Basic Electronic Components
Some basic components of electronic circuits are active components; they can increase the power of an electrical signal because they are powered by a source of electricity separate from the electrical signal. Transistors and oscillators are examples of active components. Other components are passive components; they help give a circuit its electrical characteristics, but do not require a separate source of electric power for their operation. Passive components include resistors, inductors, and capacitors. Resistors are important in controlling the voltage in different parts of a circuit. Capacitors temporarily store electric charge and help oppose voltage changes in a circuit. Inductors store energy in magnetic fields and help oppose changes of current.
Active components in turn are divided into two fundamental groups: (1) semiconductor, or solid-state, devices; and (2) electron tubes. The operation of semiconductor devices depends on the behavior of electrons in a semiconductor, a material whose ability to conduct electricity is between that of a conductor and that of an insulator. Most electronic devices in use today are semiconductor devices. The operation of electron tubes depends on the behavior of electrons moving through a vacuum or gas inside a closed container.
The most common semiconducting material used in electronics is silicon to which small amounts of certain other chemical elements have been added. The addition of these elements, a process called doping, improves the degree to which a semiconductor's ability to conduct electricity can be controlled.
Doping is used to produce either of two types of semiconductors: n-type or p-type semiconductors. An n-type semiconductor contains a large number of electrons that are free to move through it. A p-type semiconductor contains a large number of holes, sites into which an electron can move.
When an electron from a nearby atom moves into a hole, it leaves a hole at its former position; this action is the same as if the hole moved from one point to another. (When a free electron moves into a hole, the electron ceases to be free and the hole ceases to exist.) Both free electrons and holes are charge carriers—that is, under the influence of a voltage, they give rise to a flow of electric charge.
The operation of most semiconductor devices depends on the electrical properties of a pn junction, the boundary between an n-type semiconductor and a p-type semiconductor. The electrical properties of a pn junction are discussed later in this section under the subtitle Semiconductor Diodes.
Various kinds of protective cases are used to house semiconductor devices. These cases are made of metal, plastic, or a ceramic material. Wires or pins that project from the case provide electrical connections to other parts of an electric circuit.
An electron tube is essentially a sealed hollow enclosure in which the movement of electrons can be carefully controlled. The enclosure is typically made of glass and contains various metal parts called electrodes for producing and regulating a beam of electrons.
An electron tube from which all gases have been removed is called a vacuum tube. In most types of vacuum tubes, one of the electrodes must be heated to emit electrons. (This emission is called thermionic emission.) The most important types of vacuum tubes are the cathode-ray tube and the X-ray tube. Such vacuum tubes as the triode, the pentode, and the vacuum-tube diode were once important, but they have been almost entirely replaced by comparable semiconductor devices that are smaller and more durable. In addition, vacuum tubes consume much more electric energy than semiconductor devices because they require electrical heating for thermionic emission.
In some electron tubes, the enclosure is filled with a gas such as mercury vapor or neon. Such gas tubes are important sources of light. They include fluorescent lamps, neon lamps, and electronic flashtubes.