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The transistor is a solid state semiconductor device used for amplification and switching. In essence, it has three terminals. A current or voltage applied through/across two terminals controls a relatively larger current through the other terminal and the common terminal. The transistor itself does not actually amplify current, although this is a common misconception. The term transistor was originally coined to denote a voltage or current-controlled resistor but this analogy is not normally helpful in understanding modern bipolar junction transistor operation.
In analog circuits, transistors are essentially used as amplifiers. Analogue circuits include, audio amplifiers, stabilised power supplies and radio frequency amplifiers. In digital circuits, transistors function essentially as electrical switches. Digital circuits include logic gates, RAM (random access memory) and microprocessors.
In broad terms, descrete transistors are categorised according to the following parameters: type (bipolar, FET) power (low, medium, high), frequency (low, medium, high), function (amplifier/switch). Thus, for example, a particular transistor may be categorised as a bipolar, low power, high frequency switch.
Transistors come in a wide range of cases (see photo), normally glass, metal, ceramic or plastic. Power transistors have relatively large cases that can be mounted on to a heat sink to dissipate heat. At the other extreme, some surface mount, high frequency transistors are so small that they look like specks of dust. Many transitors, especially power transistors, have one terminal, normally the collector or drain, internally connected to the case to aid heat conduction.
The first transistors were made from germanium(Ge) but now most are made from silicon(Si). Some high performance types are made from gallium arsinide(GaAs).
Although millions of individual (discrete) transistors find applications, the vast majority of transistors are embodied in integrated circuits (chips). The number of transistors in a single chip ranges from about twenty (simple logic gate) to, currently, about fifty million (microprocessor or dynamic RAM).
Along with diodes, resistors, capacitors and inductors, transistors can be integrated on to a semiconductor chip using a highly automated process to produce complete electronic functions, either analogue or digital. Sometimes both analogue and digital functions are integrated on to the same chip. The cost of designing and developing an integrated circuit is very high but when this cost is spread across across millions of prouction chips the individual costs can be minimised.
The term 'chip' is used rather loosely today: originally it tended to refer to the actual piece of semiconductor before packaging. Once the chip had been packaged it was called an integrated circuit and sometimes a 'bug'. Chip and integrated circuit are now used interchangeably while bug has gone out of fashion. The term 'solid state' is used to describe a device which does not control charge flow through a vacuum (ie valve) or gas. In the same vein a circuit or equipment may be descibed as 'solid state'.
Transistor was also the common name in the sixties for a transistor radio, a pocket-sized portable radio that used transistors (rather than vacuum tubeIn electronics, a vacuum tube (American English) or thermionic) valve (British English) is a device generally used to amplify a signal. Once used in most electronic devices, vacuum tubes are now used only in specialized applications. For most purposes, ths) as its active electronic components. This is still one of the dictionary definitions of transistor.
The transistor is considered by many to be one of the greatest discoveries or inventions in modern history, ranking with banking and the printing pressThe printing press is a mechanical device for printing many copies of a text on rectangular sheets of paper. First invented in China in 1041, the printing press as we know it today was invented in the West by a German goldsmith and eventual printer, Johan. Key to the importance of the transistor in modern society is its ability to be produced in huge numbers using simple techniques, resulting in vanishingly small prices. Computer " chipsAn integrated circuit (IC is a thin chip consisting of thousands or millions of interconnected semiconductor devices, mainly transistors, as well as passive components like resistors. As of 2004, typical chips are of size 1 cm2 or smaller, but larger ones" consist of millions of transistors and sell for dollars, with per-transistor costs in the thousandths-of-pennies.
The low cost has meant that the transistor has become an almost universal tool for non-mechanical tasks. Whereas a common device, say a refrigerator, would have used a mechanical device for control, today it is often less expensive to simply use a few million transistors and the appropriate computer programA computer program (often simply called a program is an example of computer software that prescribes the actions (" computations") that are to be carried out by a computer. Most programs consist of a loadable set of instructions which determines how the c to carry out the same task through "brute force". Today transistors have replaced almost all electromechanicalIn engineering, electromechanical devices are those that combine electrical and mechanical parts. These include electric motors and mechanical devices powered by them, such as calculators and adding machines; switches, solenoids, relays, crossbar switches devices, most simple feedback systems, and appear in huge numbers in everything from computers to cars.
Hand-in-hand with low cost has been the increasing move to "digitizing" all information. With transistorized computers offering the ability to quickly find (and sort) digitalA digital system is one that uses discrete values rather than a continuous spectrum of values: compare analog. The word comes from the same source as the word digit: the Latin word for finger (counting on the fingers) as these are used for discrete counti information, more and more effort was put into making all information digital. Today almost all media in modern society is delivered in digital form, converted and presented by computers. Common "analog" forms of information such as television or newspapers spend the vast majority of their time as digital information, being converted to analog only for a small portion of the time.
The transistor was invented at Bell Laboratories in December 1947 (first demonstrated on December 23) by John Bardeen, Walter Houser Brattain, and William Bradford Shockley, who were awarded the Nobel Prize in physics in 1956. Ironically, they had set out to manufacture a field-effect transistor (FET) predicted by Julius Edgar Lilienfeld as early as 1925 but eventually discovered current amplification in the point-contact transistor that subsequently evolved to become the bipolar junction transistor (BJT).
In a bipolar junction transistor (BJT), an electrical current is fed into the base (B) and modulates the current flow between the other two terminals known as the emitter (E) and collector (C).
The BJT can also be used to measure temperature as well as compute analog logarithms.
In field-effect transistors (FET)s, the three terminals are called gate (G), source (S) and drain (D) respectively, and it is the voltage applied to the gate terminal that modulates the current between source and drain. In normal operation the NPN transistor operates with a positive voltage on its collector (with respect to the emitter), whereas the PNP operates with a negative voltage on its collector.
Power MOSFETs become less conductive with increasing temperature and can therefore be thought of as n-channel devices by default. Silicon devices that use electrons, rather than holes, as the majority carriers are slightly faster and can carry more current than their P-type counterparts. The same is true in GaAs devices.
Substituting the p-n-junction with a Schottky junction gives a MESFET (Metal Semiconductor Field Effect Transistor), used for GaAs and other III-V semiconductor materials. Using bandgap engineering in a ternary semiconductor, like AlGaAs gives a HEMT (High Electron Mobility Transistor).
The FET is simpler in concept than the bipolar transistor and can be constructed from a wide range of materials.
The most common use of MOSFET transistors today is the CMOS (complementary metallic oxide semiconductor) integrated circuit which is the basis for most digital electronic devices. These use a totem-pole arrangement where one transistor (either the pull-up or the pull-down) is on while the other is off. Hence, there is no DC drain, except during the transition from one state to the other, which is very short. As mentioned, the gates are capacitive, and the charging and discharging of the gates each time a transistor switches states is the primary cause of power drain.
The C in CMOS stands for 'complementary.' The pull-up is a P-channel device (using holes for the mobile carrier of charge) and the pull-down is N-channel (electron carriers). This allows busing of the control terminals, but limits the speed of the circuit to that of the slower P device (in silicon devices). The bipolar solutions to push-pull include 'cascode' using a current source for the load. Circuits that utilize both unipolar and bipolar transistors are called Bi-Fet. A recent development is called 'vertical P.' Formerly, BiFet chip users had to settle for relatively poor (horizontal) P-type FET devices. This is no longer the case and allows for quieter and faster analog circuits.
A clever variant of the FET is the dual-gate device. This allows for two opportunities to turn the device off, as opposed to the dual-base (bipolar) transistor which presents two opportunities to turn the device on.
FETs can switch signals of either polarity, if their amplitude is significantly less than the gate swing, as the devices (especially the parasitic diode-free DFET) are basically symmetrical. This means that FETs are the most suitable type for analog multiplexing. With this concept, one can construct a solid-state mixing board, for example.
The power MOSFET has a 'parasitic diode' (back-biased) normally shunting the conduction channel that has half the current capacity of the conduction channel. Sometimes this is useful in driving dual-coil magnetic circuits (for spike protection), but in other cases it causes problems.
The high impedance of the FET gate makes it rather vulnerable to electrostatic damage, though this is not usually a problem after the device has been installed.
A more recent device for power control is the insulated-gate bipolar transistor, or IGBT. This has a control structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These have become quite popular.
(For more on FET's, and MOSFETs in particular, see Field effect transistor.)
Before the development of the transistor, the thermionic valve, or vacuum tube, was the main active component in electronic equipment. The key advantages that have allowed transistors to replace their valve predecessors in almost all applications are:
Some audio amplifiers still use valves, their enthusiasts claiming that their sound is superior.
In particular, some argue that the larger numbers of electrons in a valve behave with greater statistical accuracy, although this ignores the facts that tubes generally have a high-impedance control terminal, and that discrete-transistor (as opposed to, say, op-amp) circuits can also be designed to use large currents. (1 milliamp of current carries about 6.24 million billions of electrons per second.)
Others detect a distinctive "warmth" to the tone. The "warmth" is actually distortion caused by the valves, but some audiophiles find a certain amount of "fuzziness" pleasing. This is 'soft-saturation' and occurs when the valves are overdriven, causing poorly designed tube amplifiers to sound better than poorly designed transistor amplifiers.
Tubes are also preferred in guitar amplifiers which are designed to be overdriven, because they have a different non-linear transfer characteristic than transistors, and create a different, more pleasing spectrum of harmonic distortion or "fuzz". Digital signal processing (DSP) can be used to achieve similar effects in the digital domain.
It is possible to mix transistors and valves in the same circuit.
Simple transistor amplifiers use emitter degeneration to achieve negative feedback, which gives a relatively predictable gain compared to the gain of the transistor itself, which varies widely.
The "second generation" of computers through the late 1950s and 1960s featured boards filled with individual transistors, and magnetic cores. Subsequently, transistors, other components (capacitors, but not high-value inductors or transformers), and the necessary wiring were integrated into a single, mass-manufactured component: the integrated circuit. In modern digital electronics, single transistors are very rare, though they remain common in power and analog applications. Recently, inroads have been made in the integration of analog circuits, also, with the advent of 'programmable analog' circuits. DSP is a technique that can (among other things) be used with A/D and D/A converters to simulate analog circuits. Linear integrated circuits got a bad reputation early on because of the difficulty of creating (high-quaity) PNP transistors, but are much better now.
Also used in power regulation and in computer PSUs, and especially in Switching power supplies . High-power transistor used in a switching power supply. Mounted on a aluminium-block for enhanced cooling.
Operationally, transistors and vacuum tubes (valves) have similar functions; they both control the flow of current.
In order to understand how a semiconductor operates, consider a glass container filled with pure water. If a pair of conductive probes are immersed in the water and a DC voltage (below the electrolysis point i.e. breakdown point for water) is applied between them, no current would flow because the water has no charge carriers. Thus, pure water is an insulator. Dissolve a pinch of table salt in the water and conduction begins, because mobile carriers ( ions) have been released. Increasing the salt concentration increases the conduction, but not very much. A dry lump of salt is non-conductive, because the charge carriers are immobile.
An absolutely pure silicon crystal is also an insulator, but when an impurity e.g. arsenic is added (called doping) in quantities minute enough not to completely disrupt the regularity of the crystal lattice, it donates free electrons and enables conduction. This is because arsenic atoms have five electrons in their outer shells while silicon atoms have only four. Conduction is possible because a mobile carrier of charge has been introduced, in this case creating n-type silicon ('n' for negative. The electron has a negative charge).
Alternatively, silicon can be doped with boron to make p-type silicon which also conducts. Because Boron has only three electrons in its outer shell another kind of charge carrier, called a 'hole', is formed in the silicon crystal lattice.
In a valve, on the other hand, the charge carriers (electrons) are emitted by thermionic emission from a cathode heated by a wire filament. Therefore, valves cannot generate holes (positive charge carriers).
Note that charge carriers of the same polarity repel one another so that, in the absence of any force, they are distributed evenly throughout the semiconductor material. However, in an unpowered bipolar transistor (or junction diode) the charge carriers tend to migrate towards a P/N junction, being attacted by their opposite charge carriers on the other side of the junction.
Increasing the doping level increases the semiconductor conductivity, providing that the crystal lattice, overall, remains intact. In a bipolar transistor the emitter has a higher doping level than the base. The ratio of emitter/base doping levels is one of the main factors that dictates the junction transistor's current gain. The level of doping is extremely low: in the order of parts per one hundred million.
The above explains conduction in a semiconductor by charge carriers, either electrons or holes, but the essence of bipolar transistor action is the way that electrons/holes seemingly make a prohibited leap across the reverse biased base/collector junction under control of the base/emitter current. The detailed physics of this are rather complicated and are perhaps beyond the scope of this article. Although a transistor may seem like two interconnected diodes, don't think you can make a bipolar transistor by connecting two discrete junction diodes together. You need to have them fabricated on the same crystal, and physically sharing a common and extremely thin base, to get bipolar transistor action.
Bipolar transistors can be turned on with light as well as electricity. Devices designed for this purpose are called phototransistors, but these can be standard transistors in a transparent package.
All transistors rely on the ability of certain materials, known as semiconductors, to change their conduction under the control of an electric field. In bipolar transistors, the semiconductor is formed into structures called p-n junctions that allow electricity to flow in only one direction through them – that is, they are a conductor when voltage is applied in one direction, and an insulator when it is applied in the other direction.
Semiconductors had been used in the electronics field for some time before the invention of the transistor. Around the turn of the 20th century they were quite common as detectors in radios, used in a device called a "cat's whisker". These detectors were somewhat troublesome, however, requiring the operator to move a small tungsten filament (the whisker) around the surface of a crystal until it suddenly started working. Then, over a period of a few hours or days, the crystal would slowly stop working and the process would have to be repeated. At the time their operation was completely mysterious. After the introduction of the more reliable and amplified vacuum tube based radios, the cat's whisker systems quickly disappeared. The "cat's whisker" is an example of a special type of diode still popular today, called a Schottky diode.
In WWII, radar research quickly pushed the frequencies of the radio receivers inside them into the area where traditional tube based radio receivers no longer worked well. On a whim, Russell Ohl of Bell Laboratories decided to try a cat's whisker. After hunting one down at a used radio store in Manhattan, he found that it worked much better than tube-based systems.
Ohl investigated why the cat's whisker functioned so well. He spent most of 1939 trying to grow more pure versions of the crystals. He soon found that with higher quality crystals the "oddness" went away, but so did their ability to operate as a radio detector. One day he found one of his purest crystals nevertheless worked well, and interestingly, it had a clearly visible crack near the middle. However as he moved about the room trying to test it, the detector would mysteriously work, and then stop again. After some study he found that the behaviour was controlled by the light in the room – more light, more conductance. He invited several other people to see this crystal, and Brattain immediately realized there was some sort of junction at the crack.
Further research cleared up the remaining mystery. The crystal had cracked because either side contained very slightly different amounts of the impurities Ohl could not remove – about 0.2%. One side of the crystal had impurities that added extra electrons (the carriers of electrical current) and made it a conductor. The other had impurities that wanted to bind to these electrons, making it an insulator. When the two were placed side by side the electrons could be pushed out of the side with extra electrons (soon to be known as the emitter) and replaced by new ones being provided (say from a battery) where they would flow into the insulating portion and be collected by the filament (the collector). However, when the voltage was reversed the electrons being pushed into the collector would quickly fill up the "holes", and conduction would stop almost instantly. This junction of the two crystals (or parts of one crystal) created a solid-state diode, and the concept soon became known as semiconduction. 'Anode' and 'Cathode' are the terms used to denote the two terminals of a diode. The mechanism of action when the diode is off has to do with the separation of charge carriers around the junction. This is called a 'depletion region.'
Armed with the knowledge of how these new diodes worked, a crash effort started to learn how to build them on demand. Teams at Purdue University, Bell Labs, MIT, and the University of Chicago all joined forces to build better crystals. Within a year germanium production had been perfected to the point where military-grade diodes were being used in most radar sets.
The key to the development of the transistor was the further understanding of the process of the electron mobility in a semiconductor. It was realized that if there was some way to control the flow of the electrons from the emitter to the collector, one could build an amplifier. For instance, if you placed contacts on either side of a single type of crystal the current would not flow through it. However if a third contact could then "inject" electrons or holes into the material, the current would flow.
Actually doing this appeared to be very difficult. If the crystal were of any reasonable size, the amount of electrons (or holes) supplied would have to be very large — making it less than useful as an amplifier because it would require a large current to start with. That said, the whole idea of the crystal diode was that the crystal itself could provide the electrons over a very small distance. The key appeared to be to place the input and output contacts very close together on the surface of the crystal.
Brattain started working on building such a device, and tantalizing hints of amplification continued to appear as the team worked on the problem. One day the system would work and the next it wouldn't. In one instance a non-working system started working when placed in water. The two eventually developed a new branch of quantum mechanics known as surface physics to account for the behaviour.
Essentially, the electrons in any one piece of the crystal would migrate about due to nearby charges. Electrons in the emitters, or the "holes" in the collectors, would cluster at the surface of the crystal where they could find their opposite charge "floating around" in the air (or water). Yet they could be pushed away from the surface from any other location with the application of a small amount of charge. So instead of needing a large supply of electrons, a very small number in the right place would do the trick.
Their understanding solved the problem of needing a very small control area to some degree. Instead of needing two separate semiconductors connected by a common, but tiny, region, a single larger surface would serve. The emitter and collector would both be placed very close together on one side, with the control lead on the other. When current was applied to the control lead, the electrons or holes would be pushed out, right across the entire block of semiconductor, and collect on the far surface. As long as the emitter and collector were very close together, this should allow enough electrons or holes between them to allow conduction to start.
They made many attempts to build such a system with various tools, but generally failed. Setups where the contacts were close enough were invariably as fragile as the original cat's whisker detectors had been, and would work briefly, if at all.
Eventually they had a practical breakthrough. A piece of gold foil was glued to the edge of a plastic wedge, and then the foil was sliced with a razor at the tip of the triangle. The result was two very closely spaced contacts of gold. When the plastic was pushed down onto the surface of a crystal and voltage applied to the other side (on the base of the crystal), current started to flow from one contact to the other as the base voltage pushed the electrons away from the base towards the other side near the contacts. The point-contact transistor had been invented, a primitive variation of the BJT.
While the device was constructed a week earlier, Brattain's notes describe the first demonstration to higher-ups at Bell Labs on the afternoon of December 23, 1947, often given as the birthdate of the transistor. The PNP point-contact germanium transistor operated as a speech amplifier with a power gain of 18 in that trial.
Shockley was upset about the device being credited to Brattain and Bardeen, who he felt had built it "behind his back" to take the glory. Matters became worse when Bell Labs lawyers found that some of Shockley's own writings on the transistor were close enough to those of an earlier patent that they thought it best that his name be left off the patent application.
Shockley was incensed, and decided to demonstrate who was the brains of the operation. Only a few months later he invented an entirely new type of transistor one day while sitting in his hotel room waiting to give a speech. This new form, the layer transistor, was considerably more robust than the fragile point-contact system, and would go on to be used for the vast majority of all transistors into the 1960s.
With the fragility problems solved, a remaining problem was purity. Making germanium of the required purity was proving to be a serious problem, and limited the number of transistors that actually worked from a given batch of material. One then-small company, Texas Instruments, decided that the solution to this problem was to use silicon rather than germanium, which should be easier to work with. They were right, and germanium disappeared from almost all transistors within only two years of silicon being introduced in the early 1950s.
Now everything was in place, and within a few years, transistor-based products, most notably radios, were appearing on the market. A major improvement in yield came when a chemist advised the fabs to use distilled water rather than tap water: calcium ions were the cause of the problem. ' Zone melting,' also known as 'Zone refining', a technique using a moving band of molten material through the crystal, makes this whole endeavour possible.
Contrary to popular belief, the portable radio was not the first "mainstream" transistor application. Even by the 1940s, ordinary consumer radios were rather sophisticated; with several tubes they used Armstrong's brilliantly ingenious superheterodyne architecture. To meet consumer expectations, it would have been necessary for a transistor radio to use similar circuitry. In those days transistors could not operate reliably as amplifiers and oscillators in the RF range, even the 540 to 1700 "kilocycle" AM broadcast band. Also, miniaturized versions of many other necessary components, such as IF transformers and multiganged tuning capacitors, were not readily available.
The first major consumer application of transistors was the hearing aid, which required only audio amplification, and retailed in a market where miniaturization was important but low price not essential. Raytheon, which had developed miniaturized and ruggedized vacuum tubes for the military, introduced the first transistorized hearing aids.
Raytheon also produced the first transistor, the CK722, that was widely available commercially. Many electronics hobbyists of the fifties have a warm place in their heart for the CK722; it was practically the only transistor available to them for almost a decade. Innumerable homebrew projects were designed around it. The CK722's that were available to hobbyists were, essentially, those that had failed QC for more demanding applications. The CK722 was Germanium based, with low current gain, relatively high collector-to-emitter leakage current and a high variation in characteristics from unit to unit. This made designing practical circuits with them challenging. They were also a bit fragile and easily 'blown up'.
Widely used general purpose low power transistor include the following complementary pairs: 2N3904(NPN)/2N3906(PNP), BC182(NPN)/BC212(PNP), BC546(NPN)/BC556(PNP), BBC547(NPN)/BC557(PNP). They come in plastic cases and cost a few cents making them popular with hobbyists.
The transitor pairs 2N2222(NPN)/2N2907A(PNP) and BFY51(NPN)/2N2905A(PNP) are popular general purpose, metal can, medium power transitors of about one watt rating.
In both the USA and Europe the bipolar power transistor work horses are the 2N3055(NPN) and its compliment the 2N2955(PNP) (MJ2955). These are rugged, 1Mhz, 15A, 60V, 115W general purpose power transistors suitable for audio power amplifier, power supply and control applications. They cost about one dollar.
A range of vastly improved bipolar and field effect transistors, for all aplications, are now available from many manufacturers at reasonable cost.
The first CMOS transistor circuit was introduced by RCA in 1963.
Another level of miniaturization later became possible with the invention of the integrated circuit, which included many transistors on one chip of silicon, and led to a new generation of devices such as pocket calculators and digital watches.
NASA was the buyer, paying $4000 in the money of the day for a quad-two-input NAND function. Now this circuit costs about two cents. It is very costly to put things in space aboard liquid-fueled rockets. Reliability was an even bigger motivation in the space program, where fewer connections meant fewer potential failure points.
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