Transistor

From Wikipedia, the free encyclopedia.

Photo of transistor types (tape measure marked in centimeters)
Enlarge
Photo of transistor types (tape measure marked in centimeters)
Transistor in the  form factor
Enlarge
Transistor in the SMD form factor

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 larger current through the other terminal and the common terminal. The term transistor was originally coined from the word transresistance.

In analog circuits, transistors are used in amplifiers. Analog 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.

Transistor was also the common name in the sixties for a transistor radio, a pocket-sized portable radio that used transistors (rather than vacuum tubes) as its active electronic components. This is still one of the dictionary definitions of transistor.

Contents

Importance

The transistor is considered by many to be one of the greatest inventions in modern history, ranking with banking and the printing press. It is the key active component in practically all modern electronics. Key to its importance 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 are relatively cheap to produce, due to the negligible per-transistor cost.

Although millions of individual (discrete) transistors are used, the vast majority of transistors are embodied in integrated circuits (chips). The number of transistors in a single chip ranges from about 20 for a logic gate to 250 million for an advanced processor (as of early 2005).

Along with diodes, resistors, capacitors and inductors, transistors can be integrated onto a semiconductor chip using a highly automated process to produce complete electronic functions, either analog or digital. Sometimes both analog 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 millions of production chips the individual costs can be minimized.

The term 'chip' is used rather loosely today: originally it referred 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 (as in a valve) or a gas and which does not use moving parts to control charge flow. In the same vein, a circuit or item of equipment may be described as 'solid state'.

The low cost of the transistor has made it an almost universal device for non-mechanical tasks. Whereas a common item, say a refrigerator, would have used a mechanical device for control, today it is often less expensive to simply use a standard integrated circuit (containing a few million transistors) and an appropriate computer program to carry out the same task through logic. Today transistors have replaced almost all electromechanical devices, are used in most simple feedback systems, and appear in huge numbers in everything from traffic lights to washing machines.

Hand-in-hand with low cost has come the trend to "digitize" all information. With transistor-utilizing computers offering the ability to quickly find, sort and process digital information, more and more effort has been put into making 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.

Types

In broad terms, discrete 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 as small as specks of dust. Many transistors, 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 arsenide (GaAs).

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?

There are two basic types of transistors, the bipolar junction transistor (BJT) and the field-effect transistor (FET), which work differently. Bipolar transistors are so named because the main conduction channel uses both electrons and holes to carry the main electric current. Field-effect transistors (also called unipolar transistors) use only one of the two types of carrier (either electrons or holes, depending on the subtype of the FET). See the articles on each type of device for more information.

Advantages of transistors over thermionic valves

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:

  • Smaller size (despite continuing miniaturization of tubes)
  • Highly automated manufacture
  • Lower cost (in volume production)
  • Lower operating voltage
  • Absence of a heater
  • Lower power dissipation (no heater and very low saturation voltage)
  • Higher reliability and greater endurance (although valves are more resistant to nuclear electromagnetic pulses (NEMP) and electrostatic discharge (ESD) )
  • Non-lifed (valves are lifed components which wear out)
  • Availability of complementary devices (allowing circuits with complementary symmetry (complementary versions of valves are not available))
  • Ability to control large currents (power transistors are available to control hundreds of amperes, while a valve to control even one ampere is quite large and expensive)
  • Non-microphonic (vibration of a valve can cause the characteristics to be modulated)

Use in audio amplifiers

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 mA of current carries about 6240 million million electrons per second.)

Others detect a distinctive "warmth" to the sound. 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.

Use in computers

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.

Transistors are also used in power regulation and in computer PSUs, especially in switching power supplies.

High-power transistor used in a switching power supply. Mounted on a  block for enhanced cooling.
Enlarge
High-power transistor used in a switching power supply. Mounted on a aluminium block for enhanced cooling.

How semiconductors work

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 attracted 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. Although a transistor may seem like two interconnected diodes, a bipolar transistor cannot be made simply by connecting two discrete junction diodes together. They need to be fabricated on the same crystal, and physically sharing a common and extremely thin base, to get bipolar transistor action.

Light sensitivity

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.

History

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.

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 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.

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.

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.'

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.

Problems with fragility and impurities

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.

Origin of name

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 the transconductance of a tube would have been "transresistance" and the device would then have been a "transresistor," or "transistor" for short. In practice 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

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 transistor 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 complement 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 applications, are now available from many manufacturers at reasonable cost.

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.

NASA was the buyer, paying about 4,000 dollars 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.

See also

External links and references


Personal tools