Transistors

The transistor, invented in 1947, has probably done more to revolutionize our lives than any other single device. A transistor is simply a triode - either an n type semiconductor sandwiched between two p type semiconductors, as in the figure below, or a p type semiconductor sandwiched between two n type semiconductors.

 

Figure 12.16: Transistor

As in the diode, as electric field is established in a certain direction in the vicinity of the junctions between the two types of semiconductors. In a transistor, a current flowing from the emitter to the collector is greatly affected by the bias of these electric fields, and these fields are controlled by a signal fed into the base. There are two main uses of transistors.

  Amplifiers: In this situation, a relatively weak signal fed into the base gets amplified into a much larger current flowing from the emitter to the collector. This amplification, which can be up to 100 times or more, is used, for example, in stereo systems to amplify the relatively weak electrical signal coming from a tape reader into a signal strong enough to drive a speaker.

Switches: A transistor can also be used as a switch: a certain type of signal into the base will cut off the current flowing from the emitter to the collector. Removing this signal will allow this current to flow again. Although relatively simple, this is the basic use of a transistor in computers, of which we now discuss.