Ecx 5239 1n
Transcript of Ecx 5239 1n
PHYSICAL ELECTRONICS
ECX 5239PRESENTATION – 01
G.V.I.S.SILVA
709062591
2012-12-15
Why is the conductivity of insulators
negligible, compared to semiconductor ?
It depends on mainly
two factors ,
Atomic bond
Energy band
structure
conductivity
conductivity
Insulator semiconductor
• valence electrons are tightly
bound to (or shared with) the
individual atoms – strongest ionic
(partially covalent) bonding.
• The energy gap is too large when
compared to semiconductor.
• Mostly covalent bonding somewhat weaker bonding
• Electrons can reach the conduction band at ordinary temperatures.
• An electron promoted into the conduction band leaves a Hole (positive charge) in the valence band, that can also participate in conduction.,
• The conductivity increases with increasing temperature.
ATOMIC BONDING
Insulator VS Semiconductor
Band structure
The energy difference
between the bottom of the
Conduction and the top of
the Valence bands is called
the Band Gap
• The highest filled state at 0 K
Fermi Energy (EF)
• The two highest energy bands
are:
• Valence band – the highest band
where the electrons are present at 0
K
• 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
Band model
Insulators:wide band gap (> 2 eV)
Semiconductors:narrow band gap (< 2 eV)
When enough energy is
supplied to the e- sitting at the
top of the valance band, e- can
make a transition to the bottom
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
as a positive charge carrier.
Magnitude of its charge is the
same with that of the electron
but with an opposite sign.
Full valanceband
Empty conductionband
+e- +e- +e- +e-Energy
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
:E:
applied field
mobility of charge carrier
SecV
cm2
is a proportionality factor
E
Vd
So is a measure how easily charge carriers move under the influence ofan applied field or determines how mobile the charge carriers are.
dV E
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 and/or energy.
• The most important sources of scattering in typical semiconductor materials, discussed below, are ionized impurity scattering.
Doping
Doping is the incorporation of [substitution] impurities into a
semiconductor according to our requirements.
In other words, impurities are introduced in a controlled
manner
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).
• The purpose of semiconductor doping is to increase thenumber of free charges that can be moved by an externalapplied voltage..
• So the crystal has no resistance to current flow andbehaves as a superconductor. The perfect periodicpotential does not impede the movement of the chargecarriers.
• However, in a real device or specimen, the presence ofimpurities, interstitials, subtitionals, temperature , etc.creates a resistance to current flow.
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 ]
Where
• 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
=1/1+exp [(0.4/0.026)]
= 2.08*10 -7
EA- EF ={EF-EV-(EA-E V)}= 0.15-0.04= -0.11eV
PROBABILITY OF ACCEPTER STATES
F(EA)=1 ⁄ 1+exp[(EA-EF) /KT ]
=1/1+exp [(-0.11/0.026)]
= 0.9856
Donors
Accepter
Thank you!