Semiconductors
Energy Band Theory
Bohr's atomic theory
Types of bands
Valence Band
Covalent Band
band/forbidden gap
the band above valence band
Band containing valence electrons
Gap between valence and conduction band
Eg=Ec-Ev
this is also the minimum energy required for conduction
Types of solids based on this theory
Conductors = overlap of Ec and Ev and Eg ~ 0
Insulators = Eg~3eV
Semiconductors= Eg~1eV
energy levels, e of unknown energy level is free electron
Types of Semiconductors
intrinsic
Extrinsic
pure elemental Si and Ge act as intrinsic semiconductors and electric field is passed through it
holes are positive charge carriers which have virtual charge that occupy the position left by electrons, similarly its direction is opposite to electron movement and indirection of field/current
on applying field electron moves in opposing direction with a certain drift velocity this leads to movement of electrons and holes
When an intrinsic semiconductor is doped it is known as extrinsic semiconductor
It is of 2 types P and N
P type is of group 13 and it has deficiency of 1 e leading to formation of 1 extra holes, here no. of holes is greater than no. of electrons. and eff. current is approx = current of holes. this is tetravalent
N type is group 15 and has an excess of 1 electron leading to negative charge. here no. of e > no. of holes and net current is approx. current of electron. this is pentavalent
nelectron=nhole
similary, I=Ih+Ie
ALL ARE ENTIRELY NEUTRAL!
No. of electrons x No. of holes = ni^2 (here ni is intrinsic carrier density)
P-N junction Diode