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!