Transistor

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Transistor :: Transistors

The transistor is a solid state semiconductor device that can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. It allows a variable current, from an external source, to flow between two of its terminals depending on the smaller voltage or current applied to a third terminal. Transistors are made either as separate components or as part of an integrated circuit.

Introduction

Transistors are divided into two main categories: bipolar junction transistors (BJTs) and field effect transistors (FETs). Transistors have three terminals where, in simplified terms, the application of current (BJT) or voltage (FET) to the input terminal increases the amount of charge in the active region, and thereby effectively increases the conductivity between the other two terminals, and hence controls current flow between those terminals. The physics of this "transistor action" is quite different for the BJT and FET; see the respective articles for further details.

In analog circuits, transistors are used in amplifiers, (direct current amplifiers, audio amplifiers, radio frequency amplifiers), and linear regulated power supplies. Transistors are also used in digital circuits where they function as electrical switches. Digital circuits include logic gates, random access memory (RAM), and microprocessors.

History

The first patents for the transistor principle were registered in Germany in 1928 by Julius Edgar Lilienfeld. In 1934 German physicist Dr. Oskar Heil patented the field-effect transistor. It is not clear whether either design was ever built, and this is generally considered unlikely.

On 22 December 1947 William Shockley, John Bardeen and Walter Brattain succeeded in building the first practical point-contact transistor at Bell Labs. This work followed from their war-time efforts to produce extremely pure germanium "crystal" mixer diodes, used in radar units as a frequency mixer element in microwave radar receivers. Early tube-based technology did not switch fast enough for this role, leading the Bell team to use solid state diodes instead. With this knowledge in hand they turned to the design of a triode, but found this was not at all easy. Bardeen eventually developed a new branch of surface physics to account for the "odd" behaviour they saw, and Bardeen and Brattain eventually succeeded in building a working device.

Bell Telephone Laboratories needed a generic name for the new invention: "Semiconductor Triode", "Solid Triode", "Surface States Triode", "Crystal Triode" and "Iotatron" were all considered, but "transistor," coined by John R. Pierce, won an internal ballot. The rationale for the name is described in the following extract from the company's Technical Memoranda calling for votes:

Bell put the transistor into production at Western Electric in Allentown, Pennsylvania. They also licensed it to a number of other electronics companies, including Texas Instruments, who produced a limited run of transistor radios as a sales tool. Another company liked the idea and also decided to take out a license, introducing their own radio under the brand name Sony. Early transistors were "unstable" and only suitable for low-power, low-frequency applications, but as transistor design developed, these problems were slowly overcome. Over the next two decades, transistors gradually replaced the earlier vacuum tubes in most applications and later made possible many new devices such as integrated circuits and personal computers.

Shockley, Bardeen and Brattain were honored with the Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect". Bardeen would go on to win a second Nobel in physics, one of only two people to receive more than one in the same discipline, for his work on the exploration of superconductivity.

In August 1948 German physicists Herbert F. Mataré (1912– ) and Heinrich Walker (ca. 1912–1981), working at Compagnie des Freins et Signaux Westinghouse in Paris, France applied for a patent on an amplifier based on the minority carrier injection process which they called the "transistron." Since Bell Labs did not make a public announcement of the transistor until June 1948, the transistron was considered to be independently developed. Mataré had first observed transconductance effects during the manufacture of germanium duodiodes for German radar equipment during WWII. Transistrons were commercially manufactured for the French telephone company and military, and in 1953 a solid-state radio receiver with four transistrons was demonstrated at the Düsseldorf Radio Fair.

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Importance

The transistor is considered by many to be one of the greatest inventions in modern history, ranking in importance with the printing press, automobile and telephone. It 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 vanishingly low per-transistor costs.

Although millions of individual transistors (known as discretes) are still used, the vast majority of transistors are fabricated into integrated circuits (also called microchips or simply chips) along with diodes, resistors, capacitors and other electronic components to produce complete electronic circuits. A logic gate comprises about twenty transistors whereas an advanced microprocessor, as of 2005, can use as many as 289 million transistors.

The transistor's low cost, flexibility and reliability have made it an almost universal device for non-mechanical tasks, such as digital computing. Transistorized circuits are replacing electromechanical devices for the control of appliances and machinery as well. It is often less expensive and more effective to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function.

Because of the low cost of transistors and hence digital computers, there is a trend to digitize information. With digital computers offering the ability to quickly find, sort and process digital information, more and more effort has been put into making information digital. As a result, today, much media data is delivered in digital form, finally being converted and presented in analog form by computers. Areas influenced by the Digital Revolution include television, radio, and newspapers.

Types

|| PNP || JFET_symbol_P.png || P-channel || NPN || JFET_symbol_N.png || N-channel BJT JFET

Transistors are categorized by:

Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide
Structure: BJT, JFET, IGFET (MOSFET), IGBT, "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 (The maximum effective frequency of a transistor is denoted by the term $f_\mathrm{T}$ , an abbreviation for "frequency of transition." The frequency of transition is the frequency at which the transistor yields unity gain).
Application: switch, general purpose, audio, high voltage, super-beta, matched pair
Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array

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

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 are named emitter, base and collector. Two p-n junctions exist inside an NPN BJT: the base/collector junction and base/emitter junction. The PNP transistor similarly has two n-p junctions. The BJT is commonly described as a current-operated device because the emitter/collector current is controlled by the current flowing between base and emitter terminals. Unlike the FET, the BJT is a low input-impedance device. The BJT has a higher transconductance than the FET. Bipolar transistors can be made to conduct with light (photons) as well as current. Devices designed for this purpose are called phototransistors.

Field-effect transistor

The field-effect transistor (FET), sometimes called a unipolar transistor, uses either electrons (N-channel FET) or holes (P-channel FET) for conduction. The three main terminals of the FET are named source, gate and drain. On some FETs a fourth connection to the body (substrate) is provided, but normally the body is connected internally to the source.

A voltage applied between the gate and source controls the current flowing between the source and drain. In FETs the source/ drain current flows through a conducting channel near the gate. This channel connects the source region to the drain region. The channel conductivity is varied by the electric field generated by the voltage applied between the gate/source terminals. In this way the current flowing between the source and drain is controlled. Like bipolar transistors, FETs can be made to conduct with light (photons) as well as voltage. Devices designed for this purpose are called phototransistors.

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), from their original construction as a layer of metal (the gate), a layer of oxide (the insulation), and a layer of semiconductor. Unlike IGFETs, the JFET gate forms a PN 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.

MESFETs are JFETs, in which the reverse biased PN junction is replaced by a semiconductor-metal Schottky-junction. These, and the HEMFETs (high electron mobility FETs), 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).

FETs are further divided into depletion-mode and enhancement-mode types. Mode refers to the polarity of the gate voltage with respect to the source at the threshold of conduction. For N-channel depletion-mode FETs the gate is negative with respect to the source while for N-channel enhancement-mode FETs the gate is positive, at the threshold of conduction. 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. Nearly all JFETs are depletion-mode types and most IGFETs are enhancement-mode types.

Other transistor types

Unijunction transistors can be used as simple pulse generators. They comprise a main body of either P-type 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 ^* illustrates just how far power semiconductor technology has advanced. Intended for three-phase power supplies, this device houses three NPN IGBTs in a case measuring 38 by 140 by 190 mm and weighing 1.5 kg. Each IGBT is rated at 1,700 volts and can handle 2,400 amperes.
Single-electron transistors (SET) consist of a gate island between two tunnelling junctions. The tunnelling current is controlled by a voltage applied to the gate through a capacitor. ^*^*
Complete list of transistor types T-Transistor.com

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). 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:

Semiconductor material characteristics
Semiconductor material	Junction forward voltage V @ 25 °C	Electron mobility m/s @ 25 °C	Hole mobility m/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

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 some 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 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 large packages that can be clamped to heat sinks for 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 microwave transistors are as small as grains of sand.

Often different packages are available for a given transistor type. Transistor packages are mainly standardized, but the assignment of a transistor's functions to the terminals is not: different transistor types can assign different 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- i.e. BC212L and BC212K).

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 speed. 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 functions.

Switches

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.

Amplifiers

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.

Transistors are commonly used in modern musical instrument amplifiers, where circuits up to a few hundred watts are common and relatively cheap. Transistors have largely replaced valves in instrument amplifiers. Some musical instrument amplifier manufacturers mix transistors and vacuum tubes in the same circuit, to utilize the inherent benefits of both devices.

Computers

The "first generation" of electronic computers used vacuum tubes, which generated large amounts of heat and were bulky, and unreliable. The development of the transistor was key to computer miniaturization and reliability. The "second generation" of computers, through the late 1950s and 1960s featured boards filled with individual transistors and magnetic memory cores. Subsequently, transistors, other components, and their necessary wiring were integrated into a single, mass-manufactured component: the integrated circuit. Transistors incorporated into integrated circuits have replaced most discrete transistors in modern digital computers.

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), and can take advantage of the higher electron mobility in a near vacuum.)
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, though this is usually a desirable part of the sound of guitar amplifiers)

" Nature abhors a vacuum tube " Myron Glass (see 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: Ubiquitous, 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.

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 BJTs are used in high-power audio amplifiers. They have been widely counterfeited^*.

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), 40 A, 100 V, 200 W, near complementary pair. For high-power amplifiers and power switches, especially in automobiles.

Transistor manufacturers

APT http://www.advancedpower.com/
Fairchild Semiconductor http://www.fairchildsemi.com/
Infineon Technologies http://www.infineon.com/
IRF http://www.irf.com/
IXYS Corporation http://www.ixys.com/
Panasonic Semiconductors http://www.panasonic.com/industrial/semi/
Powerex http://www.pwrx.com/
Rohm http://www.rohm.com/products/index.html
Sanyo Transistors http://www.sanyo.com/semiconductors/transistors/
Semikron http://www.semikron.com/
STMicroelectronics http://www.st.com/
Toshiba Semiconductor (English) http://www.semicon.toshiba.co.jp/eng/index.html
Zetex Semiconductors http://www.zetex.com/

References

Patents

— J. Bardeen et. al.
— W. Shockley

Books

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

Other

External links

AudioUK's Milestones. Photograph of first working transistor
Transistorized. Historical and technical information from the Public Broadcasting Service
IEEE Virtual Museum, Let's Get Small: The Shrinking World of Microelectronics. All about the history of transistors and integrated circuits.
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
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 the Regency radio.
The Bell Systems Memorial on Transistors.
Jerry Russell's Transistor Cross Reference Database.
Pictures of TO and SOT packages
How transistors work
Transistor Flow Control - Scientific American Magazine (October 2005)

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