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A transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Some transistors are packaged individually but most are found in integrated circuits.

The transistor is the fundamental building block of modern electronic devices, and its presence is ubiquitous in modern electronic systems.

History

A replica of the first working transistor.


Physicist Julius Edgar Lilienfeld filed the first patent for a transistor in Canada in 1925, describing a device similar to a Field Effect Transistor or "FET". However, Lilienfeld did not publish any research articles about his devices, and in 1934, German inventor Oskar Heil patented a similar device.

In 1947, John Bardeen and Walter Brattain at AT&T's Bell Labsmarker in the United Statesmarker observed that when electrical contacts were applied to a crystal of germanium, the output power was larger than the input. Solid State Physics Group leader William Shockley saw the potential in this, and over the next few months worked to greatly expand the knowledge of semiconductors, and thus could be described as the "father of the transistor". The term was coined by John R. Pierce. According to physicist/historian Robert Arns, legal papers from the Bell Labs patent show that William Shockley and Gerald Pearson had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles.

The first silicon transistor was produced by Texas Instrumentsmarker in 1954. This was the work of Gordon Teal, an expert in growing crystals of high purity, who had previously worked at Bell Labs. The first MOS transistor actually built was by Kahng and Atalla at Bell Labs in 1960.

Importance

The transistor is considered by many to be one of the greatest inventions of the twentieth century.The transistor is the key active component in practically all modern electronics. Its importance in today's society rests on its ability to be mass produced using a highly automated process (fabrication) that achieves astonishingly low per-transistor costs.

Although several companies each produce over a billion individually-packaged (known as discrete) transistors every year,the vast majority of transistors produced are in integrated circuits (often shortened to IC, microchips or simply chips) along with diodes, resistors, capacitors and other electronic components to produce complete electronic circuits. A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, as of 2006, can use as many as 1.7 billion transistors (MOSFETs)."About 60 million transistors were built this year [2002] ... for [each] man, woman, and child on Earth."

The transistor's low cost, flexibility, and reliability have made it a ubiquitous device. Transistorized mechatronic circuits have replaced electromechanical devices in controlling appliances and machinery. It is often easier and cheaper to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function.

Usage

The bipolar junction transistor, or BJT, was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of manufacture. Desirable properties of MOSFETs, such as their utility in low-power devices, usually in the CMOS configuration, allowed them to capture nearly all market share for digital circuits; more recently MOSFETs have captured most analog and power applications as well, including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters, motor drivers, etc.

Simplified operation

The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain. A transistor can control its output in proportion to the input signal, that is, can act as an amplifier. Or, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.

The two types of transistors have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing from the base to the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.

The image to the right represents a typical bipolar transistor in a circuit. Charge will flow between emitter and collector terminals depending on the current in the base. Since internally the base and emitter connections behave like a semiconductor diode, a voltage drop develops between base and emitter while the base current exists. The size of this voltage depends on the material the transistor is made from, and is referred to as VBE.

Transistor as a switch

BJT used as an electronic switch, in grounded-emitter configuration.


Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates.

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:

VRC = ICE × RC, the voltage across the load (the lamp with resistance RC)
VRC + VCE = VCC, the supply voltage shown as 6V


If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off, or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Transistor as an amplifier

Amplifier circuit, standard common-emitter configuration.


The common-emitter amplifier is designed so that a small change in voltage in (Vin) changes the small current through the base of the transistor and the transistor's current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout.

It is important that the operating values of the transistor are chosen and the circuit designed such that as far as possible the transistor operates within a linear portion of the graph, such as that shown between A and B, otherwise the output signal will suffer distortion.

Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive.

Some musical instrument amplifier manufacturers mix transistors and vacuum tubes in the same circuit, as some believe tubes have a distinctive sound.

Comparison with vacuum tubes

Prior to the development of transistors, vacuum tube (or in the UK "thermionic valves" or just "valves") were the main active components in electronic equipment.

Advantages

The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are
  • Small size and minimal weight, allowing the development of miniaturized electronic devices.
  • Highly automated manufacturing processes, resulting in low per-unit cost.
  • Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
  • No warm-up period for cathode heaters required after power application.
  • Lower power dissipation and generally greater energy efficiency.
  • Higher reliability and greater physical ruggedness.
  • Extremely long life. Some transistorized devices have been in service for more than 30 years.
  • Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
  • Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.


Limitations

  • Silicon transistors do not operate at voltages higher than about 1,000 volts (SiC devices can be operated as high as 3,000 volts). In contrast, electron tubes have been developed that can be operated at tens of thousands of volts.
  • High power, high frequency operation, such as used in over-the-air television broadcasting, is better achieved in electron tubes due to improved electron mobility in a vacuum.
  • On average, a higher degree of amplification linearity can be achieved in electron tubes as compared to equivalent solid state devices, a characteristic that may be important in high fidelity audio reproduction.




Types

PNP P-channel
NPN N-channel
BJT JFET

P-channel
N-channel
JFET MOSFET enh MOSFET dep

Transistors are categorized by Thus, a particular transistor may be described as silicon, surface mount, BJT, NPN, low power, high frequency switch.

The 'BC' letters in a common transistor name like BC547B means

Bipolar junction transistor

The bipolar junction transistor(BJT) was the first type of transistor to be mass-produced. Bipolar transistors are so named because they conduct by using both majority and minority carriers. The three terminals of the BJT are named emitter, base, and collector. The BJT consists of two p-n junctions: the base–emitter junction and the base–collector junction, separated by a thin region of semiconductor known as the base region (two junction diodes wired together without sharing an intervening semiconducting region will not make a transistor). "The [BJT] is useful in amplifiers because the currents at the emitter and collector are controllable by the relatively small base current." In an NPN transistor operating in the active region, the emitter-base junction is forward biased (electrons and holes recombine at the junction), and electrons are injected into the base region. Because the base is narrow, most of these electrons will diffuse into the reverse-biased (electrons and holes are formed at, and move away from the junction) base-collector junction and be swept into the collector; perhaps one-hundredth of the electrons will recombine in the base, which is the dominant mechanism in the base current. By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled. Collector current is approximately β (common-emitter current gain) times the base current. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications.

Unlike the FET, the BJT is a low–input-impedance device. Also, as the base–emitter voltage (Vbe) is increased the base–emitter current and hence the collector–emitter current (Ice) increase exponentially according to the Shockley diode modeland the Ebers-Moll model. Because of this exponential relationship, the BJT has a higher transconductancethan the FET.

Bipolar transistors can be made to conduct by exposure to light, since absorption of photons in the base region generates a photocurrent that acts as a base current; the collector current is approximately β times the photocurrent. Devices designed for this purpose have a transparent window in the package and are called phototransistors.

Field-effect transistor

The field-effect transistor(FET), sometimes called a unipolar transistor, uses either electrons (in N-channel FET) or holes (in P-channel FET) for conduction. The four terminals of the FET are named source, gate, drain, and body(substrate). On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description.

In FETs, the drain-to-source current flows via a conducting channel that connects the sourceregion to the drainregion. The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals; hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate–source voltage (Vgs) is increased, the drain–source current (Ids) increases exponentially for Vgsbelow threshold, and then at a roughly quadratic rate (I_{ds} \propto (V_{gs}-V_T)^2) (where VTis the threshold voltage at which drain current begins)in the "space-charge-limited" region above threshold. A quadratic behavior is not observed in modern devices, for example, at the 65 nmtechnology node.

For low noise at narrow bandwidththe higher input resistance of the FET is advantageous.

FETs are divided into two families: junction FET(JFET) and insulated gate FET(IGFET). The IGFET is more commonly known as a metal–oxide–semiconductor FET(MOSFET), reflecting its original construction from layers of metal (the gate), oxide (the insulation), and semiconductor. Unlike IGFETs, the JFET gate forms a PN diodewith the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid state equivalent of the vacuum tube triodewhich, similarly, forms a diode between its gridand cathode. Also, both devices operate in the depletion mode, they both have a high input impedance, and they both conduct current under the control of an input voltage.

Metal–semiconductor FETs (MESFETs) are JFETs in which the reverse biased PN junctionis replaced by a metal–semiconductor Schottky-junction. These, and the HEMTs (high electron mobility transistors, or HFETs), in which a two-dimensional electron gas with very high carrier mobility is used for charge transport, are especially suitable for use at very high frequencies (microwave frequencies; several GHz).

Unlike bipolar transistors, FETs do not inherently amplify a photocurrent. Nevertheless, there are ways to use them, especially JFETs, as light-sensitive devices, by exploiting the photocurrents in channel–gate or channel–body junctions.

FETs are further divided into depletion-modeand enhancement-modetypes, depending on whether the channel is turned on or off with zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can "enhance" the conduction. For depletion mode, the channel is on at zero bias, and a gate potential (of the opposite polarity) can "deplete" the channel, reducing conduction. For either mode, a more positive gate voltage corresponds to a higher current for N-channel devices and a lower current for P-channel devices. Nearly all JFETs are depletion-mode as the diode junctions would forward bias and conduct if they were enhancement mode devices;most IGFETs are enhancement-mode types.

Other transistor types



Construction

Semiconductor material

The first BJTs were made from germanium(Ge). Silicon(Si) types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductormaterial gallium arsenide(GaAs) and the semiconductor alloysilicon germanium(SiGe). Single element semiconductor material (Ge and Si) is described as elemental.

Rough parameters for the most common semiconductor materials used to make transistors are given in the table below; it must be noted that these parameters will vary with increase in temperature, electric field, impurity level, strain, and sundry other factors:

Semiconductor material characteristics

The junction forward voltageis the voltage applied to the emitter-base junction of a BJT in order to make the base conduct a specified current. The current increases exponentially as the junction forward voltage is increased. The values given in the table are typical for a current of 1 mA (the same values apply to semiconductor diodes). The lower the junction forward voltage the better, as this means that less power is required to "drive" the transistor. The junction forward voltage for a given current decreases with increase in temperature. For a typical silicon junction the change is −2.1 mV/°C.

The density of mobile carriers in the channel of a MOSFET is a function of the electric field forming the channel and of various other phenomena such as the impurity level in the channel. Some impurities, called dopants, are introduced deliberately in making a MOSFET, to control the MOSFET electrical behavior.

The electron mobilityand hole mobilitycolumns show the average speed that electrons and holes diffuse through the semiconductor material with an electric fieldof 1 volt per meter applied across the material. In general, the higher the electron mobility the speedier the transistor. The table indicates that Ge is a better material than Si in this respect. However, Ge has four major shortcomings compared to silicon and gallium arsenide:
  • Its maximum temperature is limited;
  • it has relatively high leakage current;
  • it cannot withstand high voltages;
  • it is less suitable for fabricating integrated circuits.
Because the electron mobility is higher than the hole mobility for all semiconductor materials, a given bipolar NPN transistortends to be swifter than an equivalent PNP transistortype. GaAs has the highest electron mobility of the three semiconductors. It is for this reason that GaAs is used in high frequency applications. A relatively recent FET development, the high electron mobility transistor(HEMT), has a heterostructure(junction between different semiconductor materials) of aluminium gallium arsenide (AlGaAs)-gallium arsenide (GaAs) which has twise the electron mobility of a GaAs-metal barrier junction. Because of their high speed and low noise, HEMTs are used in satellite receivers working at frequencies around 12 GHz.

Max. junction temperaturevalues represent a cross section taken from various manufacturers' data sheets. This temperature should not be exceeded or the transistor may be damaged.

Al-Si junctionrefers to the high-speed (aluminum-silicon) semiconductor-metal barrier diode, commonly known as a Schottky diode. This is included in the table because some silicon power IGFETs have a parasiticreverse Schottky diode formed between the source and drain as part of the fabrication process. This diode can be a nuisance, but sometimes it is used in the circuit.

Packaging



Transistors come in many different packages (chip carriers) (see images). The two main categories are through-hole(or leaded), and surface-mount, also known as surface mount device(SMD). The ball grid array(BGA) is the latest surface mount package (currently only for large transistor arrays). It has solder "balls" on the underside in place of leads. Because they are smaller and have shorter interconnections, SMDs have better high frequency characteristics but lower power rating.

Transistor packages are made of glass, metal, ceramic, or plastic. The package often dictates the power rating and frequency characteristics. Power transistors have larger packages that can be clamped to heat sinksfor enhanced cooling. Additionally, most power transistors have the collector or drain physically connected to the metal can/metal plate. At the other extreme, some surface-mount microwavetransistors are as small as grains of sand.

Often a given transistor type is available in sundry packages. Transistor packages are mainly standardized, but the assignment of a transistor's functions to the terminals is not: other transistor types can assign other functions to the package's terminals. Even for the same transistor type the terminal assignment can vary (normally indicated by a suffix letter to the part number, q.e. BC212L and BC212K).

See also



References

  1. Lilienfeld, Julius Edgar, "Method and apparatus for controlling electric current" 1930-01-28 (filed in Canada 1925-10-22, in US 1926-10-08).
  2. Heil, Oskar, "Improvements in or relating to electrical amplifiers and other control arrangements and devices", Patent No. GB439457, European Patent Office, filed in Great Britain 1934-03-02, published 1935-12-06 (originally filed in Germany 1934-03-02).
  3. J. Chelikowski, "Introduction: Silicon in all its Forms", Silicon: evolution and future of a technology (Editors: P. Siffert, E. F. Krimmel), p.1, Springer, 2004 ISBN 3540405461.
  4. Grant McFarland, Microprocessor design: a practical guide from design planning to manufacturing, p.10, McGraw-Hill Professional, 2006 ISBN 0071459510.
  5. W. Heywang, K. H. Zaininger, "Silicon: The Semiconductor Material", Silicon: evolution and future of a technology (Editors: P. Siffert, E. F. Krimmel), p.36, Springer, 2004 ISBN 3540405461.
  6. FETs/MOSFETs: Smaller apps push up surface-mount supply
  7. Intel Multi-Core Processor Architecture Development. Retrieved December 19, 2008
  8. Turley, J. (December 18, 2002). The Two Percent Solution. Embedded.com.
  9. apart from a small value due to leakage currents
  10. 071003 bcae1.com
  11. IGBT Module 5SNA 2400E170100
  12. Single Electron Transistors


Further reading

  • The invention of the transistor & the birth of the information age


External links



Datasheets

A wide range of transistors has been available since the 1960s and manufacturers continually introduce improved types. A few examples from the main families are noted below. Unless otherwise stated, all types are made from silicon semiconductor. Complementary pairs are shown as NPN/PNP or N/P channel. Links go to manufacturer datasheets, which are in PDFformat. (On some datasheets the accuracy of the stated transistor category is a matter of debate.)
  • 2N3904/ 2N3906, BC182/ BC212 and BC546/ BC556: Ubiquitous, BJT, general-purpose, low-power, complementary pairs. They have plastic cases and cost roughly ten cents US in small quantities, making them popular with hobbyists.
  • AF107: Germanium, 0.5-watt, 250 MHz PNP BJT.
  • BFP183: Low power, 8 GHz microwave NPN BJT.
  • LM394: "supermatch pair", with two NPN BJTs on a single substrate.
  • 2N2219A/ 2N2905A: BJT, general purpose, medium power, complementary pair. With metal cases they are rated at about one watt.
  • 2N3055/ MJ2955: For years, the venerable NPN 2N3055 has been the "standard" power transistor. Its complement, the PNP MJ2955 arrived later. These 1 MHz, 15A, 60V, 115W BJTs are used in audio power amplifiers, power supplies, and control.
  • 2N7000 is a typical small-signal field-effect transistor.
  • 2SC3281/2SA1302: Made by Toshiba, these BJTs have low-distortion characteristics and are used in high-power audio amplifiers. They have been widely counterfeited[5156].
  • BU508: NPN, 1500 V power BJT. Designed for television horizontal deflection, its high voltage capability also makes it suitable for use in ignition systems.
  • MJ11012/MJ11015: 30 A, 120 V, 200 W, high power Darlington complementary pair BJTs. Used in audio amplifiers, control, and power switching.
  • 2N5457/ 2N5460: JFET (depletion mode), general purpose, low power, complementary pair.
  • BSP296/BSP171: IGFET (enhancement mode), medium power, near complementary pair. Used for logic level conversion and driving power transistors in amplifiers.
  • IRF3710/ IRF5210: IGFET (enhancement mode), 40A, 100V, 200W, near complementary pair. For high-power amplifiers and power switches, especially in automobiles.


Part numbers starting with "2S" are from Japan. Transistors with part numbers beginning with 2SA or 2SB are PNP BJTs. Transistors with part numbers beginning with 2SC or 2SD are NPN BJTs. Transistors with part numbers beginning with 2SJ are P-channel FETs (both JFETs and MOSFETs). Transistors with part numbers beginning with 2SK are N-channel FETs (both JFETs and MOSFETs).

Patents



Prefix class Usage
BC Small signal transistor ("allround")
BF High frequency, many MHz
BD Withstands higher current and power
BA Germanium
Semiconductor
material
Junction forward
voltage
V @ 25 °C

Electron mobility
m2/(V·s) @ 25 °C
Hole mobility
m2/(V·s) @ 25 °C
Max. junction temp.
°C
Ge
0.27 0.39 0.19 70 to 100
Si
0.71 0.14 0.05 150 to 200
GaAs
1.03 0.85 0.05 150 to 200
Al-Si junction
0.3 150 to 200

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