2. Why is the conductivity of insulators
negligible, compared to semiconductor ?
two factors ,
• valence electrons are tightly • Mostly covalent bonding
bound to (or shared with) the somewhat weaker bonding
individual atoms – strongest ionic • Electrons can reach the
(partially covalent) bonding. conduction band at ordinary
• The energy gap is too large when
• An electron promoted into the
compared to semiconductor. conduction band leaves a Hole
(positive charge) in the valence
band, that can also participate in
• The conductivity increases with
5. • The highest filled state at 0 K
Fermi Energy (EF) Band structure
• The two highest energy bands The energy difference
are: between the bottom of the
• Valence band – the highest band Conduction and the top of
where the electrons are present at 0 the Valence bands is called
K the Band Gap
• Conduction band - a partially filled
or empty energy band where the
electrons can increase their energies
by going to higher energy levels
within the band when an electric
field is applied
7. When enough energy is
supplied to the e- sitting at the
top of the valance band, e- can conduction
make a transition to the bottom band
of the conduction band.
When electron makes such a
transition it leaves behind a
missing electron state.
This missing electron state is
called as a hole. Hole behaves Energy e- e- e- e-
+ + + +
as a positive charge carrier. Full
Magnitude of its charge is the valance
same with that of the electron
but with an opposite sign.
8. Electron mobility
• Characterizes how quickly an electron can move
through a metal or semiconductor, when pulled by
an electric field, in semiconductors .
• When an electric field E is applied across a piece of
material, the electrons respond by moving with an
average velocity called the drift velocity V, Then the
electron mobility μ is defined as
|v| = μE
9. Vd E
E: applied field
: mobility of charge carrier
is a proportionality factor
So is a measure how easily charge carriers move under the influence of
an applied field or determines how mobile the charge carriers are.
10. How mobility depend on doping?
• Mobility is dependent on the drift velocity. The main
factor determining drift velocity (other than effective
mass) is scattering time. How long the carrier
is accelerated by the electric field until it scatters
(collides) with something that changes its direction
• The most important sources of scattering in typical
semiconductor materials, discussed below, are ionized
Doping is the incorporation of [substitution] impurities into a
semiconductor according to our requirements.
In other words, impurities are introduced in a controlled
Impurities change the conductivity of the material so that it
can be fabricated into a device
Doped crystals are extrinsic semiconductors. “adding minute
amounts of suitable impurities to the pure crystals”
Crystals are doped to be n type or p type
n type semiconductors have few minority carriers (holes).
p type semiconductors have few minority carriers (electrons).
12. • The purpose of semiconductor doping is to increase the
number of free charges that can be moved by an external
• So the crystal has no resistance to current flow and
behaves as a superconductor. The perfect periodic
potential does not impede the movement of the charge
• However, in a real device or specimen, the presence of
impurities, interstitials, subtitionals, temperature , etc.
creates a resistance to current flow.
13. probability of occupation
• The Fermi level or Fermi energy is the energy, at which the
probability of occupation by an electron (or hole) is exactly ½. In
semiconductor, usually, Fermi level is in the band gap.
• F(E )=1 ⁄ 1+exp[(E-EF) /KT ]
• K = Boltzmann constant
• E F =Fermi energy or Fermi level
• T =0k
• F(E)=The probability that an electron state having
energy E is occupied