Elec.330 First Day Presentation(ppt)

woundcallousSemiconductor

Nov 1, 2013 (3 years and 9 months ago)

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29P

Electron


Isolated copper Atom

Conductor

Valence orbit

has only one

Electron and is

loosely bound

to core

Core

Isolated silicon atom

Electron

Semiconductor

Valence orbit

has four electrons

14P

r
1

r
2

r
3

Center of core

r
1

r
2

r
3

Energy

Energy levels in a

single atom

Electrons 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 electron

shared by two

neighboring atoms

to form a covalent

bond.

This way an atom

can have a stable

structure with

eight valence band

electrons.

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

1
st

band

2
nd

band

Valence band

Conduction band

Higher band higher energy

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 holes

In valence band

13P

14P

14P

14P

14P

P
-
type semiconductor

Aluminum atom


Hole

Has few free electrons in conduction band and many holes

In 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 type

P 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 N

And
electrons from N to P

causes

a formation of
depletion layer


This gives rise to
barrier potential(E
γ
)


preventing further migration

of

holes and electrons

Anode

Cathode

-

-

-

-

+

+

+

+

Energy

P

N

Depletion layer



Energy bands in a unbiased diode

Conduction band

Valence band

P type

N type

+

+

+

+

-

-

-

-

+

-

Forward Biased diode

R

V
B

Anode

Cathode

+

-

V
γ

Energy

P

N

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




V
B

has to overcome
V
γ

in order for the diode to


conduct



P type

N type

+

-

+

+

+

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

-

-

-

Reverse biased diode

Larger depletion layer


Anode

Cathode

V
B

Energy

P

N

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 V
B


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