EXTRINSIC SEMICONDUCTOR

In an extrinsic semiconducting material, the charge

carriers originate from impurity atoms added to the original

material is called impurity [or] extrinsic semiconductor.

• This Semiconductor obtained by doping TRIVALENT and

PENTAVALENT impurites in a TETRAVALENT

semiconductor. The electrical conductivity of pure

semiconductors may be changed even with the addition

of few amount of impurities.

DOPING

The method of adding impurities to a pure

semiconductor is known as DOPING, and the impurity

added is called the dopping agent(Ex-Ar,Sb,P,Ge and

Al).

The addition of impurity would increases the no.

of free electrons and holes in a semiconductor and

hence increases its conductivity.

SORTS OF SEMICONDUCTOR according to ADDITION OF IMPURITIES

n-type semiconductor

p-type semiconductor

N – type semiconductor

When pentavalent impurity is added to the intrinsic

semiconductors, n type semi conductors are formed.

n - type semiconductor

Valence band

Conduction band

Ev

Eg

Ed

Ec

Donors levels occupied

At T = 0K

When small amounts of pentavalent impurity such as

phosphorous are added during crystal formation, the impurity

atoms lock into the crystal lattice[ see above Fig).

Consider a silicon crystal which is doped with a fifth

column element such as P, As or Sb.

Four of the five electrons in the outermost orbital of the

phosphorus atom take part in the tetrahedral bonding with the

four silicon neighbours.

The fifth electron cannot take part in the discrete covalent

bonding. It is loosely bound to the parent atom.

It is possible to calculate an orbit for the fifth electron

assuming that it revolves around the positively charged

phosphorus ion, in the same way as for the “1s” electron around

the hydrogen nucleus.

The electron of the phosphorus atom is moving in the

electric field of the silicon crystal and not in free space, as is the

case in the hydrogen atom.

This brings in the dielectric constant of the crystal into the

orbital calculations, and the radius of the electron orbit here

turns out to be very large, about 80 Å, as against 0.5 Å for the

hydrogen orbit. Such a large orbit evidently means that the fifth

electron is almost free and is at an energy level close to the

conduction band.

At 0°K, the electronic system is in its lowest energy state, all

the valence electron will be in the valence band and all the

phosphorous atoms will be un-ionised.

The energy levels of the donor atoms are very close to the

conduction band.

In the energy level diagram, the energy level of the fifth

electron is called donor level. The donor level is so close to the

bottom of the conduction band.

Most of the donor level electrons are excited into the

conduction band at room temperature and become majority

charge carriers.

If the thermal energy is sufficiently high, in addition to the

ionization of donor impurity atoms, breaking of covalent

bonds may also occur thereby giving rise to generation of

electron hole pair.

Valence band

Conduction band

Ev

Eg

Ed

Ec

Donors levels ionised

Valence band

Conduction band

Ev

Eg

Ed

Ec

Donors levels ionised

At T = 300KAt T > 0K

P -Type Semiconductor

When trivalent impurity is added to intrinsic

semiconductor, P type semi conductors are formed.

Al has three electrons in the outer orbital. While

substituting for silicon in the crystal, it needs an extra-

electron to complete the tetrahedral arrangement of bonds

around it.

The extra electron can come only from one of the

neighboring silicon atoms, thereby creating a vacant electron

site (hole) on the silicon.

The aluminum atom with the extra electron becomes a

negative charge and the hole with a positive charge can be

considered to resolve around the aluminum atom.

p - type semiconductor

Valence band

Conduction band

Ev

Eg

Ea

Ec

At T = 0K

Since the trivalent impurity accepts an electron, the

energy level of this impurity atom is called acceptor level.

This acceptor level lies just above the valence bond.

Even at relatively low temperatures, these acceptor

atoms get ionized taking electrons from valence bond and

thus giving to holes in the valence bond for conduction.

Due to ionization of acceptor atoms, only holes and no

electrons are created.

Valence band

Conduction band

Ev

Eg

Ea

Ec

Acceptors have accepted electronsfrom valence band

If the temperature is sufficiently high, in addition to the

above process, electron-hole pairs are generated due to the

breaking of covalent bonds.

Thus holes are more in number than electrons and hence

holes are majority carriers and electrons are minority carriers

(a) At T > 0K (b) At T = 300K

Hall Effect

When a piece of conductor (metal or semi conductor)

carrying a current is placed in a transverse magnetic field,

an electric field is produced inside the conductor in a

direction normal to both the current and the magnetic field.

This phenomenon is known as the Hall Effect and the

generated voltage is called the Hall voltage.

Z

Y

X

A

B

C

D

E

F

G

O

BI

EH

Hall effect

Consider a conventional current flow through the strip

along OX and a magnetic field of induction B is applied along

axis OY.

Case – I: If the Material is N-Type Semi Conductor

(or) Metal

If the strip is made up of metal or N-type semiconductor,

the charge carriers in the strip will be electrons.

As conventional current flows along OX, the electrons

must be moving along XO.

If the velocity of the electrons is `v’ and charge of the

electrons is `e’, the force on the electrons due to the magnetic

field

F = Bev, which acts along OZ.

This causes the electrons to be deflected and the

electrons accumulate at the face ABEF.

Face ABEF will become negative and the face OCDG

becomes positive.

A potential difference is established across faces ABEF

and OCDG, causing a field EH.

Z

Y

X

A

B

C

D

E

F

G

O

B

I

Force on

electron

B

F

v

Hall effect for n type semiconductor

This field gives rise to a force of `eEH’ on the electrons in

the opposite direction. (i.e, in the negative Z direction)

At equilibrium, eEH = Be (or) EH = B

If J is the current density, then, J = ne

where `n’ is the concentration of current carriers.

ne

J

Substitute the value of `’ in eqn

ne

BJ

EH =

v =

The Hall Effect is described by means of the Hall

coefficient `RH’ in terms of current density `J’ by the relation,

EH = RHBJ (or) RH = EH/ BJ

neneBJ

BJRH

1

All the three quantities EH, J and B are measurable,

the Hall coefficient RH and hence the carrier density `n’ can

be found out.

Case – (ii) If the material is a P-type semi conductor

If the strip is a P-type semiconductor, the charge

carriers in the strip will be holes.

The holes will constitute current in the direction of

conventional current.

Holes move along the direction of the conventional

current itself along ox

Z

Y

X

A

B

C

D

E

F

G

O

B

I

Force on

hole

B

F

v

Hall effect for p type semiconductor

If `e’ is the charge of the hole, the force experienced by

the holes due to magnetic field is, F = Be , which acts along

OZ.

This causes the holes to accumulate on the face ABEF

– making it positive, and leaving the face OCDG as negative.

P-type semiconductor, RH = 1/pe , where p = the density of

holes.

Determination of Hall coefficient

The Hall coefficient is determined by measuring

the Hall voltage that generates the Hall field.

If `w’ is the width of the sample across which the Hall

voltage is measured, then

EH = VH/ w

We know that, RH = EH/ BJ

Substituting the value of EH in the above eqn

RH = VH/ wBJ (or) VH = RHwBJ

If the thickness of the sample is `t’, the its cross sectional

area A = wt, and the current density,

wt

I

A

IJ=

Substitute the value of `J ’

VH = wt

I.B.wRH

t

BIRH= (or) RH = IB

tVH

VH will be opposite in sign for P and N type semiconductors.

A rectangular slab of the given material having

thickness `t’ and width `w’ is taken.

A current of `I’ amperes is passed through this sample by

connecting it to a battery, `Ba’.

The sample is placed between two pole pieces of an

electromagnet such that the field `B’ is perpendicular to I

Z

Y

X

A B

C

D

EF

G

O

B

I

VH

wt

Ba A Rh

Z

Y

X

A B

C

D

EF

G

O

B

I

VH

wt

Ba A Rh

Experimental setup for the measurement of Hall voltage

The hall voltage `VH’ is then measured by placing

two probes at the two side faces of the slab. If the

magnetic flux density is `B’ and `VH’ is the hall voltage, then

the Hall coefficient,

RH = VHt / IB (m3/coulomb)

For n-type material, n = nee (or) Hnn

e R.ne

For p-type material, p = p e h (or)Hp

p

h R.pe

Applications of Hall effect

(1) Determination of N-type of semiconductor

For a N-type semiconductor, the Hall coefficient

is negative whereas for a P-type semiconductor, it is positive.

Thus from the direction of the Hall voltage developed, one

can find out the type of semiconductor.

(2) Calculation of carrier concentration

Once Hall coefficient RH is measured, the

carrier concentration can be obtained,

HH eRpor

eRn

1)(

1

(3)Determination of mobility

We know that, conductivity, n = n e e (or)

Also, P = p e h (or) Hp

p

n Rpe

.

(4) Measurement of magnetic flux density.

Using a semiconductor sample of known `RH’, the

magnetic flux density can be deduced from, RH =

Thus by measuring `’ and RH, ’ can be calculated.

IR

tVB

H

H

DILUTE MAGNETIC SEMICONDUCTORS

Introduction

• Integrated circuits and high-frequency devices made of

semiconductors, used for information processing and

communications, have had great success using the charge of

electrons in semiconductors.

Mass storage of information–indispensable for

information technology–is carried out by magnetic

recording (hard disks, magnetic tapes, magneto optical

disks) using spin of electrons in ferromagnetic materials.

Dilute or diluted magnetic semiconductors (DMS)

also referred to as semi magnetic semiconductors, are

alloys whose lattices are made up in part of

substitutional magnetic atoms.

DMS= Semiconductors with dilute concentration of

magnetic dopants.

The most important feature of these materials is carrier

mediated magnetism which can be easily controlled with

voltage. The advantage is that, unlike the conventional

magnets, DMS are compatible with semiconductors and can

be used as efficient sources for spin injection.

Three types of semiconductors: (A) a magnetic semiconductor(e.g.

some spinels), in which a periodic array of magnetic element is

present, (B) a dilute magnetic semiconductor(e.g. (GaMn)As,(InMn)P,

ZnCoO etc), an alloy between nonmagnetic semiconductor and

magnetic element and (C) a non-magnetic semiconductor(e.g. GaAs,

InP, Cu2O, NiO etc), which contains no magnetic ions.

Materials

The most common SMSC are II-VI compounds (like CdTe,

ZnSe, CdSe, CdS, etc.), with transition metal ions (e.g. Mn, Fe

or Co) substituting their original cations. There are also

materials based on IV-VI (e.g. PbTe, SnTe) and recently III-V

(e.g. GaAs, InSb) crystals.

The wide variety of both host crystals and magnetic atoms

provides materials which range from wide gap to zero gap

semiconductors, and which reveal many different types of

magnetic interaction.

Formation of DMS

Several of the properties of these materials may be tuned

by changing the concentration of the magnetic ions.

The most relevant feature of DMS, is the coexistence and

interaction of two different electronic sub systems:

delocalized conduction (s-type) and valence (p-type) band

electrons and localized (d or f-type) electrons of magnetic

ions.

In particular the spd exchange interaction leads to strong

band splitting, which result in giant magneto optical effects .

The most studied III-V DMS system is GaxMn1-xAs

with x up to 0.07. The solubility limit of magnetic elements in

III-V semiconductors is very low, but in order to have

ferromagnetism in DMS, a sizable amount of magnetic ions

are needed.

This can only be accomplished by means of

nonequilibrium crystal growth techniques, such as low

temperature molecular beam epitaxy (MBE).

The upper concentration limit of magnetic ions is

around 10 %.

The highest conclusively reported Tc of DMS is around

110 K for 5 % doped GaAs. It is of great technological

importance to find DMS systems with Tc above room

temperature, before one attempts to make a DMS based

device.

Applications

Diluted magnetic semiconductors (DMS) are expected to

play an role in interdisciplinary materials science and future

electronics because charge and spin degrees of freedom

accommodated into a single material exhibits interesting

magnetic, magneto-optical, magnetoelectronic and other

properties.

It is expected that magnetoelectronic important chips

will be used in quantum computers.

An inherent advantage of magnetoelectronics over

electronics is the fact that magnet tend to stay magnetized for

long.

Hence this arises interest in industries to replace the

semiconductor-based components of computer with magnetic

ones, starting from RAM.

These DMS materials are very attractive for integration of

photonic (light-emitting diodes), electronic (field effect

transistors), and magnetic (memory) devices on a single

substrate.

Some important application areas of DMS are listed below.

•Photonics plus spintronics (Spin+electronics = Spintronics)

•Improved spin transistor

•Transistors spin toward quantum computing

•Magnetic spins to store quantum information

•Microscope to view magnetism at atomic level

•Ballistic magneto resistance

•Missile guidance

•Fast accurate position and motion sensing of

mechanical components in precision aengineering and

in robotics

•In automotive sensors

Importance of DMS based devices

Information is stored (written) into spins as a particular spin

orientation (up or down)

The spins, being attached to mobile electrons, carry the

information along a conductor

The information can be stored or is read at a terminal.

Spintronics devices are attractive for memory storage and

magnetic sensors applications

Spin-based electronics promises a radical alternative to

charge-based electronics, namely the possibility of logic

operations with much lower power consumption than

equivalent charge-based logic operations

THANK YOU