The semiconductors are just average conductors, a
higher resistance than metal conductors, but a lower resistance than
insulators. The two most commonly used semiconductor elements are Silicon and
Germanium. Their +4 valence electrons mean that they have a very stable
covalent bond structure.
Intrinsic semiconductors are pure semiconductors that contain no
impurities. When temperature increases, the conduction property of the
intrinsic type also increases. This is because, at high temperatures, electrons
are excited to higher energy levels and create holes. These holes are
positively charged and flow in the direction opposite to that of electrons thus
causing electricity. In an intrinsic semiconductor, the number of holes and
electrons are equal. Other causal agent of electricity in this type is crystal
defects.
Extrinsic semiconductors when impurities are added to intrinsic
semiconductors, extrinsic semiconductors are formed, meaning they are
not in their natural form. The process of adding impurities to the
semiconductor is called doping.
Doping a pure semiconductor with a small
amount of material with a valence electron of
+5 (which inclues Phosphorus, Arsenic, and Anitmony) creates an n-type
semiconductor. It is referred to this because of the excess of free electrons
in the material.
Similiarly, doping a pure semiconductor
with a small amount of material with valence electron of +3 (Boron, Aluminum,
Gallium, and Indium) creates a p-type semiconductor. This results because of a
hole that is left by the absence of an electron in the covalent bond structure.
Note that doping a semiconductor does not add or
remove any charge. The resulting product is still electrically neutral. Doping
simply redistributes valence electrons so more or less free charges are
available for conduction.
The PN Junction is formed by joining n-type and p-type
semiconductor. The extra electrons in the n-type semiconductor attempt to move
over into available holes in the p-type semiconductor. At the same time, some
of the holes in the p-type seiconductor end up moving over to the n-type to
meet up with electrons.When this happens, we end up with an excess of electrons
on the p-type side and extra electrons on the n-type side, creating an
electrical imbalance. This electrical imbalance is known as the barrier
potential.
Forward Bias
When we apply a positive voltage to the p-type semiconductor
and a negative voltage to the n-type semiconductor, we are applying a forward
bias to the semiconductor. First, the negative voltage at the n-type semiconductor
is going to attempt to push electrons towards the junction in the middle. The
positive voltage at the p-type semiconductor will push the holes towards the
barrier as well. This reduces the barrier potential. If the barrier potential
is reduced enough, the charge carriers can move through the barrier and out the
other side. This means that current flows.
Reverse Bias
Applying a reverse voltage to our semiconductor
material is known as reverse bias. In this condition, the electrons are pulled
away from the barrier on the n-type side and the holes are pulled away from the
barrier on the p-type side. This results in a larger barrier, which creates a
much greater resistance for charges to flow through. The net result is that no
current flows through the barrier.