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.

Semiconductor, or solid-state, diodes are relatively simple devices. They have two terminals. Their principal function is to allow current to flow in only one direction. A device that performs this function is called a rectifier.

Most semiconductor diodes are junction diodes. A junction diode consists of a small single crystal of silicon that has been doped in such a way that one side is a p-type semiconductor and the other side an n-type semiconductor. The electrical properties of the pn junction where the two types of material meet give the diode its ability to function as a rectifier.

Despite the presence of holes in a p-type semiconductor and the presence of free electrons in an n-type semiconductor, both materials are normally electrically neutral—that is, the number of protons in each material is balanced by an equal number of electrons. Near a pn junction, however, free electrons from the n-type region pass into the neighboring p-type region, where they combine with holes and form negative ions. Holes from the p-type region pass into the n-type region, where they combine with free electrons and form positive ions. These actions leave the p-type material near the junction with a negative charge and the n-type material near the junction with a positive charge. The redistribution of electric charge creates an electrical barrier that opposes any further movement of charge carriers across the junction.

The operation of a typical semiconductor diode is shown in the illustration A Semiconductor Diode. When a voltage is applied across the diode so that the p-type region is at a higher voltage than the n-type region, the electrical barrier across the junction is reduced. The application of such a voltage across a pn junction is called a forward bias. Free electrons from the n-type region and holes in the p-type region are drawn toward the junction, where they unite. The diode therefore conducts an electric current, since this process will continue indefinitely and electrons will continually enter one end of the diode and leave the other.

The application of a reverse voltage across a pn junction is called a reverse bias. With a reverse bias, the electrical barrier at the junction is raised and it blocks the passage of both free electrons from the n-type region and holes from the p-type region. Under these conditions, there is no flow of charge, and the diode does not conduct.

A Schottky diode is formed by placing a metal in contact with an n-type semiconductor. The junction formed in this manner is especially useful in high-frequency circuits such as high-speed logic circuits.

This type of diode provides the circuit with capacitance, an electrical property that allows a device to store electric charge. In a circuit, capacitance works to resist changes in voltage. The value of the capacitance is controlled by the amount of reverse bias applied to the diode. Varactor diodes are useful in tuning circuits.

If a high reverse voltage is applied to a semiconductor diode, the electrical insulating barrier formed by the pn junction will break down and the diode will suddenly conduct a large current. A zener diode is doped in such a way that it will begin conducting at a specific, relatively low reverse voltage. This voltage is called the diode's breakdown voltage. Zener diodes are useful in regulating the voltage in a circuit and for protecting circuits from excessive voltage.

Some diodes are used to detect light or to generate electricity from light. For information about these types of diodes, see the subtitle Photoelectric Cell. Semiconductor lasers and light-emitting diodes (LED's) are diodes used as sources of light.

A transistor is a semiconductor device that contains three electrical terminals. One of the terminals serves to control the voltage or current between the other two. Transistors are used primarily to amplify electrical signals or to act as switches in electronic circuits. Transistors can be found in almost every piece of modern electronic equipment.

A transistor forms part of two separate circuits, both of which have a source of electric power. The electrical properties of the transistor are such that a small change in the current or voltage of one of the circuits produces a very large change in the current or voltage of the other. When a transistor is used as an analog device, the change in the second circuit is proportional to the change in the first. When a transistor is used as a digital circuit, the first circuit essentially serves as a switch that turns the second circuit on or off.

A bipolar junction transistor contains three layers of semiconductors. An npn transistor is formed by a thin layer of p-type semiconductor between two layers of n-type semiconductor; a pnp transistor is formed by a thin layer of p-type material between two layers of p-type material. The operation of both types is conceptually similar, so the operation of only one, the npn transistor, will be discussed here.

In an npn transistor, one n-type region is called the emitter and the other the collector; the p-type region is called the base. The terminals are connected to DC power sources in such a way that the base is kept at a higher voltage than the emitter, and the collector is kept at a higher voltage than the base. With this arrangement, a current readily flows from the emitter to the base (the pn junction between them is forward biased), but is blocked from flowing between the base and the collector (the pn junction between them is reversed biased).

As current flows between the emitter and base, free electrons enter the base from the emitter. Because the collector is maintained at a higher voltage than the base, these electrons are then readily drawn from the base into the collector. The base is made to be extremely thin and with a low concentration of holes so that most of the free electrons that enter the base from the emitter will be drawn into the collector. In this way, a small current in the external circuit to which the emitter and base are connected will give rise to a much larger current in the circuit to which the emitter and collector are connected.

The operation of a bipolar junction transistor typically consists of controlling the current in the emitter-collector circuit by varying the amount of current that flows in the emitter-base circuit. Weak input signals applied to the emitter-base circuit are amplified, producing much stronger output signals in the emitter-collector circuit.

In a field-effect transistor, the output current is controlled by an electric field that is varied by changing the voltage of one of the transistor's terminals. A field-effect transistor requires much less input current than a bipolar junction transistor.

There are two major types of field-effect transistors: junction-gate field-effect transistors and metal-oxide semiconductor field-effect transistors (MOSFET's). MOSFET's are the most widely used transistors today.

The operation of a typical MOSFET can be explained with the assistance of the illustration Field-effect Transistor. The three terminals, made of metal, are called the source, gate, and drain. The source and drain are connected to two separate regions of n-type semiconductor. The two n-type regions are separated from each other by a region of p-type semiconductor. Along one side of the p-type region there is a thin layer of silicon dioxide. The layer serves as a support for the gate terminal and it insulates the gate from the p-region.

In this type of MOSFET, a current cannot normally flow between the source and the gate because of the n-type region between them. However, when a positive voltage is applied to the gate, the electric field the gate produces extends through the silicon dioxide layer. The field draws electrons from the p-type region into a thin channel along the oxide layer. The channel forms a conductive path in the p-type region between the source and the drain and allows electric charge to flow between them.

A small change in the voltage applied to the gate will give rise to a large change in the current between the source and the drain. A MOSFET is typically used as a switch: either no voltage is applied to the gate so that the transistor completely blocks any current in the source-drain circuit, or a voltage is applied to the gate that will effectively eliminate any resistance to the current in the source-drain circuit.

A cathode-ray tube (CRT) is a vacuum tube in which one or more beams of electrons are used to form an image on a relatively flat glass screen at one end of the tube. The image is essentially a visual record of a varying electrical signal applied to the tube. CRT's are commonly used as picture tubes for television sets and as computer monitors. They are also used for radar displays, oscilloscope displays, and other applications.

Each beam of electrons in a cathode-ray tube is produced by an electron gun. An electron gun contains a negative electrode called the cathode, which serves as a source of the electrons; and a positive electrode, called an anode, which accelerates the electrons. A negatively charged element called the control grid controls the intensity of the beam.

The electron beam is then focused so that it will come to a point on the screen at one end of the tube. The location at which the beam strikes the screen is controlled by bending the beam vertically and horizontally. In some cathode-ray tubes, such as those used in oscilloscopes, the beam is bent as it passes between pairs of charged metal plates that exert electrostatic forces. In most cathode-ray tubes, the beam is bent by electromagnets mounted around the central part of the tube. Both types are shown in the illustration Cathode-ray Tubes.

The inner face of the screen is covered with a coating of substances called phosphors; the phosphor coating glows at the spot where it is struck by the electrons from the electron gun. A visual image is formed as the beam of electrons passes across the screen; the brightness of each spot struck by the beam is controlled by varying the intensity of the beam. Because the screen is transparent, the image formed is visible from outside the tube.

Microwave tubes are vacuum tubes that produce extremely short radio waves called microwaves. Travelling-wave tubes and klystrons are two other types. Microwave tubes are used in microwave ovens as well as in radar transmitters and other kinds of equipment.

Vacuum tubes that are used to produce a type of penetrating radiation called X rays are called X-ray tubes. Such tubes contain two electrodes; the X rays are produced by bombarding a metal target on one of the electrodes with high-energy electrons from the other electrode. The voltage between the electrodes must be large, ranging from tens of thousands to several million volts, depending upon the intended use of the X rays.

Thyristors are semiconductor devices with three terminals and four or more alternating layers of p-type and n-type semiconductors. Thyristors are typically used as switches in AC high-power circuits—circuits designed to handle large currents or voltages. Because they contain several pn junctions, thyristors can withstand much higher reverse voltages and currents than transistors and semiconductor diodes can.

During operation, a thyristor normally blocks an electric current. However, a voltage applied to one of the thyristor's terminals (the gate) will cause the thyristor to begin carrying a current. Most thyristors will continue to conduct the current until the current reverses direction. The amount of current that a thyristor conducts can be controlled by turning on the thyristor at a specific point in each cycle of the alternating current.

The silicon-controlled rectifier (SCR) is one of the most common kinds of thyristors. One use of the SCR is to provide the DC motors of an electric locomotive with a regulated amount of DC power from overhead AC power lines. An SCR allows current to flow in only direction. The triac is a similar device but can be turned on to allow current to flow through it in either direction.

Photoelectric cells, or photocells, are devices whose electrical characteristics vary according to the amount of light that strikes them. There are two basic types made from semiconductors: photovoltaic cells and photoconductive cells. A phototube, an older type of photoelectric cell, is a type of electron tube.

A photovoltaic cell is a semiconductor device that converts light into electric energy. The photovoltaic cells in a solar battery typically consist of silicon wafers treated to form an n-type semiconductor covered by a very thin layer of p-type material. When light strikes the wafer, it creates pairs of holes and free electrons. Some of the light penetrates the surface of the wafer and creates holes and free electrons near the pn junction. The electric field that naturally exists across the junction tends to push electrons into the n-type region and holes into the p-type region. If the two regions are connected through an external circuit, electrons will flow from the n-type region through the external circuit to the p-type region.

Photoconductive cells are light-sensitive semiconductor devices that do not have a pn junction. When light strikes the photoconductive cell, it creates free electrons and holes, lowering the semiconductor's electrical resistance. Although the cell does not generate electricity, it can be used with an external power source to produce an electric signal because the resistance of the semiconductor changes in proportion to the amount of light that strikes it. Photoconductive cells are used in certain types of exposure meters in light switches that turn on automatically at nightfall, and in other devices.

are vacuum tubes that conduct a current only when light strikes them. Phototubes have been used in a variety of scientific instruments and other devices, but have been largely replaced by photoconductive cells, which perform similar functions.

Triodes are vacuum tubes that, with the proper circuitry, will produce amplification of an electrical signal. They are little used today, but are historically important. A triode contains three electrodes—a cathode, grid, and anode—sealed in an evacuated container. The grid is a cylindrical screen made of metal mesh that surrounds the cathode. The anode is a cylindrical metal plate that encloses both of the other elements.

During the operation of a triode, the cathode is heated to emit electrons. The heat is provided by an encircling wire filament that becomes hot when a current is sent through it. The plate is always operated at a higher voltage than the cathode, so it will attract the electrons emitted by the cathode.

The movement of the electrons between the cathode and plate is regulated by varying the voltage on the grid. When the grid is held at a voltage lower than that of the cathode, it blocks the flow of electrons from the cathode to the plate. As the voltage of the grid approaches that of the cathode, fewer electrons are blocked and the triode current increases. At voltage levels just below the cathode voltage, very small changes in the grid voltage produce very large changes in the triode current. In this way, weak signals fed to the grid can be amplified by the cathode-plate circuit.

Image-recording electronic components are important in video cameras and a number of other electronic applications. These components include various kinds of picture tubes and semiconductor devices called charge-coupled devices (CCD's).

Lasers and masers are electronic devices that produce beams of very uniform electromagnetic waves. A laser produces a beam of light; a maser, a beam of microwave radiation.

In addition to the triode, there are various other kinds of electron tubes that were widely used before the development of semiconductor devices. These devices include the vacuum-tube diode (whose function is similar to that of a semiconductor diode), the pentode (a five-electrode vacuum tube whose function is similar to that of a transistor), and the thyratron (a vacuum tube whose function is similar to that of a thyristor).

Although an electronic product may contain a large number of components, or parts, the number of different kinds of components is relatively small. The same basic components can be combined in specific ways to perform a great diversity of functions. This section describes many of the various fundamental electronic devices and circuit elements that serve as basic electronic components.