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A transistor is a Semiconductor Device , commonly used as an amplifier or an electrically controlled switch. The transistor is the fundamental building block of the Circuit ry that governs the operation of Computer s, Cellular Phones , and all other modern Electronics . Because of its fast response and accuracy, the transistor may be used in a wide variety of Digital and Analog functions, including Amplification , Switch ing, Voltage Regulation , signal Modulation , and Oscillator s. Transistors may be packaged individually or as part of an Integrated Circuit , which may hold a billion or more transistors in a very small area. INTRODUCTION Modern transistors are divided into two main categories: Bipolar Junction Transistor s (BJTs) and Field Effect Transistor s (FETs). Application of current in BJTs and voltage in FETs between the input and common terminals increases the Conductivity between the common and output terminals, thereby controlling current flow between them. The transistor characteristics depend on their type. See Transistor Models . The term "transistor" originally referred to the Point Contact type, but these only saw very limited commercial application, being replaced by the much more practical Bipolar Junction types in the early 1950s. Today's most widely used Schematic Symbol , like the term "transistor", originally referred to these long-obsolete devices.Ralph S. Carson, ''Principles of Applied Electronics'', McGraw–Hill 1961. For a short time in the early 1960s, some manufacturers and publishers of electronics magazines started to replace these with symbols that more accurately depicted the different construction of the bipolar transistor, but this idea was soon abandoned. In Analog Circuit s, transistors are used in Amplifiers , (direct current amplifiers, audio amplifiers, radio frequency amplifiers), and linear Regulated Power Supplies . Transistors are also used in Digital Circuit s where they function as electronic switches, but rarely as discrete devices, almost always being incorporated in monolithic Integrated Circuits . Digital circuits include Logic Gate s, Random Access Memory (RAM), Microprocessor s, and Digital Signal Processors (DSPs). IMPORTANCE The transistor is considered by many to be the greatest invention of the twentieth century.1 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 (known as ''discrete'') transistors are still used, the vast majority of transistors are fabricated into Integrated Circuits (often abbreviated as ''IC'' and also called ''microchips'' or simply ''chips'') along with Diode s, Resistors , Capacitors and other Electronic Components to produce complete electronic circuits. A Logic Gate consists of about twenty transistors whereas an advanced microprocessor, as of 2006, can use as many as 1.7 billion transistors ( MOSFET s) {Link without Title} . The transistor's low cost, flexibility and reliability have made it a universal device for non-mechanical tasks, such as digital computing. Transistorized circuits have replaced 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 Newspaper s. Advantages of transistors over vacuum tubes Prior to the development of transistors, Vacuum (electron) Tube s (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:
" ''Nature abhors a vacuum tube'' " Myron Glass (see John R. Pierce ), Bell Telephone Laboratories , circa 1948. HISTORY See Also: Transistor history The first three patents for the field-effect transistor principle were registered in Germany in 1928 by physicist Julius Edgar Lilienfeld , but Lilienfeld published no research articles about his devices, and they were ignored by industry. In 1934 German physicist Dr. Oskar Heil patented another field-effect transistor. There is no direct evidence that these devices were built, but later work in the 1990s show that one of Lilienfeld's designs worked as described and gave substantial gain. Legal papers from the Bell Labs patent show that Shockley and 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 Other Transistor, R. G. Arns On s 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" behavior they saw, and Bardeen and Brattain eventually succeeded in building a working device. At the same time some European scientists were led by the idea of solid-state amplifiers. In August 1948 German physicists Herbert F. Mataré (1912– ) and Heinrich Welker (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. , won an internal ballot. The rationale for the name is described in the following extract from the company's Technical Memorandum calling for votes: Pierce recalled the naming somewhat differently: Bell immediately put the point-contact transistor into limited production at Western Electric in Allentown, Pennsylvania . Prototypes of all-transistor AM radio receivers were demonstrated, but were really only laboratory curiosities. However, in 1950 Shockley developed a radically different type of solid-state amplifier which became known as the Bipolar Junction "transistor". Although it works on a completely different principle to the Point-contact "transistor" , this is the device which is most commonly referred to as a "transistor" today. These were also licensed to a number of other electronics companies, including Texas Instruments , who produced a limited run of Transistor Radio s as a sales tool. Early transistors were chemically unstable and only suitable for low-power, low-frequency applications, but as transistor design developed, these problems were slowly overcome. There are numerous claimants to the title of the first company to produce practical transistor radios. Texas Instruments had demonstrated all-transistor AM radios as early as 1952, but their performance was well below that of equivalent battery tube models. A workable all- Transistor Radio was demonstrated in August 1953 at the Düsseldorf Radio Fair by the German firm Intermetall. It was built with four of Intermetall's hand-made transistors, based upon the 1948 invention of Herbert Mataré and Heinrich Welker {Link without Title} . However, as with the early Texas units (and others) only prototypes were ever built; it was never put into commercial production. The production of the first commercially successful transistor radio is often incorrectly attributed to Sony (originally Tokyo Tsushin Kogyo). However the Regency TR-1 , made by the Regency Division of I.D.E.A. (Industrial Development Engineering Associates) of Indianapolis, Indiana, was the first practical transistor radio made in any significant numbers. The TR-1 was announced on October 18, 1954 and put on sale in November 1954 for $49.95 (the equivalent of about $361 in year-2005 dollars) and sold about 150,000 units. The TR-1 used four Texas NPN transistors and had to be powered by a 22.5 Volt battery, since the only way to get adequate Radio Frequency performance out of early transistors was to run them close to their collector-to-emitter breakdown voltage. This made the TR-1 very expensive to run, and it was far more popular for its novelty or status value that its actual performance, rather in the fashion of the first MP3 Player s. Still, aside from its indifferent performance, the TR-1 was a very advanced product for its time, using Printed Circuit Board s, and what were then considered micro-miniature components. Masaru Ibuka , co-founder of the Japanese firm Sony , was visiting the USA when Bell Labs announced the availability of manufacturing licenses, including detailed instructions on how to manufacture junction transistors. Ibuka obtained special permission from the Japanese Ministry of Finance to pay the $50,000 license fee, and in 1955 the company introduced their own five-transistor "pocket" radio, the TR-55, under the new brand name Sony . (The term "pocket" was a matter of some interpretation, as Sony allegedly had special shirts made with oversized pockets for their salesmen) This product was soon followed by more ambitious designs, but it is generally regarded as marking the commencement of Sony's growth into a manufacturing superpower. The TR-55 was quite similar to the Regency TR-1 in many ways, being powered by the same sort of 22.5Volt battery, and was not much more practical. Very few were sold in the USA. It was not until 1957 that Sony produced their ground-breaking "TR-7" 7-transistor portable, a much more advanced design that ran on three ordinary flashlight cells and could compete favorably with vacuum tube portables. However, by this time similar designs were being produced in most industrialized countries. Over the next two decades, transistors gradually replaced the earlier Vacuum Tube s in most applications and later made possible many new devices such as Integrated Circuit s and Personal Computer s. 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 . The commercial uses of germanium transistors were limited by their sensitivity to temperature and humidity. Silicon, a semiconductor with crystal structure identical to germanium, looked promising but attempts over several years to make useful transistors were unsuccessful. In early 1954, M. Tanenbaum et. al.(Jl. of Applied Physics, 26, 686 (1955)) at Bell Labs made a high performance silicon transistor using npn junctions produced by growth rate fluctuations during crystal growing. A few months later, working independently at Texas Instruments, G. Teal (unpublished) made similar devices using sequential doping. While these devices had much superior temperature and environmental properties compared to gemanium transistors, the doping processes were difficult to control. That problem was solved by Tanenbaum and Fuller (Bell Sys. Tech. Jl., 35, 1 (1956)) using gas diffusion techniques to produce npn silicon transistors. The resulting diffused base silicon transistor was the subject of the second Bell Labs symposium. The diffusion process was easy to control, quickly adopted by the semiconductor industry and was the basis for the later invention of the integrated circuit initiating the "silicon age". TYPES
Bipolar transistors can be made to conduct by light, since absorption of photons in the base region generates a photocurrent that acts as a base current; the collector current is approximately beta times the photocurrent. Devices designed for this purpose have a transparent window in the package and are called Phototransistor s. 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 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. A Voltage applied between the gate and source (body) controls the current flowing between the drain and source. As the gate/source voltage () is increased the drain/source current () increases roughly parabolically (). In FETs the drain/source current flows through a conducting channel near the ''gate''. This channel connects the ''drain'' region to the ''source'' 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 drain and source is controlled. 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. Metal–semiconductor FETs (MESFETs) are JFETs in which the Reverse Biased PN Junction is 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-mode and '''enhancement-mode''' types, 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
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: 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 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 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:
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 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 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 frequencies 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 damaged. 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. This diode can be a nuisance, but sometimes it is used in the circuit. Packaging s)]] Transistors come in many different packages () (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 Sink s 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 a given transistor type is available in different packages. 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, desirable properties of MOSFETs, such as their utility in low-power devices, have made them the ubiquitous choice for use in digital circuits and a very common choice for use in analog circuits. 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 Phone s to Television s, vast numbers of products include Amplifier s 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, in which circuits up to a few hundred Watt s are common and relatively cheap. Transistors have largely replaced valves (electron tubes) 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, were bulky, and were unreliable. The development of the transistor was key to computer miniaturization and reliability. The "second generation" of computers, through the late . EXTERNAL LINKS TO 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 PDF format. (On some datasheets the accuracy of the stated transistor category is a matter of debate.)
SEE ALSO
REFERENCES Patents
Books Other EXTERNAL LINKS
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