Bipolar Junction Transistor
  • JFET: Junction Field-Effect Transistor
  • IGBT: Insulated-Gate Bipolar Transistor
  • Other types (e.g., FinFET, HEMT, etc.)

• Semiconductor Material (Dopants):

  • Elemental Semiconductors:
    • Germanium (Ge): First used in transistors (1947)
    • Silicon (Si): Predominant material today (first used in 1954) - available in amorphous, polycrystalline, and monocrystalline forms.
  • Compound Semiconductors:
    • Gallium Arsenide (GaAs): (1966) – used for high-frequency applications
    • Silicon Carbide (SiC): (1997) – used for high-power, high-temperature applications
  • Semiconductor Alloys:
    • Silicon-Germanium (SiGe): (1989) – used for high-speed applications
  • Other Materials:

    • Graphene: (research ongoing since 2004) - promising for high-speed and flexible electronics

  Dopants: Impurities intentionally added to a semiconductor material to modify its electrical conductivity. Dopants can be either n-type (donating electrons) or p-type (accepting electrons, creating holes).

• Electrical Polarity:

  • BJTs:
    • NPN: Uses electrons as primary charge carriers.
    • PNP: Uses holes as primary charge carriers.
  • FETs:
    • N-channel: Uses electrons as primary charge carriers.
    • P-channel: Uses holes as primary charge carriers.

• Maximum Power Rating:

  • Low Power: For small signal applications.
  • Medium Power: For intermediate power levels.
  • High Power: For power switching and amplification applications.

• Maximum Operating Frequency:

  • Low Frequency
  • Medium Frequency
  • High Frequency
  • Radio Frequency (RF): For radio communication applications.
  • Microwave Frequency: For microwave communication and radar applications.

    Transition Frequency (f_T): The frequency at which a transistor’s current gain becomes unity (1) in a common-emitter or common-source configuration. It represents the upper limit of the transistor’s useful frequency range for amplification.

• Application:

  • Switch: For digital circuits and power switching.
  • General Purpose: For a wide range of applications.
  • Audio: For audio amplifiers.
  • High Voltage: For high-voltage switching applications.
  • Super-Beta: BJTs with very high current gain.
  • Matched Pair: Two transistors with closely matched characteristics, used in differential amplifiers and current mirrors.

• Physical Packaging:

  • Through-hole: Leaded packages designed to be inserted into holes on a printed circuit board (PCB).
    • Metal packages
    • Plastic packages
  • Surface Mount (SMT): Packages designed to be soldered directly onto the surface of a PCB.
  • Ball Grid Array (BGA): Surface mount package with solder balls instead of leads.
  • Power Modules: Packages designed for high-power applications, often with integrated heat sinks.

• Amplification Factor:

  • h_FE, β_F (Transistor Beta): Current gain for BJTs in common-emitter configuration. It’s the ratio of collector current to base current.
  • g_m (Transconductance): Voltage-to-current gain for FETs. It represents how much the drain current changes for a given change in gate voltage.

• Working Temperature:

  • Standard Temperature Transistors: Typically operate in the range of -55 to 150 °C (-67 to 302 °F).
  • Extreme Temperature Transistors:
    • High-Temperature Transistors: Designed to operate above 150 °C (302 °F), up to 250 °C (482 °F) and beyond.
    • Low-Temperature Transistors: Designed to operate below -55 °C (-67 °F).

A transistor can be described using a combination of these classifications, for example: “silicon, surface-mount, BJT, NPN, low-power, high-frequency switch.”

Mnemonics for Transistor Type

A helpful mnemonic for remembering the direction of the arrow in BJT transistor symbols:

• NPN: “Not Pointing In” – The arrow on the emitter terminal of an NPN transistor symbol points outward, away from the base.
• PNP: “Points In Proudly” – The arrow on the emitter terminal of a PNP transistor symbol points inward, towards the base.

Note that this mnemonic is specific to BJTs and might not apply to all FET symbols, where arrow conventions can differ.

Field-Effect Transistors (FETs) in Detail

Field-effect transistors (FETs), also sometimes called unipolar transistors, utilize either electrons (in n-channel FETs) or holes (in p-channel FETs) as the primary charge carriers for conduction. A typical FET has four terminals:

• Source (S)
• Gate (G)
• Drain (D)
• Body (B) or Substrate (often internally connected to the source)

In an FET, current flow between the source and drain occurs through a conducting channel. The conductivity of this channel is modulated by an electric field created by the gate voltage (V_GS). By changing V_GS, we can control the drain-source current (I_DS).

The relationship between V_GS and I_DS in an FET is generally as follows:

• Subthreshold Region (Weak Inversion): For V_GS below the threshold voltage (V_T), I_DS increases exponentially with V_GS.
• Linear (Triode) Region: For V_GS above V_T and small drain-source voltage (V_DS), I_DS increases linearly with V_DS and approximately quadratically with (V_GS - V_T). Ideal quadratic behavior is not always observed, especially in modern nanoscale devices.
• Saturation Region (Active Region): For V_GS above V_T and larger V_DS, I_DS becomes relatively constant and saturates, becoming less dependent on V_DS and primarily controlled by V_GS. This region is used for amplification.

Threshold Voltage (V_T): The gate-source voltage at which a MOSFET starts to conduct significantly. It’s the voltage required to create a conducting channel in the MOSFET.

FETs are advantageous for low-noise applications at narrow bandwidths due to their high input resistance.

Families of FETs:

• Junction Field-Effect Transistor (JFET): The gate of a JFET forms a p-n junction diode with the channel. This is similar in function to a vacuum tube triode, where the grid and cathode also form a diode junction. JFETs typically operate in depletion-mode (channel is normally on and is depleted by gate voltage), have high input impedance, and are voltage-controlled current devices.

  Depletion-Mode FET: A type of FET where the channel is conductive when the gate-source voltage is zero. Applying a gate voltage of the appropriate polarity reduces the channel conductivity, “depleting” the channel of charge carriers.

• Insulated-Gate Field-Effect Transistor (IGFET): The gate is insulated from the channel by a dielectric layer, typically silicon dioxide (SiO₂). The most common type of IGFET is the Metal-Oxide-Semiconductor FET (MOSFET).

• Metal-Semiconductor Field-Effect Transistor (MESFET): Similar to a JFET, but the p-n junction gate is replaced by a metal-semiconductor junction (Schottky barrier). MESFETs and High-Electron-Mobility Transistors (HEMTs) (also known as HFETs) are particularly well-suited for high-frequency applications (GHz range). HEMTs use a two-dimensional electron gas (2DEG) with very high carrier mobility for charge transport.

  High-Electron-Mobility Transistor (HEMT): A type of FET designed for very high-frequency operation. It utilizes a heterostructure of different semiconductor materials to create a two-dimensional electron gas with exceptionally high electron mobility, resulting in faster switching speeds and lower noise.

Depletion-Mode vs. Enhancement-Mode FETs:

FETs can be further classified into depletion-mode and enhancement-mode types:

• Depletion-Mode FET: The channel is on when the gate-source voltage is zero. A gate voltage of the opposite polarity is needed to deplete the channel and reduce conduction. JFETs are almost always depletion-mode.

• Enhancement-Mode FET: The channel is off when the gate-source voltage is zero. A gate voltage of the correct polarity is needed to enhance conduction by creating a channel. Most IGFETs (MOSFETs) are enhancement-mode.

  Enhancement-Mode FET: A type of FET where the channel is non-conductive when the gate-source voltage is zero. Applying a gate voltage of the appropriate polarity creates a conducting channel, “enhancing” the channel conductivity.

For n-channel devices, a more positive gate voltage increases current in both depletion and enhancement modes. For p-channel devices, a more negative gate voltage increases current.

Metal-Oxide-Semiconductor FET (MOSFET) - The Workhorse Transistor

The metal-oxide-semiconductor field-effect transistor (MOSFET) is the dominant transistor type in modern electronics. It is fabricated by controlled oxidation of a semiconductor, typically silicon, creating a silicon dioxide insulating layer between the metal gate and the semiconductor channel.

The voltage applied to the insulated gate controls the conductivity of the channel. This voltage-controlled conductivity is the basis for amplification and switching applications.

MOSFETs account for approximately 99.9% of all transistors produced worldwide, making them the most common transistor type by a vast margin. They are the fundamental building blocks of most modern electronics due to their scalability, low power consumption, and high density.

Bipolar Junction Transistors (BJTs) in Detail

Bipolar junction transistors (BJTs) are named “bipolar” because they utilize both electrons (negative charge carriers) and holes (positive charge carriers) for conduction. BJTs were the first type of transistor to be mass-produced.

A BJT is essentially a combination of two p-n junction diodes connected back-to-back. It is formed by sandwiching a thin layer of one type of semiconductor material between two layers of the opposite type. This results in two configurations:

• NPN Transistor: A thin layer of p-type semiconductor (base) is sandwiched between two n-type semiconductors (emitter and collector).
• PNP Transistor: A thin layer of n-type semiconductor (base) is sandwiched between two p-type semiconductors (emitter and collector).

These structures create two p-n junctions:

• Base-Emitter Junction
• Base-Collector Junction

These junctions are separated by the thin base region. Simply wiring two diodes together will not create a transistor; the crucial element is the shared, intervening semiconductor base region.

BJTs have three terminals corresponding to the three semiconductor layers:

• Emitter (E)
• Base (B)
• Collector (C)

BJTs are effective amplifiers because a small base current can control much larger emitter and collector currents.

Operation in the Active Region (Amplification):

For an NPN transistor operating in the active region (used for amplification):

1. Base-Emitter Junction is Forward-Biased: A small positive voltage is applied between the base and emitter. This forward-biases the junction, causing electrons to flow from the emitter into the base and holes from the base into the emitter. Recombination of electrons and holes occurs at the junction.
2. Base-Collector Junction is Reverse-Biased: A larger positive voltage is applied between the collector and emitter (collector more positive than base). This reverse-biases the junction. Electrons and holes are generated at and move away from the junction.
3. Electron Injection into the Base: Electrons injected from the emitter into the base diffuse across the thin base region.
4. Collector Current: Most of these electrons reach the reverse-biased base-collector junction and are swept into the collector region due to the electric field. Only a small fraction of electrons recombine within the base region. This recombination in the base is the primary component of the base current.
5. Current Gain (β): The collector current is approximately β (beta) times the base current. Beta (also denoted as h_FE) is the common-emitter current gain of the transistor. For small-signal transistors, β is typically greater than 100, but it can be lower in high-power transistors.

Current Gain (β or h_FE): In a bipolar junction transistor (BJT), the ratio of collector current to base current in a common-emitter configuration. It indicates how much the collector current is amplified for a given base current.

The lightly doped and narrow base region is crucial for high current gain. It minimizes recombination in the base, allowing more charge carriers to reach the collector. By controlling the base current, we effectively control the collector current.

BJT Characteristics:

• Low Input Impedance: BJTs have lower input impedance compared to FETs, meaning they draw more current from the input signal source.

• Exponential Relationship: The base-emitter current and collector-emitter current (I_CE) increase exponentially with the base-emitter voltage (V_BE), following the Shockley diode model and Ebers-Moll model.

• Higher Transconductance: Due to the exponential relationship, BJTs generally have higher transconductance than FETs, making them potentially better for voltage gain in certain amplifier applications.

  Transconductance (g_m): For a transistor, the ratio of change in output current to the change in input voltage. It’s a measure of the transistor’s effectiveness in converting input voltage changes into output current changes.

Phototransistors: BJTs can be made sensitive to light. When light photons are absorbed in the base region, they generate a photocurrent that acts as a base current. The collector current is then amplified by β times the photocurrent. Phototransistors are designed with a transparent window in the package to allow light to reach the base region.

Usage of MOSFETs and BJTs: A Comparison

• MOSFETs:
  • Dominant in Modern Electronics: MOSFETs are the most widely used transistor type, accounting for 99.9% of transistors globally.
  • Digital Circuits: MOSFETs are the primary transistor for digital circuits due to their excellent switching characteristics, low power consumption, and high integration density.
  • Analog Circuits: MOSFETs are also widely used in analog circuits, including power MOSFETs, LDMOS, and RF CMOS.
• BJTs:
  • Historically Significant: BJTs were the most common transistor type from the 1950s to 1960s.
  • Analog Applications (Historically): BJTs were favored for many analog circuits like amplifiers due to their greater linearity.
  • Power Electronics (Partially Replaced): While BJTs were initially used in power electronics, MOSFETs (especially power MOSFETs, LDMOS, and RF CMOS) have largely replaced them in many power applications since the 1980s due to MOSFETs’ advantages in switching speed and efficiency.
  • Specific Analog Applications: BJTs still find use in some specialized analog circuits where their specific characteristics (like higher transconductance or predictable V_BE) are advantageous.

In integrated circuits, MOSFETs have overwhelmingly dominated digital circuits since the 1970s. Discrete MOSFETs, particularly power MOSFETs, are used in a wide range of applications, including analog circuits, voltage regulators, amplifiers, power transmitters, and motor drivers.

Other Transistor Types: A Diverse Landscape

Beyond MOSFETs and BJTs, a vast array of specialized transistor types exists, tailored for specific applications and performance requirements. Some notable examples include:

Field-Effect Transistor (FET) Family:

• Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET):
  • p-type MOS (PMOS): Uses p-channel MOSFETs.
  • n-type MOS (NMOS): Uses n-channel MOSFETs.
  • Complementary MOS (CMOS): Combines PMOS and NMOS transistors for low power digital circuits.
    • RF CMOS: Optimized for radio frequency (RF) applications (amplification, reception).
  • Multi-gate Field-Effect Transistor (MuGFET):
    • Fin Field-Effect Transistor (FinFET): 3D structure with source/drain regions shaped like fins on the silicon surface, enhancing gate control and density.
    • GAAFET (Gate-All-Around FET): Similar to FinFET but uses nanowires instead of fins, with the gate surrounding the nanowires on all four sides for even better gate control.
    • MBCFET (Multi-Bridge-Channel FET) / RibbonFET / Horizontal Nanosheet Transistor: Variant of GAAFET using horizontal nanosheets instead of nanowires, further improving density and performance.
  • Thin-Film Transistor (TFT): Used in LCD and OLED displays, often made from amorphous silicon, LTPS (Low-Temperature Polycrystalline Silicon), LTPO (Low-Temperature Polycrystalline Oxide), or IGZO (Indium Gallium Zinc Oxide).
  • Floating-Gate MOSFET (FGMOS): Used in non-volatile memory (flash memory) for data storage.
  • Power MOSFET: Optimized for power electronics applications (high current, high voltage switching).
    • Lateral Diffused MOS (LDMOS): A type of power MOSFET commonly used in RF power amplifiers.
  • Carbon Nanotube Field-Effect Transistor (CNFET or CNTFET): Uses carbon nanotubes as the channel material, offering potentially high mobility and performance.
  • Ferroelectric Field-Effect Transistor (FeFET): Utilizes ferroelectric materials in the gate stack, enabling non-volatile memory and other unique functionalities.
  • Junction Gate Field-Effect Transistor (JFET): Gate insulated by a reverse-biased p-n junction.
  • Metal-Semiconductor Field-Effect Transistor (MESFET): Similar to JFET but uses a Schottky junction as the gate.
    • High-Electron-Mobility Transistor (HEMT): Specialized MESFET using heterostructures for very high-frequency applications, often based on materials like GaN (Gallium Nitride), SiC (Silicon Carbide), Ga₂O₃ (Gallium Oxide), or GaAs (Gallium Arsenide).
  • Negative-Capacitance FET (NC-FET): Uses ferroelectric materials to achieve sub-60mV/decade subthreshold swing, enabling ultra-low power operation.
  • Inverted-T Field-Effect Transistor (ITFET): A novel FET architecture.
  • Fast-Reverse Epitaxial Diode Field-Effect Transistor (FREDFET): Integrates a fast-recovery diode for improved switching performance.
  • Organic Field-Effect Transistor (OFET): Uses organic semiconductors, enabling flexible and low-cost electronics.
  • Ballistic Transistor: A theoretical transistor where electrons travel ballistically (without scattering) through the channel, potentially achieving very high speeds.
  • FETs for Sensing:
    • Ion-Sensitive Field-Effect Transistor (ISFET): Used to measure ion concentrations in solutions (biosensing, chemical sensing).
    • Electrolyte-Oxide-Semiconductor Field-Effect Transistor (EOSFET): Used in neurochips and biosensors.
    • Deoxyribonucleic Acid Field-Effect Transistor (DNAFET): Used for DNA detection and analysis.
    • Bio-FET (Field-Effect Transistor-based Biosensor): General category of FET-based biosensors.

Bipolar Junction Transistor (BJT) Family:

• Heterojunction Bipolar Transistor (HBT): Uses different semiconductor materials for emitter, base, and collector to achieve very high frequencies (hundreds of GHz), common in ultrafast and RF circuits.
• Schottky Transistor: Integrates a Schottky diode to improve switching speed and prevent saturation.
• Avalanche Transistor: Designed to operate in avalanche breakdown mode for fast pulse generation.
• Darlington Transistor: Configuration of two BJTs connected to provide very high current gain.
• Insulated-Gate Bipolar Transistor (IGBT): Combines a MOSFET input stage with a BJT output stage, offering high input impedance and high power handling capability. Widely used in power electronics (inverters, motor drives).
• Phototransistor: BJT sensitive to light, used as a light sensor.
• Emitter-Switched Bipolar Transistor (ESBT): A monolithic combination of a high-voltage BJT and a low-voltage power MOSFET.
• Multiple-Emitter Transistor: Used in transistor-transistor logic (TTL) and integrated current mirrors.
• Multiple-Base Transistor: Used for amplifying very-low-level signals in noisy environments.

Other Transistor Types:

• Tunnel Field-Effect Transistor (TFET): Switches by modulating quantum tunneling through a barrier, potentially offering lower power consumption than MOSFETs.
• Diffusion Transistor: Formed by diffusing dopants into a semiconductor substrate. Can be either BJT or FET type.
• Unijunction Transistor (UJT): A two-layer, three-terminal device used as a simple pulse generator and in trigger circuits.
• Single-Electron Transistor (SET): Operates by controlling the tunneling of single electrons, enabling extremely low power operation and high sensitivity.
• Nanofluidic Transistor: Controls ion movement in water-filled channels at the nanoscale, a concept in nanoelectronics and bioelectronics.
• Multigate Devices:
  • Tetrode Transistor
  • Pentode Transistor
  • Trigate Transistor: (Intel prototype) Early form of FinFET.
  • Dual-Gate Field-Effect Transistors: Have two gates controlling a single channel, optimized for high-frequency amplifiers, mixers, and oscillators.
• Junctionless Nanowire Transistor (JNT): Uses a uniformly doped nanowire with a gate to modulate current flow, simplifying fabrication.
• Nanoscale Vacuum-Channel Transistor: Prototype vacuum transistor at the nanoscale, potentially offering high speed and radiation hardness.
• Organic Electrochemical Transistor (OECT): Uses organic materials and electrochemical processes.
• Solaristor (Solar Cell Transistor): A two-terminal, gate-less, self-powered phototransistor that integrates solar energy harvesting and transistor functionality.
• Emerging Materials-Based Transistors: Research is ongoing into transistors based on materials like Germanium-Tin, Wood, Paper, Carbon-doped Silicon-Germanium, Diamond, Aluminum Nitride, and Super-lattice castellated structures, exploring new performance and application possibilities.

Device Identification: Transistor Naming Standards

Identifying transistors can be complex due to the variety of naming standards and manufacturer-specific codes. Three major standards are commonly used:

Joint Electron Device Engineering Council (JEDEC) - American Standard

The JEDEC (Joint Electron Device Engineering Council) part numbering system is a US-based standard that evolved in the 1960s. JEDEC transistor numbers usually start with:

• 2N: Indicating a three-terminal device (standard transistor).
• 3N: Indicating a four-terminal device (e.g., dual-gate FETs).

The prefix is followed by a 2, 3, or 4-digit number. The number itself generally does not directly indicate device properties, though lower numbers often correspond to older germanium devices.

Examples:

• 2N3055: A silicon NPN power transistor (widely used general-purpose power transistor).
• 2N1301: A PNP germanium switching transistor (an older type).

A letter suffix (e.g., “A,” “B,” etc.) is sometimes appended to indicate a newer variant or improved version of the original device. Gain groupings are rarely indicated by suffixes in JEDEC numbers.

Japanese Industrial Standard (JIS) - Japanese Standard

The JIS (Japanese Industrial Standard) semiconductor designation system labels transistors starting with:

• 2S: e.g., 2SD965, 2SC1815.

Often, the “2S” prefix is omitted on the device package itself. For example, a 2SD965 might be marked simply “D965,” and a 2SC1815 as “C1815.” Suppliers may also list them without the “2S” prefix.

JIS series often include suffixes like “R,” “O,” “BL” (for red, orange, blue, etc.) to denote variants with tighter tolerances or different h_FE (gain) groupings.

European Electronic Component Manufacturers Association (EECA) - European Standard

The EECA (European Electronic Component Manufacturers Association) numbering scheme, inherited from Pro Electron (which merged with EECA in 1983), uses a two-letter prefix followed by a three-digit sequence number (or one letter and two digits for industrial types).

• First Letter: Indicates the semiconductor material:
  • A: Germanium
  • B: Silicon
  • C: Materials other than Ge or Si, like GaAs.
• Second Letter: Denotes the intended application or device type:
  • A: Diode
  • C: General-purpose transistor
  • … (other letters for specific types)

The three-digit sequence number originally indicated the case type in early devices. Suffixes are common:

• Letters (e.g., “C” in BC549C) often indicate higher h_FE (gain).
• Other codes can denote gain ranges (e.g., BC327-25) or voltage ratings (e.g., BUK854-800A).

Common EECA Prefixes:

• BC: Silicon, general-purpose transistor (e.g., BC547, BC548, BC549)
• BD: Silicon, power transistor
• BF: Silicon, RF transistor
• BA: Germanium, diode
• AC: Germanium, general-purpose transistor

Proprietary Numbering Systems

Manufacturers may also use their own proprietary numbering systems. For example, the CK722 was a well-known early germanium transistor with a proprietary designation.

While some prefixes like “MPF” (originally Motorola FET) might suggest a manufacturer, second-sourcing (multiple manufacturers producing the same or similar devices) has made manufacturer prefixes less reliable indicators of the actual producer.

Some proprietary naming schemes borrow elements from standard schemes. For instance, a PN2222A might be a plastic-cased version of the JEDEC 2N2222A (possibly from Fairchild Semiconductor). However, variations exist; a PN108 is a plastic version of a BC108 (EECA standard), not a 2N108 (which is a completely different device). The PN100 is unrelated to other “xx100” devices.

Military part numbers are often assigned their own codes, like the British Military CV Naming System.

Large-volume purchasers may request parts with “house numbers,” which are specific to their purchasing specifications and not necessarily registered or standardized device numbers. For example, an HP part number might be assigned to a standard JEDEC device like the 2N2218 and also get a military CV number.

Naming Problems and Ambiguity

The existence of multiple independent naming schemes and the practice of abbreviating part numbers on device packages can lead to ambiguity.

Example: “J176” could refer to either:

• J176: A low-power JFET.
• 2SJ176: A higher-power MOSFET.

As older “through-hole” transistors get surface-mount counterparts, numerous new part numbers are often created by manufacturers to accommodate different pinout arrangements and options (e.g., dual or matched NPN + PNP devices in a single package). Even well-established devices like the 2N3904, originally standardized, have numerous non-standardized surface-mount versions with varied naming conventions.

Construction of Transistors

Semiconductor Materials

The first transistors were made from germanium (Ge). While germanium was crucial in early transistor development, silicon (Si) is now the dominant semiconductor material for transistors due to its superior temperature stability, abundance, and suitability for integrated circuit fabrication. However, germanium still finds niche applications and is used in conjunction with silicon in silicon-germanium (SiGe) alloys for high-speed transistors.

Advanced microwave and high-performance transistors increasingly utilize compound semiconductor materials like gallium arsenide (GaAs) and silicon carbide (SiC).

Elemental Semiconductor Material: A semiconductor material made of a single element, such as germanium (Ge) or silicon (Si). Compound Semiconductor Material: A semiconductor material composed of two or more elements from different groups in the periodic table, such as gallium arsenide (GaAs) or silicon carbide (SiC). Semiconductor Alloy: A semiconductor material formed by mixing two or more semiconductor elements or compounds, such as silicon-germanium (SiGe).

Rough Parameter Comparison of Common Semiconductor Materials:

Key Material Properties:

• Junction Forward Voltage: The voltage required to forward-bias a p-n junction and initiate current flow. Lower voltage is generally better as it reduces power consumption for driving the transistor. It decreases with increasing temperature (approximately -2.1 mV/°C for silicon).
• Electron Mobility & Hole Mobility: Measure how easily electrons and holes move through the material under an electric field. Higher mobility generally leads to faster transistor operation. Germanium has higher electron and hole mobility than silicon, while gallium arsenide has the highest electron mobility among these three.

Germanium vs. Silicon vs. Gallium Arsenide:

• Germanium (Ge):
  • Advantages: Higher electron and hole mobility than silicon. Lower junction forward voltage.
  • Disadvantages: Lower maximum operating temperature, higher leakage current, lower voltage breakdown, less suitable for integrated circuits.
• Silicon (Si):
  • Advantages: Good balance of properties, high maximum operating temperature, low leakage current, good voltage breakdown, excellent for integrated circuits, abundant and cost-effective.
  • Disadvantages: Lower electron and hole mobility than germanium and gallium arsenide. Higher junction forward voltage than germanium.
• Gallium Arsenide (GaAs):
  • Advantages: Highest electron mobility among the three, enabling very high-frequency operation.
  • Disadvantages: Lower hole mobility, higher junction forward voltage, more expensive and complex to process than silicon.

Silicon is dominant for most transistor applications due to its overall balance of properties and suitability for mass production of integrated circuits. Gallium arsenide is preferred for high-frequency applications where its superior electron mobility is crucial.

High-Electron-Mobility Transistors (HEMTs): HEMTs based on materials like aluminum gallium arsenide (AlGaAs)-gallium arsenide (GaAs) and gallium nitride (GaN) offer even higher electron mobility, enabling even higher frequency operation and lower noise. They are used in applications like satellite receivers and high-power RF electronics.

Maximum Junction Temperature: This is the maximum temperature the transistor’s semiconductor junction can withstand without damage. It is crucial to stay within this limit to ensure reliable operation.

Al-Si Junction (Schottky Diode): The table also mentions “Al-Si junction,” referring to the Schottky diode, a high-speed metal-semiconductor diode. Some silicon power MOSFETs have a parasitic reverse Schottky diode formed between the source and drain during fabrication. This diode can sometimes be a nuisance but is also utilized in certain circuit designs.

Packaging

Transistors are available in various semiconductor packages, broadly categorized as:

• Through-Hole (Leaded): Packages with leads designed to be inserted into holes on a PCB and soldered on the other side.
• Surface-Mount (SMD - Surface Mount Device): Packages designed to be soldered directly onto the surface of a PCB. SMDs are smaller and have shorter interconnections, offering better high-frequency performance but often lower power ratings than through-hole packages.
• Ball Grid Array (BGA): A type of SMD package with solder balls on the underside instead of leads, enabling very high pin counts and compact size.

Through-Hole Package: A type of electronic component package with leads designed to be inserted through holes in a printed circuit board (PCB) for soldering. Surface-Mount Package (SMD): A type of electronic component package designed to be mounted directly onto the surface of a printed circuit board (PCB) without leads passing through holes. Ball Grid Array (BGA): A type of surface-mount package (SMD) that uses an array of solder balls on its underside for electrical connections to a printed circuit board (PCB).

Transistor packages are made from materials like glass, metal, ceramic, or plastic. The package type significantly influences the transistor’s power rating and frequency characteristics.

Power transistors typically come in larger packages with provisions for attaching heat sinks to dissipate heat effectively. In many power transistors, the collector (BJT) or drain (FET) is electrically connected to the metal enclosure for improved heat dissipation.

At the opposite extreme, some microwave transistors in surface-mount packages can be incredibly small, almost like grains of sand, to minimize parasitic capacitances and inductances for high-frequency operation.

A given transistor type may be available in multiple package styles. While transistor packages are mostly standardized in form factor, the terminal assignment (pinout) is not always consistent. Different transistor types can use the same package but have different pin functions. Even within the same transistor type, terminal assignments can vary, often indicated by a suffix letter in the part number (e.g., BC212L vs. BC212K).

Common Through-Hole Transistor Packages (Alphabetical Order):

ATV, E-line, HRT, MRT, SC-43, SC-72, TO-3, TO-18, TO-39, TO-92, TO-126, TO-220, TO-247, TO-251, TO-262, ZTX851. (TO - Transistor Outline)

Unpackaged Transistor Chips (Die):

Transistor chips (die) can also be used unpackaged in hybrid devices or assembled using techniques like direct chip attach (DCA) and chip-on-board (COB). The IBM SLT module from the 1960s is an example of a hybrid circuit module using glass-passivated transistor and diode die.

Flexible Transistors:

Researchers have developed flexible transistors, including organic field-effect transistors (OFETs), which are useful in applications like flexible displays and other flexible electronics.

See Also

(Refer to the original Wikipedia article for related topics.)

References

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Further Reading

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External Links

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