Copyright © 2007 Dorling Kindersley
Electronic circuits operate nearly every modern machine—microwave ovens, cars, and computers. In an electronic circuit, an electric signal carries information. Signals are controlled and changed by components made from materials called SEMICONDUCTORS.
A signal is a current or voltage change that carries information. Changes can represent instructions, numbers, sounds, or pictures in the form of a code. A digital signal is either on or off. An analog signal is continuously flowing electricity, which increases and decreases to represent the information.
Electronic circuits use components (parts) to control electric signals. These include resistors, capacitors, diodes, and transistors. Resistors control how much current flows through them, capacitors store electric charges and release them when necessary, and diodes let current pass through in only one direction.
Transistors are electronic components that can change and control electronic signals. A transistor can work as a switch—turning a signal on or off—or as an amplifier—increasing the current or voltage in a circuit.
A semiconductor is a material that conducts electricity less well than a metal, but better than an insulator, and can act as both a conductor and an insulator. The chemical elements silicon and germanium are the most important semiconductors for making electronic components.
The current in a semiconductor is carried by positive holes and by negative electrons. A hole is an empty space left in the orbit of an atom when an electron has escaped. Holes move through a semiconductor in the opposite direction from electrons, hopping from atom to atom.