Download Semiconductor Theory

TOPIC: Semiconductor Theory.
Introduction
There are three basic types of materials that we are
concerned with in electronics. These are conductors,
semiconductors and insulators.
Materials that have very low electrical resistivity (in the
order of 1 x 10-6 ohm-metres) are called conductors.
Materials that have very high electrical resistivity (in the
order of 1 x 1013 ohm-metres) are called insulators.
Semiconductors are materials that have resistivity values in
between those of conductors and insulators, they are neither
good conductors nor good insulators.
Examples of
conductors:
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Copper
Aluminium
Silver
Gold
Examples of
insulators:
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Rubber
PVC
Paper
Mica
Examples of
Semiconductors:
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Silicon
Germanium
Semiconductor materials are used to make a range of
devices that are used in modern electronic circuits. In order
to understand how these devices work we must first gain an
understanding of the electrical properties of naturally
occurring (intrinsic) semiconductors. We then need to
learn about the electrical properties of extrinsic
semiconductors. Extrinsic semiconductor material is just a
naturally occurring pure semiconductor material that has
been modified by a manufacturing process.
Intrinsic semiconductor materials
The naturally occurring semiconductor materials that are
used to manufacture electronic devices are Silicon and
Germanium (Germanium is an older choice of material
which is less used today).
Pure Silicon
First a very pure crystal of silicon must be produced. The
atomic structure of the silicon can be represented by the
diagram below. Silicon like all semiconductors is a group 4
element and its atoms have only four electrons in the outer
shell ( 4 valence electrons). It takes eight electrons to fill
the outer shell and make it stable. The atoms share their
valence electrons with neighbouring atoms so that each
atom effectively contains eight electrons in the outer shell.
This sharing of valence electrons with neighbouring atoms
forms covalent bonds. It is these covalent bonds that bind
the atoms together.
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The silicon atoms
form a square lattice
Each silicon nucleus
has four electrons in
its outer shell
These electrons are
paired with the
corresponding
electrons in adjacent
atoms.
These are called
covalent bonds.
Covalent bonds are
what binds the
material together
The net result is that
each nuclei (along
with the electrons in
the inner shells) are
surrounded by eight
outer electrons tightly
bound in the atomic
structure.
Note this is a simplified diagram showing a 2 dimensional
representation of the structure of silicon. Obviously silicon
has a 3 dimensional structure and the covalent bonds do
not really lie in a single plane as shown in the diagram.
The actual arrangement of covalent bonds forms a shape
called a tetrahedron.
This diagram does give a good representation of how the
electrons are bound to the atoms. This reflects the fact that
there are no free electrons to produce an electrical current if
a voltage is applied to the material.
The Doping of Semiconductors
The addition of a small percentage of foreign atoms in the
regular crystal lattice of silicon or germanium produces
dramatic changes in their electrical properties, producing ntype and p-type semiconductors.
Pentavalent impurities
Impurity atom with 5 valence electrons produce n-type
semiconductors by contributing extra electrons.
Trivalent impurities
Impurity atoms with 3 valence electrons produce p-type
semiconductors by producing a "hole" or electron
deficiency.
Semiconductor Current
Both electrons and holes contribute to current flow in an
semiconductor.
CURRENT FLOW in an N-TYPE MATERIAL is
similar to conduction in a copper wire. That is, with voltage
applied across the material, electrons will move through the
crystal toward the positive terminal just like current flows
in a copper wire.
CURRENT FLOW in a P-TYPE MATERIAL is by
positive holes, instead of negative electrons. Unlike the
electron, the hole moves from the positive terminal of
the P material to the negative terminal.