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Transistor

The transistor is an amplifying and/or switching semiconductor device. The transistor is the key component in all modern electronics. In digital circuits, transistors are used as electrical switches, and arrangements of transistors can function as logic gates, RAM-type memory and other devices. In analog circuits, transistors are essentially used as amplifiers.

The transistor was invented at Bell Laboratories in December 1947 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).

Table of contents

How Does a Transistor Work?

A transistor is electrically a three-terminal device. In a 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). In FETs, 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.

Bipolar Junction Transistor (BJT)


The schematic symbols for pnp- and npn-type BJTs.

Conceptually, one can understand a bipolar junction transistor as two diodes placed back to back, connected so they share either their positive or their negative terminals. The forward-biased emitter-base junction allows charge carriers to easily flow out of the emitter. The base is made thin enough so that most of the injected carriers will reach the collector rather than recombining in the base. Since small changes in the base current affect the collector current significantly, the transistor can work as an electronic amplifier. The rate of amplification, usually called the current gain (β), is roughly one hundred for most types of BJTs. That is, one milliampere of base current usually induces a collector current of about a hundred milliamperes. BJTs prevail in all sorts of amplifiers from audio to radio frequency applications and are also popular as electronic switching devices.

Field-Effect Transistor (FET)


The schematic symbols for p- and n-channel MOSFETs. The symbols to the right include an extra terminal for the transistor body whereas in those to the left the body is implicitly connected to the ground.

The most common variety of field-effect transistors, the enhancement-mode MOSFET (metal-oxide semiconductor field-effect transistor) can also be viewed as two back-to-back diodes that separate the source and drain terminals. The volume in between is covered by an extremely thin insulating layer that carries the gate electrode. When a voltage is applied between gate and source, an electric field is created in that volume, causing a thin conductive channel to form between the source and drain and allowing current to flow across. The amount of this current can be modulated, or completely turned off, by varying the gate voltage. Because the gate is insulated, no DC current flows to or from the gate electrode. This lack of a gate current (as compared to the BJT's base current), and the ability of the MOSFET to act like a switch, allows particularly efficient digital circuits to be created. Hence, MOSFETs have become the dominant technology used in computing hardware such as microprocessors and memory devices such as RAM.

Advantages of Transistors over Thermionic Valves

Before the transistor, the thermionic valve or vacuum tube, was the main component of an amplifier. The key advantages that have allowed transistors to replace their valve predecessors in almost all applications are

  • smaller size,
  • simpler manufacture and, hence,
  • lower cost,
  • lower operating voltages,
  • absence of a heated filament and, as a consequence,
  • lower power dissipation,
  • higher reliability and greater endurance.

Valves are still used in very high-power applications such as broadcast radio signal amplification. Some audio amplifiers also use them, 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. Other 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.

The "second generation" of computers through the late 1950s and 1960s featured boards filled with individual transistors. Subsequently, transistors, other components and the necessary wiring, were integrated into a single, mass-manufactured component in the integrated circuit. In modern digital electronics, single transistors are very rare, though they remain common in power and analog applications.

History

All transistors rely on the ability of certain materials, known as semiconductors, to change their electrical resistance 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 the 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.

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.

He then started to try to figure out why they were so "odd". He spent most of 1939 trying to grow more pure versions of the crystals, and soon found that the "oddness" went away when they were of better quality — 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 it, 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.

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[?], 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.

The first transistor took some time to construct. 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. 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.

Such a system was of limited practical use, no better physically than the cat's whisker of old. Soon newer methods and understanding allowed for much more robust versions. Within a few years transistor-based products, most notably radios, were appearing on the market.



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