1. Electrical conductivity of semiconductors less than
that of metals
have unique useful electrical characteristics
sensitive to the presence of even minute
concentrations of impurities
Two types : 1) Intrinsic – pure materials
2) Extrinsic –impurities added
2. band structure
relatively narrow forbidden band
gap (<2 eV)
Eg : Si (1.1eV) and Ge(0.7eV)
Also compounds composed of
Groups IIIA and VA – GaAs, In Sb
Groups IIB and VIA- Cd S, Zn Te
3. In above compounds ,
->dissimiar electronegativities
-> atomic bonding becomes more ionic
-> band gap energy increases—more
insulative
4. a vacant electron state in the VB created because of
e- from VB excited to CB
Treating a missing electron from the valence band
as a positively charged particle called
a hole.
Charge of hole= +1.6x10^-19 C
In the presence of an electric field, excited
electrons and holes move in opposite directions
5.
6. Two types of charge
carrier (free electrons
and holes)
p is the number of
holes per cubic meter
µh is the hole mobility
ni is known as the
intrinsic carrier
concentration
7. impurity concentration determines the
electrical behavior of semiconductors
One atom in 10^12 is sufficient to render
silicon extrinsic at room temperature.
Types : 1) n-type
2) p-type
8. Si has 4 e-s : covalent bonded with other
adjacent atoms
If we add a Grp VA impurity , 4e-s form
bonds , there will be 1 e- free .
It is free e- / conducting e-
For these free e-s , energy state is in
forbidden band gap just below CB
If energy is given,e- get excited to CB
Hence this type impurity is termed a ‘donor’
No hole in VB
9. At room temp, we can easily excite
large no. of e-s from donor states
No.of e-s in CB >>no.of holes in
VB
This type material is n-type
extrinsic semiconductors
e- are majority carriers
Holes are minority charge carriers
n-type : Fermi level shifted
upward in band gap