29P

Electron

Isolated copper Atom

Conductor

Valence orbithas only oneElectron and isloosely bound to core

Core

Isolated silicon atom

Electron

Semiconductor

Valence orbithas four electrons

14P

r1r2

r3

Center of core

r1

r2

r3

Energy

Energy levels in a single atomElectrons in the same orbit has same energy

A silicon crystal is formed by zillions of silicon atoms

14P

14P

14P 14P

14P

Covalent Bond

Electron

Silicon crystal

An electronshared by twoneighboring atomsto form a covalentbond.This way an atomcan have a stablestructure witheight valence bandelectrons.

14P

Energy bands

Electron(in conduction band)Hole (in valence band)

In a crystal, electrons in the same orbit do not have the same energy and thus form energy bands

1st band

2nd band

Valence band

Conduction band

Hig

her

band

hig

her

ener

gy

14P

14P

14P 14P

14P

Electron

Hole

Thermal energy produces free electron and hole pair

(in valence band)

(in conduction band)

14P

14P

14P 14P

14P

Electron

Hole

Recombination of free electron and hole

(in valence band)

(in conduction band)

A

B

C D

E

F

Free Electron (in conduction band)++++++

+

+

+

+

------

-

-

-

-

Hole/electron flow through a semiconductor

The hole moves A-B-C-D-E-F (pseudo movement)

The electron moves F-E-D-C-B-A

Hole

14P 14P 14P

14P 14P 14P

(in valence band)

Intrinsic and extrinsic semiconductor

Intrinsic = pure

Extrinsic = impure or doped

Doping• Doping means mixing a pure semiconductor with impurities to increase its electrical conductivity

Can be done in two ways:

• Increasing the number of electrons by mixing pentavalent elements such as phosphorous, arsenic, antimony (means adding donor impurities)

• Increasing the number of holes by mixing trivalent elements such as aluminum, boron, gallium (means adding acceptor impurities)

15P

14P

14P 14P

14P

Free Electron

N-type semiconductor

Phosphorous atom

Has many free electrons in conduction band and few holesIn valence band

13P

14P

14P 14P

14P

P-type semiconductor

Aluminum atom Hole

Has few free electrons in conduction band and many holesIn valence band

Majority and minority carriers

Electrons are• Majority carriers in N-type semiconductor• Minority carriers in P-type semiconductor

Holes are• Majority carriers in P-type semiconductor• Minority carriers in N-type semiconductor

A diode is formed by putting a N-type and P-type of semiconductor together

N typeP type

Note: Both N and P-type of materials are electrically neutral

Anode Cathode

P-N Junction

P type N type++++

----

Migration of holes from P to NAnd electrons from N to P causes

a formation of depletion layer

This gives rise to barrier potential(Eγ) preventing further migration of

holes and electrons

Anode Cathode----

++++

Energy

PN

Depletion layer

Energy bands in a unbiased diode

Conduction band

Valence band

P type N type

++++

----

+ -

Forward Biased diode

R

VB

Anode Cathode

+ - Vγ

Energy

PN

Smaller depletion layer

Energy bands of a forward biased diode

Conduction band

Valence band

Forward Biased diode

•The diode behaves like a ‘ON’ switch in this mode

• Resistance R and diode’s body resistance limits the current through the diode

• VB has to overcome Vγ in order for the diode to conduct

P type N type

+-

++++

++++

++++

----

----

----

Reverse biased diodeLarger depletion layer

Anode Cathode

VB

Energy

PN

Larger Depletion layer

Valence band

Conduction band

Energy bands in a reverse biased diode

Reverse Biased diode

•The diode behaves like a ‘OFF’ switch in this mode

• If we continue to increase reverse voltage VB

breakdown voltage of the diode is reached

• Once breakdown voltage is reached diode conducts heavily causing its destruction

Breakdown • Diode breakdown is caused by thermally generated electrons in the depletion region

•When the reverse voltage across diode reaches breakdown voltage these electrons will get sufficient energy to collide and dislodge other electrons

• The number of high energy electrons increases in geometric progression leading to an avalanche effect causing heavy current and ultimately destruction of diode