Semiconductor
Classification of Metals , Semiconductor & Insulator :-
On the basis of relative values of electrical conductivity (d) or resistivity (1) ,
the solid
are broadly classified as : -
i) Metal : - They posses very low resistivity (or high conductivity) for Example ,
Al , Cu , Ag
.
etc Silver is the best metal conductor .
. 10-10-12m
9 and 0 : 102-108sm-
ii) Semiconductor: -
They have resistivity or
conductivity intermediate to metals and
insulators .
. 10-5-10°
9 m and G: 105-10-Sm-1
iii) Insulator : -
They have high resistivity or low conductivity.
Ex : Plastic putty and rubber etc
,
.
,
10-10-19 Sm+
9 10"-10191m
. and 0 :
Energy Bands in Solid :-
According to Bohr's atomic model and concept of electronic configuration in an
atom , the electrons have definite discrete amount of corresponding to diff-
energy
- event shells and subshells it well defined energy levels of electrons in an atom .
But in a
crystal
due to interatomic interaction ,
valence electrons shared by more
are
than atom Due to this , splitting of level takes place . The collection
one .
energy
of closely spaced energy levels is called
these band. These bands
Energy are formed
due to continuous energy variation in different energy levels .
These different energy levels for different electrons are formed because inside the
each electron has two electrons
crystal ,
a unique position and no are exactly at the same
pattern of surrounding charges .
Valence Band : -
·
The Energy band which includes the energy levels
of the valence elections is called Valence band .
This band be
partially completely filled
may or
·
with electrons but is never .
empty
·
It does not contribute to electric current
.
Conduction Band : -
·
The energy band above the valence band is called conduction band .
·
At room temperature , this band is either
empty or partially filled with electrons.
·
This band contribute to the electric current .
Energy Band Gap : -
The minimum energy required for shifting electrons from valence band to conduction
band is called
energy band gap (Eg).
It is the between the top .
of the valence band and bottom of Conduction band
gap
·
It can be zero , small or
large depending upon the material .
Eg =
nv =
T
hC
temperature thermal energy of
By increasing the , the
valence electron increases due to which it breaks
free away from the covalent bond and becomes free
to conduct electricity This electron leaves behind a
.
vacancy of a electron with an effective positive
charge called Hole
.
In intrinsic semiconductors: -
ni = ne =
nn
Position of a hole can also be changed . When an electron jumps from site 2 to the
hole in site 1, a hole is created at site 2 and sites
gets occupied.
Once free ,
the electron does not take part in motion of the hole
.
The free electron give rise to electron current Ic ,
under the
applied Electric force .
Under the
applied field the holes
,
move in the direction of field
due tojumping of elections acts Dive
.
They as
charge car as
to hole current In
Hence ,
the total current is : -
I = [c + In
Extrinsic Semiconductors :-
The conductivity of intrinsic semiconductors is very low at room temperature
.
But , it can be increased if some pentavalent or trivalent impurity is mixed
,
with it . Those semiconductors in which some atoms are embedded
impurity
are known as Extrinsic or
impurity semiconductors
.
They process of addition of a desirable impurity to a
pure semiconductor to
its conductivity is called
increase Doping
.
The
impurity atoms added are called .
Dopant
They are of 2
types :
-
i) Pentavalent (As Sb, P) ,
ii) Trivalent (In B All , ,
Types of Extrinsic Semiconductor are of 2
types :
-
i) N-type Semiconductor
ii) P-type Semiconductor .
