Transistor

A transistor is a small, solid device capable of amplifying and switching electrical signals. A transistor can be manufactured using a wide variety of materials; most transistors utilize the unique semiconducting properties of silicon or germanium that has been selectively contaminated with other elements (e.g., arsenic, phosophorus, boron, gallium). A transistor is controlled by voltages communicated to it through three or more metallic contacts. Transistors are active devices, meaning that they must be supplied with power to function. Virtually all electronic devices contain transistors, from a handful to many millions; to the extent that our civilization has come to depend on computers and electronic communications, therefore, it depends on the transistor. The term transistor is a shortening of TRANSfer ResISTOR.

A transistor can perform a variety of useful electrical tasks because its resistance (the ease with which an electrical current flows through it) can be adjusted using a low-power control signal applied to one of a transistor's three metallic terminals. The resulting change in resistance between the other two terminals of the transistor—through one of which current enters the transistor, leaving through the other—changes the current passed through the transistor. This current can in turn be converted into a voltage by passing it through a resistor (a passive or unpowered circuit device that simply dissipates energy); the change in voltage across this "load" resistor can be many times greater than the change in voltage that was used to alter the resistance of the transistor itself. This increase in amplitude or strength is called amplification, one of the most basic processes in electronics.

Transistor amplification can be compared to controlling a powerful flow of water through a pipe by turning a valve: in this analogy, the force applied to the valve represents the transistor's control voltage, while the water flowing through the pipe represents its output current. A small, varying force applied to the valve—a back-and-forth wiggle—causes matched variations in the greater force carried by the water passing through the pipe; a small signal thus generates another that varies identically in time but is larger in amplitude. Vacuum tubes, which were developed before transistors but perform the same functions, are termed "valves" in British usage in reference to this analogy.

When a transistor acts as an amplifier it does not create the additional energy appearing in its output signal, just as a valve does not create water it dispenses. Rather, a transistor modulates the energy flowing from a battery or power supply in a way that is similar to a valve adjusting the flow rate from a source of pressurized water.

It is also clear from the valve analogy that instead of wiggling the valve one might choose instead to operate it in only two positions, open and shut, avoiding partial settings completely. Vacuum tubes and transistors can also be operated in this way, switching an electrical current on and off in response to a two-valued control signal. The on-off signals generated by this technique, switching, are the basis of digital electronics; the constantly-varying signals involved in amplification are the basis of analog electronics. Both rely on transistors, which are cheaper, smaller, and more reliable than other devices that can perform these functions.

Bipolar junction transistors

If the same crystal is doped so that each end is n-type and the very thin slice in the center is p-type, the resulting sandwich forms a bipolar junction transistor or n-p-n transistor. In an n-p-n transistor one of the n-type regions is termed the collector, the other n-type region is termed the emitter. (The emitter emits majority charge carriers, and the collector collects them.) The very thin slice of p-type material in the center is termed the base of the transistor. In a p-n-p transistor the collector and emitter regions are made from p-type semiconductor and the base has the characteristics of n-type material. Both n-p-n and p-n-p transistors are in common use but these two transistor types are not directly interchangeable since they require different power-supply polarities. Many circuits employ both n-p-n and p-n-p transistors, but the circuitry must supply the correct voltages. It is common to connect the two types of transistors together in an arrangement that is called complementary symmetry.

Transistors are able to amplify signals because their design permits the supply of charge carriers to be adjusted electrically. A transistor will have a high electrical resistance when it is starved for charge carriers but it will conduct quite well when a control signal injects extra carriers that can be used to support increased current.