Q.5. Describe
extrinsic semiconductor with energy diagram.
Ans. Extrinsic Semiconductors:- The conduction in intrinsic semiconductor is due to
inherent crystal properties. Contrary to this, conduction in extrinsic
semiconductors occurs due to the presence of foreign impurities. Deliberate
impuring of intrinsic semiconductors is done by adding impurities to obtain
extrinsic semiconductor. The impuring agents are called dopants, and the
process is known as doping. The dopants are elements either of the third column
or the fifth column of the periodic table. They increase the conductivity of
semiconductors considerably. Based on the type of dopants used, extrinsic
semiconductors are classified as
(i) n - type and (ii) p - type.
The n - type semiconductor is also called negative or
electron carrier type, and p - type as positive or hole carrier type.
n - type semiconductors
and their energy diagram:-
In this
case, the fifth column elements such as As, Sb or P are used as dopants. Figure
a shows a germanium crystal doped with phosphorus. Four of the five electrons
in the outermost orbit of phosphorus atom forms a tetrahedral bond with four
germanium neighbours. The fifth electron remains loosely bound
Fig. - n - type
semiconductor (a) germanium doped with fifth column phosphorus, and (b) energy
diagram showing donar energy level.
to its parent atom, and moves
in the electric field of germanium crystal with an electron orbit of large
radius. The energy of fifth electron is close to the conduction band, and is
shown in figure. This energy level is known as donor energy level ED as the dopant
has donated one of its electron to the semiconductor.
Law of mass
action:-
Now the
cross-over of fifth electron into the conduction band is much easier than it
would have been from the valence band. The energy needed to excite the fifth
electron into the conduction band is known as ionization energy EI. The number of donar level electrons excited into the
conduction band at room temperature is large, as EI << Eg. The number of electrons in the conduction band are
more than the number of holes nh in the valence band (i.e. ne > nh). The electrons remain the majority charge carriers.
However the law of mass action (ne x ne = constant)
holds good.
p-Type
semiconductors and their energy diagram:-
An
intrinsic crystal can also be doped by elements of third column such as Ga, In
or Al. Doping of a silicon crystal by an aluminium atom is illustrated in figure
(a). Aluminium requires a fourth electron in addition to three electrons in its
outer orbit to form a tetrahedral bond. The fourth electron comes from the
neighbouring silicon atom. Migration of electron from silicon atom creates a
hole in it. The hole thus created (in silicon) moves around aluminium atom with
a positive charge.
Acceptor energy level. Calculations reveal that the holes orbit at a radius
of about , and are close to the valence band. The energy level possessed by
them is known as acceptor energy level EA, and is shown in figure (b). The holes are majority
charge carriars in this case (nh > ne), and the law of mass action holds goods here also
- The conductivity of extrinsic semiconductors, both n - type and p - type, may be found from eq. (s = neeeme + nhehmh) knowing that
Fig - p-type semiconductor
(a) silicon doped by third column aluminium, and (b) energy diagram showing
acceptor energy level.
