A transistor is a semiconductor device used to increase the strength of electrical signals or control the flow of electricity. It is a key part of many modern electronic devices. A transistor is made from semiconductor material and usually has three terminals that connect to an electronic circuit. When a voltage or current is applied to one pair of terminals, it controls the current flowing through another pair. This allows the output power to be greater than the input power, which means a transistor can amplify a signal. Some transistors are used individually, while others are built into integrated circuits. Because transistors are essential in most modern electronics, many people believe they are one of the greatest inventions of the 20th century.

In 1925, physicist Julius Edgar Lilienfeld suggested the idea of a field-effect transistor (FET), but it could not be built at that time. The first working transistor, called a point-contact transistor, was created in 1947 by John Bardeen, Walter Brattain, and William Shockley at Bell Labs. These scientists won the 1956 Nobel Prize in Physics for this invention. The most common type of transistor, the metal–oxide–semiconductor field-effect transistor (MOSFET), was developed at Bell Labs between 1955 and 1960. Transistors changed the field of electronics and made smaller, more affordable devices like radios, calculators, and computers possible.

Most transistors are made from very pure silicon, and some are made from germanium. Other semiconductor materials are sometimes used as well. A field-effect transistor uses only one type of charge carrier, while a bipolar junction transistor uses two types. Compared to vacuum tubes, transistors are smaller and use less power. However, some vacuum tubes, like traveling-wave tubes and gyrotrons, work better at very high frequencies or high voltages. Many types of transistors are produced to standard specifications by different manufacturers.

History

The thermionic triode, a vacuum tube invented in 1907, helped create amplified radio technology and long-distance telephone systems. However, the triode was a delicate device that used a lot of power. In 1909, physicist William Eccles discovered the crystal diode oscillator. In 1925, physicist Julius Edgar Lilienfeld filed a patent for a field-effect transistor (FET) in Canada, designed as a solid-state replacement for the triode. He filed similar patents in the United States in 1926 and 1928. However, he did not publish research about his devices, and his patents did not describe working prototypes. Because high-quality semiconductor materials were not available at the time, Lilienfeld’s ideas would not have been useful in the 1920s and 1930s, even if the devices had been built. In 1934, inventor Oskar Heil patented a similar device in Europe.

From November 17 to December 23, 1947, John Bardeen and Walter Brattain at Bell Labs in Murray Hill, New Jersey, experimented with a crystal of germanium. They found that applying two gold point contacts to the crystal produced a signal with more power than the input. William Shockley, leader of the Solid State Physics Group, recognized the importance of this discovery and expanded knowledge about semiconductors. The word "transistor" was created by John R. Pierce as a short form of "transresistance." According to Lillian Hoddeson and Vicki Daitch, Shockley suggested that Bell Labs’ first patent for a transistor should focus on the field-effect and that he be named as the inventor. However, Bell Labs lawyers noted that Lilienfeld’s earlier patents had already described a similar idea. Instead, Bardeen, Brattain, and Shockley invented the first point-contact transistor in 1947. For this achievement, they shared the 1956 Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect."

Shockley’s team first tried to build a field-effect transistor (FET) by changing the conductivity of a semiconductor, but they failed due to problems with surface states, dangling bonds, and materials like germanium and copper. Investigating these challenges led them to develop the bipolar point-contact and junction transistors instead.

In 1948, physicists Herbert Mataré and Heinrich Welker independently invented the point-contact transistor while working at Compagnie des Freins et Signaux Westinghouse in Paris. Mataré had experience making crystal rectifiers from silicon and germanium during World War II. By June 1948, he and Welker produced consistent results using germanium samples, similar to Bardeen and Brattain’s work in 1947. Learning that Bell Labs had already invented the transistor, the company quickly began producing its own device, called a "transistron," for use in France’s telephone network. They filed their first patent application on August 13, 1948.

In 1948, Bell Labs’ William Shockley applied for a patent (2,569,347) for a bipolar junction transistor. On April 12, 1950, Bell Labs chemists Gordon Teal and Morgan Sparks successfully made a working bipolar NPN junction amplifying germanium transistor. Bell Labs announced this new "sandwich transistor" in a press release on July 4, 1951.

The first high-frequency transistor was the surface-barrier germanium transistor developed by Philco in 1953. It could operate at frequencies up to 60 MHz. These transistors were made by etching small depressions into an n-type germanium base with jets of indium(III) sulfate until it was very thin. Indium electroplated into the depressions formed the collector and emitter.

AT&T first used transistors in telecommunications equipment in the No. 4A Toll Crossbar Switching System in 1953, for selecting trunk circuits from routing information encoded on translator cards. Before this, the Western Electric No. 3A phototransistor read mechanical encoding from punched metal cards.

The first prototype pocket transistor radio was shown by INTERMETALL, a company founded by Herbert Mataré in 1952, at the Internationale Funkausstellung Düsseldorf from August 29 to September 6, 1953. The first production-model pocket transistor radio was the Regency TR-1, released in October 1954. It was made by Regency Division of Industrial Development Engineering Associates and Texas Instruments. The TR-1 had four transistors and one germanium diode. It was designed by Painter, Teague and Petertil in Chicago and came in six colors: black, ivory, mandarin red, cloud grey, mahogany, and olive green. Other colors were added later.

The first production all-transistor car radio was developed by Chrysler and Philco and announced in The Wall Street Journal on April 28, 1955. Chrysler offered the Mopar model 914HR as an option starting in fall 1955 for its 1956 Chrysler and Imperial cars, which reached dealers in October 1955.

The Sony TR-63, released in 1957, was the first mass-produced transistor radio. It led to the widespread use of transistor radios. Seven million TR-63s were sold worldwide by the mid-1960s. Sony’s success with transistor radios caused transistors to replace vacuum tubes as the main electronic technology in the late 1950s.

The first working silicon transistor was developed at Bell Labs on January 26, 1954, by Morris Tanenbaum. Texas Instruments announced the first production commercial silicon transistor in May 1954. This was done by Gordon Teal, an expert in growing high-purity crystals who had previously worked at Bell Labs.

The basic idea of the field-effect transistor (FET) was first proposed by physicist Julius Edgar Lilienfeld in 1926 and 1928. The FET concept was also later theorized by engineer Osk

Importance

Transistors are the most important parts in nearly all modern electronic devices. Because of this, many people believe they are one of the greatest inventions of the 20th century.

The first transistor was created in 1947 at Bell Labs. This invention was recognized as an IEEE Milestone in 2009. Other important milestones include the creation of the junction transistor in 1948 and the MOSFET in 1959.

The MOSFET is the most commonly used type of transistor. It is used in many devices, such as computers, smartphones, and communication tools. It is considered the most important transistor and a key invention in electronics. Since the late 20th century, the MOSFET has been the foundation of modern digital electronics, helping to create the digital age. The US Patent and Trademark Office describes it as a "groundbreaking invention that changed life and culture worldwide." Its ability to be made in large quantities using automated manufacturing processes from simple materials makes it very affordable. By 2018, more than 13 sextillion MOSFETs had been produced, making them the most numerous artificial objects ever made.

Many companies produce over a billion individual MOSFETs each year. However, most are made as part of integrated circuits, which also include other components like diodes, resistors, and capacitors. These circuits are used to create complete electronic systems. A simple logic gate may use up to 20 transistors, while an advanced microprocessor, as of 2023, can contain up to 92 billion transistors on a single chip. When using two chips, this number can double to 184 billion. Some special chips, as of 2020, may contain up to 2.6 trillion transistors. Transistors in microprocessors are often grouped into logic gates to perform calculations.

Transistors are widely used because they are inexpensive, versatile, and dependable. They have replaced older mechanical systems in controlling machines and appliances. It is often easier and less expensive to use a standard microcontroller and write a computer program to control a device than to design a mechanical system for the same purpose.

Simplified operation

A transistor uses a small signal between one pair of its terminals to control a much larger signal at another pair of terminals. This ability is called gain. It can make a stronger output signal, such as voltage or current, that is related to a weaker input signal. This makes it act as an amplifier. It can also be used as an electrically controlled switch, where the amount of current depends on other parts of the circuit.

There are two main types of transistors, with slight differences in how they are used:

• A bipolar junction transistor (BJT) has three terminals labeled base, collector, and emitter. A small current at the base terminal, which flows between the base and emitter, can control or switch a much larger current between the collector and emitter.

• A field-effect transistor (FET) has three terminals labeled gate, source, and drain. A voltage at the gate can control the current between the source and drain.

The top image in this section shows a typical bipolar transistor in a circuit. Electric charge flows between the emitter and collector terminals based on the current at the base. The base and emitter connections behave like a semiconductor diode, causing a voltage drop between them. The size of this drop, determined by the transistor's material, is called V BE (Base Emitter Voltage).

Transistors are often used in digital circuits as electronic switches that can be either on or off. They are used in both high-power applications, like switched-mode power supplies, and low-power applications, like logic gates. Important factors for this use include the amount of current switched, the voltage handled, and the speed of switching, measured by how quickly the signal changes.

In a switching circuit, the goal is to make the switch behave like an ideal switch. When off, it acts like an open circuit, and when on, it acts like a short circuit. The switch should change states quickly, with the off state having very little current (leakage) and the on state having very low resistance. These properties ensure the switch does not harm the circuit.

In a grounded-emitter transistor circuit, such as a light-switch circuit, as the base voltage increases, the emitter and collector currents increase rapidly. The collector voltage decreases because the resistance between the collector and emitter becomes smaller. If the voltage between the collector and emitter were zero (or nearly zero), the collector current would only depend on the load (like a light bulb) and the power supply. This is called saturation, meaning the current flows freely from the collector to the emitter. When saturated, the switch is on.

To use bipolar transistors for switching, the transistor must be biased to operate between the off-state (cut-off region) and the on-state (saturation region). This requires enough base current. Because transistors provide current gain, a small base current can control a much larger collector current. The ratio of these currents depends on the type of transistor and the collector current. In a light-switch circuit, the resistor is chosen to provide enough base current to ensure the transistor is saturated. The resistor value is calculated using the power supply voltage, the transistor’s C-E voltage drop, the collector current, and the amplification factor beta.

A common-emitter amplifier is designed so that a small change in input voltage (V in) causes a small change in the base current. This change, combined with the transistor’s current amplification and the circuit’s properties, leads to large changes in the output voltage (V out).

Many different single-transistor amplifier designs are possible. Some provide current gain, some provide voltage gain, and some provide both.

From mobile phones to televisions, many products use amplifiers for sound, radio, and signal processing. Early transistor-based amplifiers produced only a few hundred milliwatts of power, but as better transistors became available and amplifier designs improved, power and sound quality increased. Today, transistor-based amplifiers capable of producing hundreds of watts are common and affordable.

Comparison with vacuum tubes

Before transistors were invented, vacuum tubes (also called thermionic valves in the UK) were the main parts used in electronic devices.

Transistors have replaced vacuum tubes in most uses because of these advantages:

• Transistors do not need a heater to produce light, which saves energy, avoids delays from warming up, and prevents problems like cathode poisoning.
• They are very small and light, making electronic devices smaller.
• Many tiny transistors can be made together on a single chip.
• They work well with low voltages, such as those from small batteries.
• They use less energy than vacuum tubes, especially in tasks like amplifying voltage.
• They can be paired with other devices to create more flexible circuit designs.
• They are not easily broken by shocks or vibrations, reducing unwanted noise in audio devices.
• They are not damaged by broken glass, gas leaks, or other physical harm.

Transistors also have some limitations:

• They do not move electrons as quickly as vacuum tubes, which is needed for high-power or high-frequency uses, such as in some TV transmitters or satellite amplifiers.
• They can be damaged by brief electrical or heat events, like static electricity, while vacuum tubes are more resistant.
• They can be harmed by radiation and cosmic rays, requiring special protection for use in space.
• In audio devices, they do not produce the same type of sound as vacuum tubes, which some people prefer.

Types

Transistors are grouped into categories based on several factors:

• Structure: MOSFET (IGFET), BJT, JFET, insulated-gate bipolar transistor (IGBT), and other types.
• Semiconductor material (dopants): Metalloids such as germanium (first used in 1947) and silicon (first used in 1954), in amorphous, polycrystalline, or monocrystalline forms. Compounds like gallium arsenide (1966) and silicon carbide (1997). Alloys such as silicon–germanium (1989) and materials like graphene (research since 2004).
• Electrical polarity: NPN and PNP (BJTs), and N-channel and P-channel (FETs).
• Maximum power rating: Low, medium, or high.
• Maximum operating frequency: Low, medium, high, radio (RF), or microwave frequency. The highest effective frequency for a transistor is called the transition frequency (fT), which is the frequency at which the transistor provides equal input and output voltage.
• Application: Switch, general purpose, audio, high voltage, super-beta, or matched pair.
• Physical packaging: Through-hole metal, through-hole plastic, surface mount, ball grid array, or power modules.
• Amplification factor: Measured by hFE, βF (transistor beta), or gm (transconductance).
• Working temperature: Traditional transistors (−55 to 150 °C) and extreme temperature transistors (below −55 °C or above 150 °C). Some high-temperature transistors can operate up to 250 °C using special materials.

A specific transistor might be described as silicon, surface-mount, BJT, NPN, low-power, high-frequency switch.

A helpful way to remember transistor types (based on their electrical symbol) is to look at the direction of the arrow. For BJT symbols, the arrow on an n–p–n transistor points away from the "N" in NPN. On a p–n–p transistor, the arrow points toward the "P" in PNP. This rule does not apply to MOSFET symbols, where the arrow direction is often reversed.

Field-effect transistors (FETs), also called unipolar transistors, use either electrons (in n-channel FETs) or holes (in p-channel FETs) to conduct electricity. FETs have four terminals: source, gate, drain, and body (substrate). On most FETs, the body is connected to the source inside the package.

In a FET, current flows from the drain to the source through a conducting channel. The conductivity of the channel is controlled by the voltage applied between the gate and source. When the gate–source voltage (VGS) increases, the drain–source current (IDS) increases rapidly below a certain threshold voltage (VT) and then more slowly above it. Modern FETs do not always follow this pattern.

FETs with high input resistance are useful for reducing noise in narrow bandwidth applications.

FETs are divided into two main groups: junction FETs (JFETs) and insulated gate FETs (IGFETs). IGFETs are more commonly called metal–oxide–semiconductor FETs (MOSFETs), named for their construction using layers of metal, oxide, and semiconductor. Unlike IGFETs, JFETs use a p–n diode between the gate and channel. JFETs operate similarly to vacuum tube triodes and have high input resistance.

Metal–semiconductor FETs (MESFETs) and high-electron-mobility transistors (HEMTs) are types of FETs that use special materials for high-frequency applications (several GHz).

FETs are also divided into depletion-mode and enhancement-mode types. Depletion-mode FETs conduct current even when no voltage is applied to the gate, while enhancement-mode FETs require a gate voltage to start conducting. For n-channel FETs, a more positive gate voltage increases current, while for p-channel FETs, it decreases current. Most JFETs are depletion-mode, and most IGFETs are enhancement-mode.

The metal–oxide–semiconductor field-effect transistor (MOSFET) is the most common type of transistor. It is made by oxidizing a semiconductor, usually silicon, and has an insulated gate that controls the device’s conductivity. MOSFETs are used for amplifying or switching electronic signals and are the foundation of most modern electronics. Over 99.9% of all transistors in the world are MOSFETs.

Bipolar transistors are called "bipolar" because they use both majority and minority carriers to conduct electricity. The first mass-produced transistor, the bipolar junction transistor (BJT), is made by combining two p–n diodes. It has three layers: n–p–n or p–n–p, with two p–n junctions (base–emitter and base–collector).

BJTs have three terminals: emitter, base, and collector. They are used in amplifiers because a small base current controls the larger currents at the emitter and collector. In an n–p–n transistor, the base is narrow and lightly doped, allowing most electrons to move from the emitter to the collector. This process controls the collector current, which is the main function of the transistor.

Device identification

Three main systems are used to identify transistor devices. Each system uses a combination of numbers and letters to describe the type of device.

The JEDEC numbering system was created in the United States during the 1960s. JEDEC numbers for transistors often start with "2N," which means the device has three parts. Transistors with four parts, such as dual-gate field-effect transistors, begin with "3N." After the prefix, a number with two, three, or four digits follows. These numbers do not describe the device's properties, though early devices with small numbers were often made of germanium. For example, 2N3055 is a silicon transistor, and 2N1301 is a germanium transistor. A letter, like "A," may follow the number to show a newer version of the device.

In Japan, the JIS-C-7012 system labels transistors with numbers starting with "2S," such as 2SD965. However, sometimes the "2S" is not shown on the package, and the device might only be marked as D965 or C1815. This system sometimes uses letters like "R," "O," or "BL" to show different versions, such as variations in performance.

The European Electronic Component Manufacturers Association (EECA) uses a numbering system that came from Pro Electron. This system starts with two letters: the first letter shows the material (A = germanium, B = silicon, C = other materials like GaAs), and the second letter shows the device's purpose (A = diode, C = general-purpose transistor). A three-digit number follows, or one letter and two digits for industrial devices. Early devices used this number to describe the package type. Letters or other codes may follow to show details like performance (e.g., BC549C) or voltage (e.g., BUK854-800A).

Some manufacturers use their own numbering systems, such as CK722. Because devices are often made by multiple companies, a prefix like "MPF" in MPF102 no longer reliably shows the maker. Some systems mix parts from other systems, like PN2222A, which is a version of 2N2222A in a plastic package.

Military devices sometimes use special codes, such as the British CV system. When companies buy large quantities of similar parts, they may assign their own numbers for internal use, such as HP 1854,0053, which refers to the JEDEC 2N2218 transistor.

Because there are many different numbering systems and part numbers are often shortened on devices, confusion can happen. For example, the number J176 might refer to two different devices: a low-power JFET and a high-power MOSFET.

As older transistors are replaced with surface-mount versions, new devices often have many different part numbers. This is because manufacturers use different systems to handle changes in design, such as different pin arrangements or packaging options. Even if a device like 2N3904 is well known, its newer versions may not follow a standard naming system.

Construction

The first BJTs were created using germanium (Ge). Today, silicon (Si) types are most commonly used, but some advanced microwave and high-performance transistors use gallium arsenide (GaAs) or the semiconductor alloy silicon–germanium (SiGe). Single-element semiconductor materials like germanium and silicon are called elemental.

Basic information about common semiconductor materials used in transistors is shown in the table next to this text. These values can change when temperature, electric field, impurity levels, strain, or other factors increase.

The junction forward voltage is the voltage applied to the emitter-base junction of a BJT to make the base carry a specific current. As the junction forward voltage increases, the current grows very quickly. The values in the table are typical for a current of 1 mA (the same values apply to semiconductor diodes). A lower junction forward voltage is better because it means less power is needed to operate the transistor. For a typical silicon junction, the junction forward voltage decreases by −2.1 mV for every degree Celsius increase in temperature. In some circuits, special components called sensistors are used to adjust for these changes.

The number of moving particles in the channel of a MOSFET depends on the electric field that forms the channel and other factors, such as the amount of impurities in the channel. Some impurities, called dopants, are added intentionally during the creation of a MOSFET to control its electrical behavior.

The electron mobility and hole mobility columns show how fast electrons and holes move through a semiconductor material when an electric field of 1 volt per meter is applied. In general, higher electron mobility means a transistor can operate faster. The table shows that germanium is better than silicon in this regard. However, germanium has four major drawbacks compared to silicon and gallium arsenide:

• It cannot handle high temperatures.
• It has higher leakage current.
• It cannot withstand high voltages.
• It is less suitable for making integrated circuits.

Since electrons move faster than holes in all semiconductor materials, a bipolar n–p–n transistor is generally faster than an equivalent p–n–p transistor. Gallium arsenide (GaAs) has the highest electron mobility among the three materials. This is why GaAs is used in high-frequency applications. A newer type of FET, the high-electron-mobility transistor (HEMT), uses a structure made from two different semiconductor materials—aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs). This structure allows electrons to move twice as fast as in a GaAs-metal barrier junction. Because of their speed and low noise, HEMTs are used in satellite receivers that operate at frequencies around 12 GHz. HEMTs made from gallium nitride and aluminum gallium nitride (AlGaN/GaN HEMTs) have even higher electron mobility and are being developed for other uses.

Maximum junction temperature values are taken from manufacturer datasheets. This temperature should not be exceeded, or the transistor may be damaged.

An Al–Si junction refers to a high-speed metal–semiconductor barrier diode, also known as a Schottky diode. This is included in the table because some silicon power IGFETs have a parasitic reverse Schottky diode formed between the source and drain during the fabrication process. This diode can cause problems, but it is sometimes used in circuits.

Discrete transistors can be individually packaged or unpackaged chips.

Transistors come in many semiconductor packages (see image). The two main categories are through-hole (or leaded) and surface-mount, also called surface-mount devices (SMD). The ball grid array (BGA) is a newer type of surface-mount package. It uses solder balls on the bottom instead of leads. Because they are smaller and have shorter connections, SMDs work better at high frequencies but have lower power ratings.

Transistor packages are made of glass, metal, ceramic, or plastic. The package often determines the power rating and frequency performance. Power transistors have larger packages that can be attached to heat sinks for better cooling. Most power transistors also connect the collector or drain to the metal enclosure. At the other extreme, some surface-mount microwave transistors are as small as grains of sand.

A specific transistor type may be available in several packages. While transistor packages are mostly standardized, the assignment of functions to the package’s terminals can vary. For example, different transistor types may assign different functions to the same terminals. Even for the same transistor type, the terminal assignments can differ (usually indicated by a letter suffix in the part number, such as BC212L and BC212K).

Today, most transistors come in a wide range of surface-mount packages. In comparison, the number of through-hole packages available is much smaller. Here is a short list of the most common through-hole transistor packages in alphabetical order: ATV, E-line, MRT, HRT, SC-43, SC-72, TO-3, TO-18, TO-39, TO-92, TO-126, TO220, TO247, TO251, TO262, ZTX851.

Unpackaged transistor chips (die) can be used in hybrid devices. The IBM SLT module from the 1960s is an example of a hybrid circuit that uses glass-passivated transistor and diode chips. Other methods for packaging discrete