Electronic signals and electrical power are amplified or switched by use of transistors, which are semiconductor devices. Transistors are one of the most fundamental building elements of contemporary electronic devices and circuits. It is made of semiconductor material and typically has at least three terminals for connecting to an external circuit, according to the manufacturer. One pair of transistor terminals are controlled by a voltage or current supplied to one pair of terminals on the other pair of transistor terminals. Because the controlled (output) power of a transistor may be greater than the controlling (input) power of the transistor, a transistor has the capability of amplification. Currently, some transistors are packed separately, but a large number of transistors are found incorporated in integrated circuits. More information about may be found on Wikipedia.

When it is used as an amplifier, it accepts a very little electric current at one end (the input current) and generates a considerably larger electric current (the output current) at the other (the output current). In other words, it acts as a current booster of some kind. That comes particularly handy in applications such as hearing aids, which was one of the earliest applications for transistors to be developed. A hearing aid is equipped with a small microphone that takes up noises from your environment and converts them into fluctuating electric currents, which are then sent to your ears. You hear a much louder version of the noises around you because the sounds are sent into a transistor that amplifies them and drives a small loudspeaker. This is how it works.

Transistors may also be used as switches in some situations. A very little electric current passing through one section of a transistor may cause a considerably larger electric current to flow through another area of the transistor, and vice versa. In other words, the tiny current causes the bigger current to become active. This is, in essence, how all computer chips do their functions. For example, a memory chip may include hundreds of millions or even billions of transistors, each of which can be turned on or off individually by the computer. Because each transistor may be in two separate states at the same time, it has the ability to store two different numbers, zero and one, in its memory. A chip can hold billions of zeros and ones, as well as almost as many conventional numbers and letters, thanks to the billions of transistors it contains (or characters, as we call them). I'll go into more detail about this later.