Page 948 - Basic Electrical Engineering

have a complete understanding of behaviour of semiconducting materials we
will first have a quick review of atomic theory.

14.2 REVIEW OF ATOMIC THEORY

An atom of any material consists of a central nucleus around which electrons
are orbiting in different shells or orbits. Electrons are held in orbits due to the

electrostatic force between them and the nucleus. Electrons are negatively
charged while the protons in the nucleus are positively charged. Because

there is an equal number of protons with positive charge and orbiting
electrons of equal but opposite charge, an atom is electrically neutral. If,
however, an atom loses an electron it will lose some negative charge and will

become a positively charged ion. Similarly, if an atom gains an electron it
will become a negatively charged ion. Different materials are made up of

different types of atoms. However, their electrons and protons are identical.
An electron from one atom can replace an electron of any other atom of

different material.
Electrons orbit in different shells around the nucleus. Each shell contains a

definite number of electrons. For example, the total number of electrons in
first, second, and third shells are 2, 8, and 18, respectively. The number of
electrons in the outermost shell of the atom determine the electrical property

of the material. The number of electrons present in the outermost shell are
called valence electrons. The outermost shell of an atom may be completely

filled or partially filled. The outermost orbit must have eight electrons. If the
outermost orbit has less than eight electrons, we call them vacancies or holes
(i.e., empty spaces).

Let us consider atoms of two semiconducting materials, silicon and
germanium. Silicon has 14 electrons orbiting in three orbits and they are

distributed as 2, 8, 4. The nucleus has 14 protons which are positively
charged. Thus, the atom is electrically neutral. Germanium has 32 electrons

orbiting in four shells as 2, 8, 18, 4. The nucleus has 32 protons, and hence
the atom is electrically neutral. Two-dimensional representations of silicon

and germanium atom has been shown in Fig. 14.1. Both silicon and
germanium have four valence electrons and four holes in their outermost