This chapter introduces the physics behind semiconductor devices. It discusses how quantum physics led to new understandings of atomic structure that contradicted earlier classical models. In particular, it describes Niels Bohr's planetary model of the atom and introduces concepts like atomic number, valence, free electrons, energy levels, and the distinction between conductors, semiconductors, and insulators based on their band gaps. The chapter lays the groundwork for understanding how semiconductor devices function by explaining these foundational quantum physics principles.

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1. Introduction
This chapter will cover the physics behind the
operation of semiconductor devices and show how
these principles are applied in several different types
of semiconductor devices. Subsequent chapters will
deal primarily with the practical aspects of these
devices in circuits and omit theory as much as
possible.
2. Quantum Physics
To say that the invention of semiconductor devices
was a revolution would not be an exaggeration. Not
only was this an impressive technological
accomplishment, but it paved the way for
developments that would indelibly alter modern
society.

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Semiconductor devices made possible miniaturized
electronics, including computers, certain types of
medical diagnostic and treatment equipment, and
popular telecommunication devices, to name a few
applications of this technology.
But behind this revolution in technology stands an even
greater revolution in general science: the field of
quantum physics. Without this leap in understanding the
natural world, the development of semiconductor devices
(and more advanced electronic devices still under
development) would never have been possible.

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Quantum physics is an incredibly complicated realm of
science, and this chapter is by no means a complete
discussion of it, but rather a brief overview. Without a
basic understanding of quantum physics, or at least an
understanding of the scientific discoveries that led to its
formulation, though, it is impossible to understand how
and why semiconductor electronic devices function. Most
introductory electronics textbooks I've read attempt to
explain semiconductors in terms of "classical" physics,
resulting in more confusion than comprehension.
Many of us have seen diagrams of atoms that look
something like this:

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• Tiny particles of matter called protons and neutrons make up the center of
the atom, while electrons orbit around not unlike planets around a star. The
nucleus carries a positive electrical charge, owing to the presence of
protons (the neutrons have no electrical charge whatsoever), while the
atom's balancing negative charge resides in the orbiting electrons. The
negative electrons tend to be attracted to the positive protons just as
planets are gravitationally attracted toward whatever object(s) they orbit, yet
the orbits are stable due to the electrons' motion. Further attempts at
defining atomic structure were undertaken, and these efforts helped pave
the way for the bizarre discoveries of quantum physics. Consider this short
description of electrons in an atom, taken from a popular electronics
textbook:
• Orbiting negative electrons are therefore attracted toward the positive
nucleus, which leads us to the question of why the electrons do not fly into
the atom's nucleus. The answer is that the orbiting electrons remain in their
stable orbit due to two equal but opposite forces. The centrifugal outward
force exerted on the electrons due to the orbit counteracts the attractive
inward force (centripetal) trying to pull the electrons toward the nucleus due
to the unlike charges.

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3. The Bohr model
The most important properties of atomic and molecular
structure may be exemplified using a simplified picture of
an atom that is called the Bohr Model. This model was
proposed by Niels Bohr in 1915; it is not completely
correct, but it has many features that are approximately
correct and it is sufficient for much of our discussion. The
correct theory of the atom is called quantum mechanics;
the Bohr Model is an approximation to quantum
mechanics that has the virtue of being much simpler.
A Planetary Model of the Atom

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The atomic number of an element indicates the number
of protons present in the nucleus of its atom.
The atomic mass of an element is the sum of the
masses of its protons and neutrons present in the
nucleus.
4. Valency
The number of bonds which an atom of an element can
form is called the bond number or valence of that
element. Puali’s exclusion principle stated that no two
electrons in an atom have the same energy content.

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5. Free Electrons
As soon as an electron left an atom and
moves to the adjacent atom it is referred to as
a free electron.

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6. Units of Energy
The electron volt is the amount of energy
required to move one electron through a
potential difference of one volt.
1eV = 1,602 x 10-19 Joule

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7. Energy levels
When electrons move from a lower to a higher
level it gains energy and if it moves from a
higher to a lower shell it loose energy.
8. Fermi energy level
To explain the behaviour of materials it is
necessary to know the numbers of electrons in
both the conduction and the valence band of
the sample. Only electrons in the conduction
band are free to move and create current flow
if an external voltage is applied.

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Conduction is influenced by temperature and
the fermi level is the lowest temperature where
conduction can takes place, namely at 0 Kelvin
or -273°C.
9. Conductors, semi-conductors and
insulators
Conductors has no forbidden gap
Semi-conductors has a small forbidden gap
when forward biased
Insulators has a wide forbidden gap, and no
current flow is possible