Semiconductors

The term electronics usually refers to devices in which electrons are conducted through a vacuum, gas, or semiconductor. Automotive applications such as electronic control units mostly use semiconductors such as diodes, transistors, and power transistors. A semiconductor’s electrical resistance is higher than that of most conductors, but lower than that of most insulators. A semiconductor’s conducting ability depends on two kinds of charge carriers:

  • The first type of charge carrier is the negative electron. This kind of semiconductor contains an excess of free electrons. They can be made to flow and carry charge.
  • The second type of charge carrier is the hole. Holes occur when an electron moves from its existing place to a new place, leaving a void where it was. Because the holes are positive, a voltage can make them move toward the negative pole. When connected in a circuit, both the electrons and the holes move, but in opposite directions.
FIGURE 36-5A
N-Type semi-conductor.
FIGURE 36-5B
P-Type semi-conductor.

The number of charge carriers, either an excess of electrons or deficiency of electrons (causing holes in a material), can be altered by doping, or adding very small quantities of impurities to a semiconductor material. A doped semiconductor always has an excess of one type of charge carrier. For example, electrons in excess make it an N-type semiconductor Figure 36-5A. N stands for negative. Holes in excess make it a P-type semiconductor Figure 36-5B. P stands for positive. Each of these materials responds to positive or negative current flow oppositely.

Semiconductor Operation

Most electronic components combine P-type and N-type semiconductors. The point where they join is called the PN junction. In this area, the depletion layer occurs, and some electrons and holes cancel each other out and few charge carriers are present. The depletion layer is very thin and acts like an insulator.

A diode is one P-type semiconductor material joined to one N-type semiconductor material with a single PN junction. If it is connected to a voltage source so that the P region is connected to a negative pole and the N region to a positive pole, then the negative pole attracts the holes, and the positive pole attracts the electrons. This enlarges the depletion layer and the insulated space. As a result, current cannot flow across the junction. Diodes are discussed further later on in the chapter.

Semiconductors are very versatile substances and are widely used to make various electronic components. Their conductivity can be manipulated and precisely controlled by doping with impurities to make transistors. They can also be designed to vary their conductance in magnetic fields to make Hall-effect devices and to vary their light to make photoelectric diodes and transistors. Semiconductor devices are replacing many other kinds of switching devices such as mechanical switches and relays. They are small, light, and use low operating voltages. They are reliable, require no maintenance, and are relatively easy to manufacture, but they are sensitive to heat and voltage spikes.

Semiconductor Materials

Many different materials have an atomic structure that can be used to make semiconductors. Silicon is the most widely used semiconductor material because it has a useful temperature range and is abundant, cheap, and easy to manufacture. Germanium was among the first semiconductor materials to be developed and is less widely used than silicon, but it is useful in very high-speed devices when alloyed with silicon. High-speed semiconductors widely use gallium-arsenide; however, it is more expensive and more difficult to manufacture. Silicon carbide has been used to create blue light-emitting diodes (LEDs), and it can withstand high operating temperatures.

AppliedScience
AS-57: Conductors: The technician can explain the difference between an electrical conductor and an insulator.

The electrical conductivity of a material refers to the freedom of the electrons within the atoms of the material to move around. Materials with high electron mobility are known as conductors, while materials with low electron mobility are called insulators. High electron mobility allows electrons to freely flow when a voltage is applied across two points of the material.

Most familiar conductors are metallic, although there are non-metallic conductors such as graphite and salt solutions. Silver is the best conductor, but copper is most commonly used for electrical wiring due to many desirable properties including high tensile strength and ductility.

An insulator is a material that resists the flow of an electrical charge. Glass and paper are both very good insulators, but polymers and plastics are more commonly used to insulate wiring. The purpose of an insulator in electrical applications is to support or separate conductors without allowing current flow through themselves.