Chapter

16

AC Electrics - Semiconductors

An Introduction to Semiconductors . . . . . . . . . . .	. .	. .	. .	. .	.	.	251
Conductors and Insulators . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.251
Semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .252
N-Type Material . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.253
P-Type Material . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	. .		.254
Current Flow							255
The P-N Junction . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	255
Reverse Bias . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	256
Forward Bias							256
The Junction Diode . . . . . . . . . . . . . . . . .	. .	. .	. .	. .	.	.	256
The Bipolar or Junction Transistor . . . . . . . . . . . .	. .	. .	. .	. .	.	.	257
Summary							259

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16 AC Electrics -Semiconductors

Semiconductors - Electrics AC 16

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AC Electrics - Semiconductors 16

An Introduction to Semiconductors

Most people own some type of hand-held or desktop calculator these days. The cost of these useful devices varies depending on sophistication; simple ones are given away free as advertising gimmicks, yet there is more computing power inside one of these tiny machines than took Neil Armstrong to the moon!!

Transistorization and miniaturization have enabled us to build ever more sophisticated electronics and package them in ever smaller units. Modern pilots rely heavily on the electronic flight systems incorporated in their aircraft and therefore must have an understanding of how transistors, or more specifically semiconductors, work.

Conductors and Insulators

Before proceeding with the explanation about how semiconductors work, let us remind ourselves about the general atomic construction of conductors and insulators.

Figure 16.1 A hydrogen atom

The most simple atom is the Hydrogen atom. It consists of a nucleus, containing one proton (positively charged) and one neutron (neutrally charged), and an electron (negatively charged) orbiting about the nucleus.

Conductors and insulators have more complex atoms with an increasing (equal) number of neutrons, protons and electrons with the latter orbiting the nucleus in multiple orbits or shells.

These atoms are held together by the bonds formed between the valence electrons in the outer shells and arrange themselves into a lattice type arrangement equidistant from each other. Electrons in the outer shells are less tightly bonded to their parent atom than those on the inner shells and are free to move from one atom to the next.

These electrons, known as free electrons, form the basis for current flow within the material. Conductors, formed by atoms held together by electrovalent bonds, possess large numbers of free electrons, and this allows current to flow easily through the material or put another way; the material has high conductivity (low resistivity). Gold, silver and copper are all examples of good conductors.

AC Electrics - Semiconductors 16

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16 AC Electrics -Semiconductors

Insulators, on the other hand, are formed by atoms held together by covalent bonds and possess few free electrons. This means that current flow is difficult; the material has low conductivity (high resistivity). Mica is one example of a good insulator.

Semiconductors

Semiconductors, as their name would imply, fall somewhere between a conductor and an insulator. Silicon and germanium are examples of semiconductors.

Both materials are formed by atoms with covalent bonds. Though each possesses some free electrons at normal temperatures, they are closer to being insulators than conductors. Thus an EMF applied across the material would give rise to an intermediate current flow, higher than that in an insulator, but less than that in a conductor.

Conductivity can be improved by the controlled addition of impurities into the silicon or germanium material using a process known as doping.

Semiconductors - Electrics AC 16

Figure 16.2 A typical atomic lattice structure

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