INVESTIGATORY PROJECT ON SEMICONDUCTOR CLASS-XII

1. GUIDED BY :- SUBMISSIONTED BY
SHIVAM JHADE
CLASS-XIIA SEC-A
ROLL NO- 24
BOARD ROLL NO-
YEAR-2020-21
PHYSICS INVESTIGATORY PROJECT
ON
SEMICONDUCTORS
NRI GLOBAL DISCOVERY SCHOOL BHOPAL
TEACHERPRINCIPAL

2. ACKNOLEGMENT
I would like to express my special
thanks of gratitude to my teacher
(Name of the teacher) as well as
our principal (Name of the
principal)who gave me the golden
opportunity to do this wonderful
project on the topic
(SEMICONDUCTOR), which also
helped me in doing a lot of
Research and i came to know
about so many new things I am
really thankful to them.
Secondly i would also like to thank
my parents and friends who
helped me a lot in finalizing this
project within the limited time
frame.
DATE:-
SHIVAM JHADE
XII A

3. INDEX
1. INTRODUCTION
2.Theory a n d Definition
3 . E f f e c t o f t e m p e r a t u r e on
conductivity o f Semiconductor
4. INTRINSIC SEMICONDUCTORS
5 . e x t r i n s i c semiconductors
· n-type semiconductor
7. p-type semiconductor
8 . e l e c t r i c a l r e s i s t i v i t y o f
semiconductors

4. INTRODUCTION
Semiconductors :-
Most of the solids can be placed in one of the two classes:
Metals and insulators. Metals are those through which
electric charge can easily flow, while insulators are those
through which electric charge is difficult to flow. This
distinction between the metals and the insulators can be
explained on the basis of the number of free electrons in
them. Metals have a large number of free electrons
which act as charge carriers, while insulators have
practically no free electrons.
There are however, certain solids whose electrical
conductivity is intermediate between metals and
insulators. They are called ‘Semiconductors’. Carbon,
silicon and germanium are examples of semi-
conductors. In semiconductors the outer most electrons
are neither so rigidly bound with the atom as in an
insulator, nor so loosely bound as in metal. At absolute
zero a semiconductor becomes an ideal insulator.

5. Theory and
Definition
 Semiconductors are the materials whose
electrical conductivity lies in between metals
and insulator.
The energy band structure of the semiconductors
is similar to the insulators but in their case, the size of
the
 forbidden energy gap is much smaller than that of the
insulator. In this class of crystals, the forbidden gap is of
 the order of about 1ev, and the two energy bands are
distinctly separate with no overlapping. At absolute
o0,
 no electron has any energy even to jump the forbidden gap
and reach the conduction band. Therefore
the
 substance is an insulator.
But when we heat the crystal and thus provide some energy to
the atoms and their electrons, it becomes an easy matter for
some electrons to jump the small (» 1 ev) energy gap and go to
conduction band. Thus at higher temperatures, the crystal
becomes a conductors. This is the specific property of the crystal
which is known as a semiconductor.

6. E f f e c t o f t e m p e r a t u r e on
conductivity o f Semiconductor
At 0K, all semiconductors are insulators. The valence
band at absolute zero is completely filled and there are
no free electrons in conduction band. At room
temperature the electrons jump to the conduction band
due to the thermal energy. When the temperature
increases, a large number of electrons cross over the
forbidden gap and jump from valence to conduction band.
Hence conductivity of semiconductor increases with
temperature.
INTRINSIC SEMICONDUCTORS
Pure semiconductors are called intrinsic semi-conductors. In
a pure semiconductor, each atom behaves as if thereare

7. 8 electrons in its valence shell and therefore the entire
material behaves as an insulator at low temperatures.
A semiconductor atom needs energy of the order of 1.1ev
to shake off the valence electron. This energy becomes
available to it even at room temperature.
Due to thermal agitation of crystal structure, electrons
from a few covalent bonds come out. The bond from
which electron is freed, a vacancy is created there. The
vacancy in the covalent bond is called a hole.
This hole can be filled by some other electron in a
covalent bond. As an electron from covalent bond moves
to fill the hole, the hole is created in the covalent bond
from which the electron has moved. Since the direction
of movement of the hole is opposite to that of the negative
electron, a hole behaves as a positive
carrier. Thus, at room temperature,
charge
a pure
semiconductor will have electrons and holes wandering
in random directions. These electrons and holes are
called intrinsic carriers.
As the crystal is neutral, the number of free electrons will
be equal to the number of holes. In an intrinsic
semiconductor, if ne denotes the electron number density
in conduction band, nh the hole number density in
valence band and ni the number density or concentration
of charge carriers, then
ne = nh =ni

8. extrinsic semiconductors
 As the conductivity of intrinsic semi-conductors is poor, so
intrinsic semi-conductors are of little practical importance. The
conductivity of pure semi-conductor can, however be
enormously increased by addition of some pentavalent or a
trivalent impurity in a very small amount (about 1 to 106 parts of
the semi-conductor). The process of adding an impurity to a
pure semiconductor so as to improve its conductivity is called
doping. Such semi-conductors are called extrinsic semi-
conductors. Extrinsic semiconductors are of two types :
i)
ii)
n-type semiconductor
p-type semiconductor

9. n-type
semiconductor
When an impurity atom belonging to group V of the
periodic table like Arsenic is added to the pure semi-
conductor, then four of the five impurity electrons form
covalent bonds by sharing one electron with each of the
four nearest silicon atoms, and fifth electron from each
impurity atom is almost free to conduct electricity.
As the pentavalent impurity increases the number of
free electrons, it is called donor impurity. The electrons
so set free in the silicon crystal are called extrinsic
carriers and the n-type Si-crystal is called n-type
extrinsic semiconductor. Therefore n-type Si-crystal will
have a large number of free electrons (majority carriers)
and have a small number of holes (minority carriers).
In terms of valence and conduction band one can think
that all such electrons create a donor energy level just
below the conduction band as shown in figure.
As the energy gap between donor energy level and the
conduction band is very small, the electrons can easily

10. raise themselves to conduction band even at room
temperature. Hence, the conductivity of n-type extrinsic
semiconductor is markedly increased.
In a doped or extrinsic semiconductor, the number
density of the conduction band (ne) and the number
density of holes in the valence band (nh) differ from that
in a pure semiconductor. If ni is the number density of
electrons is conduction band, then it is proved that
ne nh =ni2
p-type semiconductor
If a trivalent impurity like indium is added in pure semi-
conductor, the impurity atom can provide only three
valence electrons for covalent bond formation. Thus a
gap is left in one of the covalent bonds. The gap acts as
a hole that tends to accept electrons. As the trivalent
impurity atoms accept electrons from the silicon crystal,it

11. is called acceptor impurity. The holes so created are
extrinsic carriers and the p-type Si-crystal so obtained is
called p-type extrinsic semiconductor. Again, as the pure
Si-crystal also possesses a few electrons and holes,
therefore, the p-type si-crystal will have a large number
of holes (majority carriers) and a small number of
electrons (minority carriers).
It terms of valence and conduction band one can think
that all such holes create an accepter energy level just
above the top of the valance band as shown in figure.
The electrons from valence band can raise themselves
to the accepter energy level by absorbing thermal
energy at room temperature and in turn create holes in
the valence band.
Number density of valence band holes (nh) in p-type
semiconductor is approximately equal to that of the
acceptor atoms (Na) and is very large as compared to the
number density of conduction band electrons (ne). Thus,
nh» Na > > ne
e l e c t r i c a l r e s i s t i v i t y o f
semiconductors
Consider a block of semiconductor of length l1 area of
cross-section A and having number density of electrons
and holes as ne and nh respectively. Suppose that on
applying a potential difference, say V, a current I flows
through it as shown in figure. The electron current (Ic)
and the hole current (Ih) constitute the current I flowing
through the semi conductor i.e.
I=Ie +Ih (i)

12. It ne is the number density of conduction
band electrons in the semiconductor and ve, the drift
velocity of electrons then
Ie = eneAve
Similarly, the hole current, Ih = enhAvh
From (i) I = eneAve + enhAvh
I=eA(neve +nhvh) (ii)
If r is the resistivity of the material of the
semiconductor, then the resistance offered by the
semiconductor to the flow of current is given by :
R=  l/A (iii)
Since V = RI, from equation (ii) and (iii) we have
V = RI =  l/A eA (neve + nh vh)
V=  le(neve +nhvh) (iv)
If E is the electric field set up across the semiconductor,
then:
E=V/l (v)
from equation (iv) and (v), we have
E = e (neve + nhvh)
1/ = e (ne ve/E + nh vh/E)
On applying electric field, the drift velocity
acquired by the electrons (or holes) per unit strength of
electric field is called mobility of electrons (or holes).
Therefore,
mobility of electrons and holes is given by :
e = ve/E and h = vh/E
1/ =e(ne e +nh h) (vi)
Also,  = 1/ is called conductivity of the material
of semiconductor
 =e(ne e +nh h) (vii)
The relation (vi) and (vii) show that the conductivity and
resistivity of a semiconductor depend upon the electron

13. and hole number densities and their mobilities. As
ne and nh increases with rise in temperature, therefore,
conductivity of semiconductor increases with rise in
temperature and resistivity decreases with rise in
temperature.
.
.

14. bibl o g r a p h y
1.www.google.com
2.www.wikipedia.com
3.www.ncert.nic.in
4.Pradeep class 12 Physics
5.HC verma class 12 Physics

15. TheEnd