Download SEMICONDUCTOR PHYSICS In solid substances electricity is

SEMICONDUCTOR PHYSICS
In solid substances electricity is conducted with the help of charge
carriers which are the free electrons, that move and transfer electric charge from
one place to another (from lower potential to higher potential). At 0 K no
thermal energy is present so electrons remain in their valence band. As
temperature increases these electrons gain sufficient kinetic energy to cross the
energy gap between valence band and conduction band and move to the
conduction band. Once energy reaches a sufficient level these electrons move in
bulk and conduct charge.
Based on charge carrier concentration, solid substances can be classified
as metals, semiconductors or insulators. In metals electrons are the only charge
carriers and their concentration is very high because of the overlapping that
takes place between the valence and the conduction band. In semiconductors
however, this concentration depends on the energy gap between bands. In
insulators this gap is too high for electrons to jump into the conduction band and
so insulators do not conduct electricity.
At room temperature these electrons that are conducted move in random
motion. Their motion is like that of gas molecules since in conductors a large
number of electrons move randomly hence they do not have a net motion in any
particular direction. Thus no current is generated. As some potential difference
is applied across the two ends of a conductor, an electric field is set up inside the
conductor and electrons start moving from the lower potential to the higher
potential point. An external force accelerates the motion of electrons inside the
conductor.
Drift velocity is the small velocity imposed due to the random motion of
electrons on the application of an electric field. It is also defined as the velocity
with which the free electrons get drifted with at the positive potential on the
application of an electric field. Given by,
vd=(eE/m)τ , τ: Time instant between successive collisions.
Relation between drift velocity and electric current density (J)
ΔQ=neAvdΔt
Or,
I = ΔQ/Δt = neAvd
Current density J is the total flow of charge per time over cross section area A.
In a semiconductor the concentration of the charge carrier depends on
the energy gap between the two bands (i.e. valence band and conduction band).
When the electrons leave the valence band in a semiconductor, it leaves behind a
positively charged vacancy, referred to as ‘hole’. These holes are mobile in the
valence band. And these electrons pair with the corresponding hole, known as
electron hole pair. Hence semiconductors have two types of charge carrierselectrons in conduction band and holes in valence bands.
An intrinsic semiconductor is one, which is composed purely of only one
type of element and is free from any defects or impurities. The electrons and
holes are always created in pairs by thermal agitation; the concentration of both
of these is the same. Whereas in extrinsic semiconductors, it is possible to
generate electrons and holes by the process known as doping in which
impurities are inserted into the substance. These exhibit greater conductivity
than the intrinsic semiconductors. Extrinsic semiconductors can be further
classified as n-type and p-type.
N-type semiconductors - doped by infusing a small amount of donor element.
Donor elements are elements with 5 valence electrons. Hence n-type
semiconductors have an excess of electrons in them.
P-type semiconductors- doped by infusing a small amount of acceptorelements
with three valence electrons. Hence p-type semiconductors have an excess of
holes.
The process of infusing the impurities in the semiconductor in order to
increase the charge carrier concentration and hence increase conductivity is the
process of diffusion. This process plays an important role in semiconductor
physics.