Transistor

The transistor is a solid-state semiconductor device used for amplification and switching, and has three terminals: a small current or voltage applied to one terminal controls the current through the other two. It is the key component in all modern electronics. In digital circuits, transistors are used as very fast 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.

Transistor was also the common name in the sixties for a transistor radio, a pocket-sized portable radio that utilized transistors (rather than vacuum tubes) as its active electronics. 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 press. 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 "chips" consist of millions of transistors and sell for dollars, with per-transistor costs in the thousanths-of-pennies. The average home might contain a few tens of light bulbs and perhaps 100 metres of paper, things many consider "cheap", but the computer you are using to read this contains millions of transistors.

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 program to carry out the same task through "brute force". Today transistors have replaced almost all electromechanical 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) digital 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 a television or newspapers spend the vast majority of their time as digitial information, being converted to analog only for a small portion of the time.

Invention

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).

How Does a Transistor Work?

A transistor is 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)

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 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.

The most common form of MOSFET transistor in use today is the CMOS (complementary metallic oxide semiconductor) which is the basis for virtually all integrated circuits produced.

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.

1900s

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.

World War II

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. 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 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 (electricity)) 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.

Development

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.

First transistor

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.

Problem

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 lawers 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 insensed, and decided to demontrate 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 speach. This new form, the layer transistor, was considerable more robust than the fragile point-contact system, and would go on to the 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.

Origin of Name

In a vacuum tube (British: valve), changes in plate current are proportional to changes in grid voltage, to a first approximation. The ratio (current / voltage) has the dimensions of a conductance. Since the current and voltage are not measured at the same terminals, it is referred to as the "transconductance" rather than "conductance," and is an important figure of merit for a vacuum tube. Had vacuum tubes been named for their function rather than their structure, they might have been called "transconductors."

John R. Pierce coined the name "transistor" in 1949. It was originally thought that the transistor could be usefully considered to be the electronic dual of a vacuum tube. The property equivalent to transconductance would have been "transresistance" and the device would then have been a "transresistor," or "transistor" for short. It transpired that the transistor was not close enough to being a vacuum-tube dual for the concept to have any quantitative usefulness, and the concept of "transresistance" lives on only in the name "transistor."

Early Consumer and Hobbyist Applications

The transistor radio was not the first "mainstream" application of transistors. Even by the 1940s, ordinary consumer radios were rather sophisticated pieces of electronics, using several tubes, and based on Armstrong's brilliantly ingenious superheterodyne technology. To meet consumer expectations, it was necessary for a transistor radio to use similar circuitry. It was not easy in the early days to get transistors to operate reliably as amplifiers and oscillators in the RF range—even the 540-1700 "kilocycle" AM broadcast band. Miniaturized versions of many necessary components such as IF transformers and multiganged tuning capacitors were not available.

The first major consumer application of transistors was the hearing aid, which required only audio amplification, and represented a market where miniaturization was important and low price was not a requirement. Raytheon, which had developed miniaturized and ruggedized vacuum tubes for the military, introduced the first transistorized hearing aids.

Raytheon also introduced the first transistor, the CK722, that was widely available through ordinary commercial channels. Electronics hobbyists of the fifties have a warm place in their heart for the CK722, essentially the only transistor available for almost a decade, and innumerable homebrew projects were designed around it. The CK722's available to hobbyists were, essentially, those that had failed QC for more demanding applications. Germanium based, with low gain and high emitter-to-collector leakage, and high variation in characteristics from unit to unit, designing practical circuits with these components was quite a challenge.

Miniaturization

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.

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