Learning objectives By the end of this chapter, you should be able to:
explain how a semiconductor operates compared to a metallic conductor
distinguish between intrinsic and extrinsic conduction
describe the structural difference between n-type and p-type semiconductors
detail, with the use of a diagram, how a p-n diode is formed
use a graph to show the effects of forward and reverse bias on a p-n diode
list the uses of semiconductors.
What is a semiconductor?
Semiconductors are used for all modern electronic equipment and have allowed for huge increases in speed and processing power of computers.
Semiconductors are used in LEDs (light-emitting diodes), thermistors and LDRs (light-dependent resistors).
While the resistivity of a semiconductor is between that of a good conductor and a good insulator, its resistivity decreases as temperature increases. Common materials used in semiconductors are silicon and germanium.
In this chapter we will look at the structure and operation of a basic semiconductor.
Silicon
A silicon (Si) atom has four valence electrons in its outer shell (Fig. 25.1). Valence is usually the number of electrons required to fill the outermost shell of an atom. These are locked into a covalent (sharing) bond with other silicon atoms. This means each Si atom has a full outer shell of eight electrons, making it stable. At zero kelvin, silicon is essentially an insulator as there are no free electrons for conduction.
FIG. 25.1 Each dot represents a valence electron. Each silicon (Si) atom covalently bonds four of its electrons with four other electrons around it
Semiconductors are materials whose resistivity is between that of a good conductor and a good insulator.
