theory

1. 1
Karmanyevaadhikaaraste maa phaleshu
kadaachana;
Maa karmaphalahetur bhoor maa te
sango’stwakarmani.

2. 2

3. What these terms are:
 Valence band
 Conduction band
 Forbidden gap
3

4. 4
FORBIDDEN GAP
Energy
Valence Band
Conduction Band

5. Categories of
Solids
There are three categories of
solids, based on their conducting
properties:
conductors
semiconductors
insulators
5

6. Electrical Resistivity and Conductivity of
Selected Materials at 293 K 6

7. Reviewing the previous table
reveals that:
 The electrical conductivity at room temperature
is quite different for each of these three kinds of
solids
 Metals and alloys have the highest conductivities
 followed by semiconductors
 and then by insulators
7

8. Conductors 8
Valence Band
Conduction Band
Energy
In conductors there is no
forbidden gap.
The valence band and the
conduction band overlap.
The electrons from valence
band freely enter into the
conduction band due to
overlapping of bands.
Therefore very low potential
difference can cause
continuous flow of current.
No forbidden gap

9. Semiconductors 9
Valence Band
Conduction Band
FORBIDDEN GAP
Around 0.7eV (Ge) and
1.1 eV (Si)
Energy
In the case of semiconductors
the forbidden gap is very
small.
At 0K the conduction band is
empty and the valence band
is completely filled.
When a small amount of
energy is supplied, the
electrons can easily jump the
forbidden gap.
The conductivity of a
semiconductor is of the order
of 10 2mho m-1
Filled Band

10. Arrangement of electrons in an isolated
Silicon atom 10

11. Electron arrangement in a silicon atom
11

12. 12

13. 13

14. The thermal energy at room
temperature is easily enough to do
this, meaning that the electron is
essentially free to move from the
moment the phosphorus atom is
introduced into the silicon crystal.
14

15. Insulators
15
FORBIDDEN GAP
Energy
When the temperature is
increased, then, some
electrons will move to go to
conduction band. This is the
reason why certain
materials which are
insulators at room
temperature become
conductors at high
temperature.
The resistivity of an insulator
lies approximately between
10 11 and 10 16 Ωm
Valence
Band
Conduction Band

16. Resistivity vs. Temperature 16
Figure: (a) Resistivity versus temperature for a typical conductor. Notice the linear rise in
resistivity with increasing temperature at all but very low temperatures. (b) Resistivity
versus temperature for a typical conductor at very low temperatures. Notice that the
curve flattens and approaches a nonzero resistance as T → 0. (c) Resistivity versus
temperature for a typical semiconductor. The resistivity increases dramatically as T → 0.

17. 17Comparison of Insulator ,Semiconductor and
Conductor

18.  PRESENTED BY AHMEDABAD
GROUP