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Extrinsic Semiconductors

The Extrinsic Semiconductors are those in which impurities of large quantity are present. Usually, the impurities can be either 3rd group elements or 5th group elements.

In extrinsic semiconductor without breaking the covalent bonds we can increase the charge carriers.

Based on the impurities present in the Extrinsic Semiconductors, they are classified into two categories.

1. N-type semiconductors 2. P-type semiconductors 2

He

Ne

Ar

Kr

Xe

Rn

inert

gase

s acc

ept

1e

acc

ept

2e

giv

e u

p 1e

giv

e u

p 2e

giv

e u

p 3e

F Li Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

He

Ne

Ar

Kr

Xe

Rn

inert

gase

s acc

ept

1e

acc

ept

2e

giv

e u

p 1e

giv

e u

p 2e

giv

e u

p 3e

F Li Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

S Mg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

For Silicon: Donors (n type): P, As, Sb Acceptors (p type): B, Al,

Ga, In

3

4

When any pentavalent element such as Phosphorous, Arsenic or Antimony is added to the intrinsic Semiconductor , four electrons are involved in covalent bonding with four neighboring pure Semiconductor atoms.

Fifth electron is superfluous, becomes free electron and enters into conduction band.

The fifth electron is weakly bound to the parent atom. And even for lesser thermal energy it is released Leaving the parent atom positively ionized.

N - type Semiconductors

5

When we add a pentavalent impurity to pure semiconductor we get n-type semiconductor.

6

As Puresi

N-typeSi

Fig 1.

7

E Ed

Ev

Valence band

Ec

Conduction band

Ec

Electronenergy

Distance

Donor levelsEg

8

9

P-type semiconductors

When a trivalent elements such as Al, Ga or Indium have three electrons in their outer most orbits , added to the intrinsic semiconductor all the three electrons of Indium are engaged in covalent bonding with the three neighboring Si atoms.

Indium needs one more electron to complete its bond. this electron maybe supplied by Silicon , there by creating a vacant electron site or hole on the semiconductor atom.

Indium accepts one extra electron, the energy level of this impurity atom is called acceptor level and this acceptor level lies just above the valence band.

These type of trivalent impurities are called acceptor impurities and the semiconductors doped the acceptor impurities are called P-type semiconductors.

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Gallium atom has 3 valence electrons.

It makes covalent bonds with adjacent three electrons of silicon atom.

There is a deficiency of one covalent bond and creates a hole.

Therefore trivalent impurity accepts one electron and becomes negative acceptor ion. Trivalent impurity known as acceptor.

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P-type Semiconductor

When we add a Trivalent impurity to pure semiconductor we get p-type semiconductor.

12

GaPure

siP-type

Si

Fig 2.

In addition to this, some of the covalent bonds break due temperature and electron hole pairs generates.

Holes are majority carriers and electrons are minority carriers.

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Carriers in P-type Semiconductor

Si

Si

SiInSi

HoleCo-Valent bonds

Impure atom (acceptor)

14

E

Ea

Ev

Valence band

Ec

Conduction band

Ec

Electronenergy

temperature

Acceptor levels

Eg

15

16

Conduction is carried out by means of

1. Drift Process. 2. Diffusion Process.

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18

Drift process

CBVB

• Electrons move from external circuit and in conduction band of a semiconductor.

• Holes move in valence band of a semiconductor.

A B

VFig 4.

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Diffusion process

X=a

• Moving of electrons from higher concentration gradient to lower concentration gradient is known as diffusion process.

Fig 5.

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+

+

+

+ + +

+

+

+ +

+

N

- -

-

-

-

- -

-

-

-

-

P Acceptor ion Donor ion

Minority electron Minority holeMajority holes Majority electrons

Fig 1.

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