Basics of Semiconductors
Transistors, diodes, integrated circuits and many more devices all have
semiconductor technology in common. Before looking at how some of these
devices work it is necessary to have a basic understanding of what
conductors, insulators and naturally what semiconductors are.
Conductors and Non Conductors
In terms of electricity there are two main classes of material: namely:
conductors and non-conductors (or insulators). From their names it can be
gathered that conductors will conduct electricity freely, whereas non-conductors
act as insulators preventing the flow of an electric current.
An electric current is made up of the flow of electrons. This means that for
a current to flow, the electrons must be able to move freely within the
material. In some materials electrons are moving freely around the lattice, not
particularly attached to a given electron. At any instance electrons are moving
freely but randomly. By placing a potential difference across the conductor the
electrons can be made to drift in one direction and this constitutes an electric
current. Metals are all conductors of electricity, and a number of other
substances also conduct it to varying degrees.
Other substances do not have electrons moving freely around the lattice.
Electrons are firmly held within their molecules and cannot escape easily.
Accordingly when a potential is placed across the substance very few electrons
will move and very little or no current will flow. These substances are called
non-conductors or insulators. They include most plastics, ceramics and many
naturally occurring substances like wood.
Semiconductors
As the name suggests a semiconductor is neither a true conductor nor an
insulator, but half way between. A number of materials exhibit this property,
and they include germanium, silicon, gallium arsenide, and a variety of other
substances.
To understand how it acts as a semiconductor it is necessary to first look at
the atomic structure of pure silicon, a good insulator. It consists of a nucleus
with three rings or orbits containing electrons, each of which has a negative
charge. The nucleus consists of neutrons that are neutral and have no charge,
and protons that have a positive charge. In the atom there are the same number
of protons and electrons so the whole atom has no overall charge.
The electrons are arranged in rings with strict numbers of electrons. The
first ring can only contain two, and the second has eight. The third and outer
ring has four. The electrons in the outer shell are shared with those from
adjacent atoms to make up a crystal lattice. When this happens there are no free
electrons in the lattice, making silicon a good insulator. A similar picture can
be seen for germanium. It has two electrons in the inner most orbit, eight in
the next, 18 in the third, and four in the outer one. Again it shares its
electrons with those from adjacent atoms to make a crystal lattice without any
free electrons.
Impurities
The properties of semiconductors change very significantly by adding very small
amounts of impurity. If traces of impurities of materials having five electrons
in the outer ring of their atoms are added they enter the crystal lattice
sharing electrons with the silicon. However as they have one extra electron in
the outer ring, one electron becomes free to move around the lattice. This
enables a current to flow if a potential is applied across the material. As this
type of material has a surplus of electrons in the lattice it is known as an
N-type semiconductor. Typical impurities that are often used to create N-type
semiconductors are phosphorous and arsenic.
It is also possible to place elements with only three electrons in their
outer shell into the crystal lattice. When this happens the silicon wants to
share its four electrons with another atom with four atoms. However as the
impurity only has three, there is a space or a hole for another electron. As
this type of material has electrons missing it is known as P-type material.
Typical impurities used for P-type material are boron, and aluminium.
Holes
It is easy to see how electrons can move around the lattice and carry a current.
However it is not quite so obvious for holes. This happens when an electron from
a complete orbit moves to fill a hole, leaving a hole where it came from.
Another electron from another orbit can then move in to fill the new hole and so
forth. The movement of the holes in one direction corresponds to a movement of
electrons in the other, hence an electric current.
From this it can be seen that either electrons or holes can carry charge or
an electric current. As a result, they are known as charge carriers, holes being
the charge carriers for a P-type semiconductor and electrons for an N-type
semiconductor.
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