Semiconductors have electrical conductivity between metals and insulators. They have unique electrical properties and their behavior can be modified by adding small amounts of impurities. There are two types of semiconductors - intrinsic, which are pure materials, and extrinsic, which have impurities added. Extrinsic semiconductors can be n-type or p-type depending on the impurity, with n-type having free electrons as majority carriers and p-type having electron holes as majority carriers.

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

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