Transistor

Transistors are categorized by:

• Semiconductor material: germanium, silicon, gallium arsenide
• Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array
• Type: BJT, JFET, IGFET (MOSFET), "other types"
• Polarity: NPN, PNP, N-channel, P-channel
• Maximum power rating: low, medium, high
• Maximum operating frequency: low, medium, high, radio frequency (RF), microwave
• Application: switch, general purpose, audio, high voltage, super-beta, matched pair

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

The maximum effective frequency of a transistor is denoted by the term <math>f_{T}</math>, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain.

Bipolar junction transistor

The bipolar junction transistor (BJT) was the first type of transistor to be commercially mass-produced. Bipolar transistors are so named because the main conduction channel uses both electrons and holes to carry the main electric current. The terminals are named emitter, base and collector. Two p-n junctions exist inside the BJT, collector-base junction and base-emitter junction. Although commonly described as a current operated device, the collector current is actually controlled by the voltage difference between base and emitter terminals. The BJT is commonly used with voltage feedback to control the base voltage, but sometimes the base is driven by a current input. In contrast to the FET, the BJT is a low input-impedance device when used without voltage feedback.

In the BJT a small amount of base-emitter current causes a larger amount of collector-emitter current. The ratio of the allowed collector-emitter current to the base-emitter current is called current gain, β or <math>h_{FE}</math>. A β value of 100 is typical for small bipolar transistors. In a typical configuration, a very small signal current flows through the base-emitter junction to control the emitter-collector current.

Bipolar transistors can be turned on with light as well as electricity. Devices designed for this purpose are called phototransistors. Even normal transistors will conduct if their semiconductor junctions are exposed to light.

Field-effect transistor

Field-effect transistors (occasionally called unipolar transistors) use only one of the two carrier types (either electrons or holes, depending on the subtype). Thbe terminals of the FET are named source, gate and drain. In the FET a small amount of voltage is applied to the gate in order to control current flowing between the source and drain. In FETs the main current appears in a narrow conducting channel formed near the gate. This channel connects the source terminal to the drain terminal. The channel conductivity can be altered by varying the voltage applied to the gate terminal, enlarging or constricting the channel and thereby controlling the main current.

FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as metal oxide semiconductor FET (MOSFET). Unlike MOSFETs, the JFET gate terminal forms a diode with the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and 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.

FETs are further divided into enhancement mode and depletion mode types. Mode refers to the polarity of the gate voltage with respect to the source when the device is conducting. For an N-channel FET: in depletion mode the gate is negative with respect to the source while in enhancement mode the gate is positive. For both modes, if the gate voltage is made more positive the source/drain current will increase. For P-channel devices the polarities are reversed. Most IGFETs are enhancement mode types and nearly all JFETs are depletion mode types.

Other transistor types

• Unijunction transistors can be used as simple pulse generators. They comprise a main body of either P- or N-type semiconductor with ohmic contacts at each end (terminals Base1 and Base2). A junction with the opposite semiconductor type is formed at a point along the length of the body for the third terminal (Emitter).
• Dual gate FETs have a single channel with two gates in cascode; a configuration that is optimized for high frequency amplifiers, mixers, and oscillators.
• Transistor arrays are used for general purpose applications, function generation and low-level, low-noise amplifiers. They include two or more transistors on a common substrate to ensure close parameter matching and thermal tracking, characteristics that are especially important for long tailed pair amplifiers.
• Darlington transistors comprise a medium power BJT connected to a power BJT. This provides a high current gain equal to the product of the current gains of the two transistors. Power diodes are often connected between certain terminals depending on specific use.
• Insulated gate bipolar transistors (IGBTs) use a medium power IGFET, similarly connected to a power BJT, to give a high input impedance. Power diodes are often connected between certain terminals depending on specific use. IGBTs are particularly suitable for heavy-duty industrial applications. The Asea Brown Boveri (ABB) 5SNA2400E170100 [1] illustrates just how far power semiconductor technology has advanced. Intended for three-phase power supplies, this awesome device houses three NPN IGBTs in a case measuring 38 by 140 by 190 mm. Each IGBT is rated at 1,700 volts and can handle 2,400 amperes.

Semiconductor material

The first BJTs were made from germanium (Ge) and some high power types still are. Silicon (Si) types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductor material gallium arsenide (GaAs) and the semiconductor alloy silicon germanium (SiGe). Germanium was largely replaced by silicon because silicon semiconductor behavior is stable at higher relative temperatures. Single element semiconductor material (Ge and Si) is described as elemental.

Characteristics of the most common semiconductor materials used to make transistors are given in the table below:

The junction forward voltage is the voltage applied to the emitter-base junction of a BJT in order to make the base conduct a specified current. 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 temperature. For a typical silicon junction the change is approximately -2.1 mV/°C.

The electron mobility and hole mobility columns show the average speed that electrons and holes diffuse through the semiconductor material with an electric field of 1 Volt per meter applied across the material. In general, the higher the electron mobility the faster 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 and 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 transistor tends to be faster than an equivalent PNP transistor type. GaAs has the fastest 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 double 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 a frequency around 12 GHz.

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

Al—Si junction refers to the high-speed (aluminum—silicon) semiconductor—metal barrier diode, commonly known as a Schottky diode. This is included in the table because most silicon power IGFETs have a parasitic reverse Schottky diode formed between the source and drain as part of the fabrication process.

Packaging

Transistors come in many different chip carriers (see images), both through-hole (or leaded) such as metal canister and dual in-line package (DIP); 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 and digital functions). It has solder 'balls' on the underside in place of leads. Because they are smaller and have shorter interconnections, SMDs have higher frequency characteristics but lower power rating.

Often several package options are offered for a given transistor. Transistor arrays are sold in the same chip carriers as integrated circuits, often in DIP or SMD packages. A transistor array consists of multiple transistors built onto a single die for use as several individual transistors. Transistor packages are made of glass, metal, ceramic or plastic. The power rating and frequency of operation often dictates the type of packaging used. Power transistors have large packages that can be clamped to a heat sink for enhanced cooling. Additionally, most power transistors have the collector or drain physically connected to the metal can/metal plate. At the other size extreme, some surface-mount microwave transistors resemble grains of sand.

Usage

In the early days of transistor circuit design, the bipolar junction transistor, or BJT, was the most commonly used transistor. Even after MOSFETs became available, the BJT remained the transistor of choice for digital and analog circuits because of their ease of manufacture and ruggedness. However, the MOSFET has several desirable properties for digital circuits, and since major advancements in digital circuits have pushed MOSFET design to state-of-the-art, MOSFETs are now commonly used for both analog and digital purposes.

Use as an electronic switch

Transistors make very good electronic switches, and this is their main use in computer and logic circuitry.

BJT Transistor used as an electronic switch - Circuit Diagram

Use in amplifiers

Circuit Diagram || Waveforms

Audio amplifiers

From mobile phones to televisions, vast numbers of products include audio amplifiers. 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.

Musical instrument amplifiers

Transistors are very commonly used in today's instrument amplifiers, where circuits upto a few hundred watts are common and relatively cheap. Transistors have largely replaced valves in instrument amplifiers. It is possible to mix transistors and vacuum tubes in the same circuit, to utilize the benefits of both devices while minimizing their disadvantages.

Use in computers

The "first generation" of electronic computers used vacuum tubes, which generated heat and were bulky and fragile and were unreliable. Transistorizing the electronic computer was key to computer miniaturization. The "second generation" of computers through the late 1950s and 1960s featured boards filled with individual transistors, and magnetic cores. Subsequently, transistors and other components and their necessary wiring were integrated into a single, mass-manufactured component: the integrated circuit. In modern digital electronics, single transistors are very rare.

More on Transistor tester

Advantages of transistors over vacuum tubes

Before the development of transistors, vacuum tubes (or in the UK thermionic valves or just valves) were the main active components in electronic equipment. The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are:

• Smaller size (despite continuing miniaturization of vacuum tubes)
• Highly automated manufacture
• Lower cost (in volume production)
• Lower possible operating voltages
• Operation without a warm-up period (most vacuum tubes need 10 to 60 seconds to "warm up")
• Lower power dissipation (no heater power, very low saturation voltage)
• Higher reliability and greater ruggedness to physical shocks (although vacuum tubes are more resistant to nuclear electromagnetic pulses (NEMP) and electrostatic discharge (ESD) )
• Much longer lifetime (vacuum tube cathodes are eventually exhausted)
• Complementary devices available (allowing circuits with complementary symmetry–complementary vacuum tubes are not available)
• Ability to control large currents (power transistors are available to control hundreds of amperes, vacuum tubes to control even one ampere are large and costly)
• Non-microphonic (vibration can modulate vacuum tube characteristics)

" Nature abhors a vacuum tube " John R. Pierce, Bell Telephone Laboratories, circa 1948.

Gallery

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 PDF format. (On some datasheets the accuracy of the stated transistor category is a matter of debate.)

• 2N3904/2N3906, BC182/BC212 and BC546/BC556: BJT, general-purpose, low-power, complementary pairs. They have plastic cases and cost roughly ten cents U.S. in small quantities, making them popular with hobbyists, and nearly ubiquitous.

• BFP183: Low power, 8 GHz microwave NPN BJT.

• LM394: So-called '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, 15 A, 60 V, 115 W BJTs are used in audio power amplifiers, power supplies and control.

• 2SC3281/2SA1302: Made by Toshiba to have low-distortion characteristics, these are used in high-power audio amplifiers. They have been widely counterfeited[2].

• BU508: NPN, 1500V power BJT. Designed for television horizontal deflection, its high voltage capability also finds use in ignition systems.

• MJ11012/MJ11015: 30A, 120V, 200W, high power Darlington complementary pair BJTs. Used in audio amplifiers and control and power switching.

• 2N5457/2N5460: JFET, general purpose, low power, complementary pair.

• BSP296/BSP171: IGFET, 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.

See also

• Avalanche transistor
• Band gap
• Bipolar junction transistor
• Compound transistor
• Darlington transistor
• Field effect transistor
• IGBT
• NPN
• PNP
• Semiconductor
• Transconductance
• Transresistance
• Transistor Models
• Wikibooks: Transistors
• Vacuum tube
• Integrated circuit vacuum tube

External links and references

• U.S. Patent 2524035 -- J. Bardeen et. al.
• U.S. Patent 2569347 -- W. Shockley
• AudioUK's Milestones. Photograph of first working transistor
• Transistorized. Historical and technical information from the Public Broadcasting Service (PBS) web site
• The Transistor Legacy Then and Now. From Lucent Technologies (Bell Telephone Laboratories) (AT&T)
• This Month in Physics History: November 17 to December 23, 1947: Invention of the First Transistor. From the American Physical Society (APS)
• 50 Years of the Transistor. From Science Friday, December 12, 1997
• The CK722 Museum. Website devoted to the "classic" hobbyist germanium transistor
• Bob's Virtual Transistor Museum & History. Treasure trove of transistor history
• 1954 to 2004, the TR-1's Golden Anniversary. In depth coverage of Regency radio.
• Jerry Russell's Transistor Cross Reference Database.
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• {{{Author|{{{Last|}}}}|1{{{1|}}}={{{3|}}}}}}|, {{{First}}}}}}}}}|1{{{1|}}}={{{3|}}}}}}| (1999)}}}|1{{{1|}}}={{{3|}}}}}}}}}}}}|.}}}|1{{{1|}}}={{{3|}}}}}}| "{{{Chapter}}}" in}} }|1{{{1|}}}={{{3|}}}}}}|{{{Editor}}} }}}|1{{{1|}}}={{{3|}}}}}}|2=[{{{URL}}}|3=}} Principals of Transistor Circuits}|1{{{1|}}}={{{3|}}}}}}|2=]|3=}}}|1{{{1|}}}={{{3|}}}}}}|, {{{Others}}}}}}|1{{{1|}}}={{{3|}}}}}}|, {{{Pages}}}}}}|1{{{1|}}}={{{3|}}}}}}|, Butterworth-Heinemann}}}|1{{{1|}}}={{{3|}}}}}}|. ISBN 0750644273}}
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• Michael Riordan, How Europe Missed the Transistor, IEEE Spectrum, November 2005 (NA Edition)
• Armand Van Dormael, "The French Transistor", Proceedings of the 2004 IEEE Conference on the History of Electronics, Bletchly Park, England June 2004. Available on the Web at http://www.ieee.org/organizations/history_center/Che2004/VanDormael.pdfar:الترنسيستورات

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