semiconductivty, types, intrinsic and extrinsic, concept of hole, n-type- material science

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