Semiconductor
Material
Most modern electronic devices are made from semiconductor in combination
with conductors and insulators. Semiconductor materials commonly used in
electronics are: Si, Ge, GaAs, B, P, Ge, In, Sb. At absolute zero temperature
the electron are bounded and it is unable to contribute to the conduction of
current in semiconductor. At finite temperature the electron acquire enough
thermal energy to break out of their covalent bond and contribute to the
current in the semiconductor.
When an electron is thermally
excited out of its covalent bond it leave behind a single vacancy called a hole
at the site of the bond. Then another electron come and fills up that hole. The
net charge of the hole vacancy is positive, hence the hole can be think of as a
positive particle moving in the direction of the electric field.
Acceptors
However, only small current are
created if we only used thermal agitation because electron and holes exist in
equal number. A semiconductor with equal number of holes and electron is called
intrinsic. We can alter this number of hole and electron by using dopant. Extra
holes can be created by adding acceptor to the semiconductor. Acceptor atom
commonly used are: Boron, Indium, Aluminum, and element from group 2 of the
periodic table. An acceptor have one less electron in their outer valence shell
than do the atom of the host crystal. A nearby electron will jump and fill up
this hole, thus creating extra hole in the semiconductor. A doped semiconductor with abundant number of
extra holes is called a p-type semiconductor.
Donors
Extra free electron can be created
by using the dopant called donors. Donors atom has one more electron in their
outer valence shell than do the atoms of the host crystal. Therefore, when
added to a semiconductor the electron will easily break away and therefore
created the extra electron. Donor atom are typically phosphorous, arsenic, and
antimony. A doped semiconductor with an abundance of extra free electron is
called an n-type semiconductor.
PN Junction Diode
When one side of the semiconductor is a p-type and another side is doped
n-type, we created the PN junction. The electron from n-type will try to
diffuse to the p-type side, and the hole from the p-type side will diffuse to
the n-type side. However, when they start diffusing to the other sides,
acceptor ion core and donor ion core are left behind on each side. And this
created the net negative charge and net positive charge on each side.
Therefore, electric field is created. The field is called built-in field, and
it will counteract the diffusion. Therefore in the end the diffusion will stop.
And the region around the junction will become depletion region and it is void
of carriers.
When an external voltage is applied
from the p side to the n side the diode is in forward biased. The external
voltage created electric field that diminishes the built in electric field.
Therefore, the hole and electron are now able to diffuse to another side. This
is called carrier injection and current flows. However, if the voltage applied
is negative, then the built in voltage will be reinforce and further suppress
the diffusion. So no current will flow. Therefore, the current will flow only
is the diode is in forward biased region.
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