Semiconductor

Physics
 Semiconductors
The materials whose electrical conductivity lies between those
of conductors and insulators, are known as semiconductors.

Silicon 1.1 eV
Germanium 0.7 eV
Cadmium Sulphide 2.4 eV

Silicon is the most widely used semiconductor.

Semiconductors have negative temperature coefficients of

resistance, i.e. as temperature increases resistivity deceases
 Energy Bands

If a band is completely filled with electrons, and the band immediately above it is empty,
the material has an energy band gap. This band gap is the energy difference between
the highest occupied state in the valence band and the lowest unoccupied state in the
conduction band. The material is either a semiconductor if the band gap is relatively
small, or an insulator if the band gap is relatively large.
The bands are actually composed of a finite number of very closely spaced electron
energy levels.
Formation of Bands
Energy Band Gap
Energy Band Diagram

Conduction
electrons
Energy Band Diagram
Forbidden energy band is small
for semiconductors.

Less energy is required for

electron to move from valence
to conduction band.

A vacancy (hole) remains when

an electron leaves the valence
band.

Hole acts as a positive charge

carrier.
 Intrinsic Semiconductor
A semiconductor, which is in its extremely pure form, is known
as an intrinsic semiconductor. Silicon and germanium are the
most widely used intrinsic semiconductors.

Both silicon and germanium are

tetravalent, i.e. each has four
electrons (valence electrons) in
their outermost shell.

Each atom shares its four

valence electrons with its four
immediate neighbours, so that
each atom is involved in four
covalent bonds.
 Intrinsic Semiconductor

When the temperature of an intrinsic semiconductor is

increased, beyond room temperature a large number of
electron-hole pairs are generated.

Since the electron and holes are generated in pairs so,

Free electron concentration (n) = concentration of holes (p)

= Intrinsic carrier concentration (ni)
 Extrinsic Semiconductor

Pure semiconductors have negligible conductivity at room

temperature. To increase the conductivity of intrinsic
semiconductor, some impurity is added. The resulting
semiconductor is called impure or extrinsic semiconductor.

Impurities are added at the rate of ~ one atom per 106 to 1010
semiconductor atoms. The purpose of adding impurity is to
increase either the number of free electrons or holes in a
semiconductor.
 Extrinsic Semiconductor
Two types of impurity atoms are added to the semiconductor

Atoms containing 5 Atoms containing 3

valance electrons valance electrons
(Pentavalent impurity atoms) (Trivalent impurity atoms)
e.g. P, As, Sb, Bi e.g. Al, Ga, B, In

N-type semiconductor P-type semiconductor

 N-type Semiconductor
The semiconductors which are obtained by introducing
pentavalent impurity atoms are known as N-type
semiconductors.

Examples are P, Sb, As and Bi. These elements have 5

electrons in their valance shell. Out of which 4 electrons will
form covalent bonds with the neighbouring atoms and the 5th
electron will be available as a current carrier. The impurity atom
is thus known as donor atom.
In N-type semiconductor current flows due to the movement of
electrons and holes but majority of through electrons. Thus
electrons in a N-type semiconductor are known as majority
charge carriers while holes as minority charge carriers.
 Donors
• We use Silicon (Si) as an example
– Substitute one Si (Group IV) atom with a
Group V atom (e.g. As or P)
– Si atoms have four valence electrons that
participate in covalent bonding
– When a Group V atom replaces a Si atom, it
will use four of its electrons to form the
covalent bonding
– What happens with the remaining electron?
 This crystal has been doped with a pentavalent impurity.

The free electrons in n type silicon support the flow of current.

 P-type Semiconductor
The semiconductors which are obtained by introducing trivalent
impurity atoms are known as P-type semiconductors.
Examples are Ga, In, Al and B. These elements have 3
electrons in their valance shell which will form covalent bonds
with the neighbouring atoms.
The fourth covalent bond will remain incomplete. A vacancy,
which exists in the incomplete covalent bond constitute a hole.
The impurity atom is thus known as acceptor atom.

In P-type semiconductor current flows due to the movement of

electrons and holes but majority of through holes. Thus holes in
a P-type semiconductor are known as majority charge carriers
while electrons as minority charge carriers.
 Acceptors
• Use Silicon (Si) as an example
– Substitute one Group III atom (e.g. Al or In) with a Si
(Group IV) atom
– Si atoms have four valence electrons that participate in
the covalent bonding
– When a Group III atom replaces a Si atom, it cannot
complete a tetravalent bond scheme
– An “electronic vacancy” – hole – is formed when an
electron from the valence band is grabbed by the atom so
that the core is negatively charged, the hole created is
then attracted t the negative core
– At T = 0 K this hole “stays” with atom – localized hole
– At T > 0 K, electron from the neighboring Si atom can
jump into this hole – the hole can then migrate and
contribute to the current
 This crystal has been doped with a trivalent impurity.

The holes in p type silicon contribute to the current.

Note that the hole current direction is opposite to electron current
so the electrical current is in the same direction
 Fermi Energy
The Fermi energy is a quantum mechanical concept and it
usually refers to the energy of the highest occupied quantum
state in a system of fermions at absolute zero temperature.
.

Fermi Level
The Fermi level (EF) is the maximum energy level, which can
be occupied by an electron at absolute zero temperature.
 Fermi Energy Diagram for Intrinsic
Semiconductors

Forbidden Fermi
Energy Level (EF)
Gap

The Fermi level (EF) lies at the middle of the forbidden energy
gap.
 Fermi Energy Diagram for N-type
Semiconductors

Fermi
Level (EF)

Energy (eV) Donor

Level

Fermi
Level (EF)

The Fermi level (EF) shifts upwards towards the bottom of the
conduction band.
 Fermi Energy Diagram for P-type
Semiconductors

Energy (eV) Acceptor

Level
Fermi
Level (EF)

Fermi
Level (EF)

The Fermi level (EF) shifts downwards towards the top of the
valance band.
 If the dopant atoms contain one less electron then there will
be an extra unoccupied band slightly higher in energy than
the valence band.  This is called the acceptor band and can
accept electrons from the valence band with lower thermal
energies than the pure crystal bands can This is called a p-
type semiconductor (P is for positive since the dopant atom
will appear to have one fewer electrons, it will look to be
"positive").  

 If the dopant atoms have extra electrons then an extra band

is created that can donate electrons into the conduction
band.  This creates an n-type semiconductor. (N is for
negative.  the dopant atom has one more electron than the
rest of the lattice and appears "negative" by comparison. 
 Mass Action Law
Addition of n-type impurities decreases the number of holes
below a level. Similarly, the addition of p-type impurities
decreases the number of electrons below a level.

It has been experimentally found that

“Under thermal equilibrium for any semiconductor, the
product of no. of holes and the no. of electrons is constant and
independent of amount of doping. This relation is known as
mass action law”
n. p  ni2

where n = electron concentration, p = hole concentration

and ni = intrinsic concentration
 Charge carrier concentration in N-type and
P-type Semiconductors
The free electron and hole concentrations are related by the
Law of Electrical Neutrality i.e.

Total positive charge density is equal to the total negative

charge density

Let ND = Concentration of donor atoms = no. of positive

charges/m3 contributed by donor ions
p = hole concentration
NA=Concentration of acceptor atoms
n = electron concentration

By the law of electrical neutrality

ND + p = NA + n
For N-Type semiconductor
NA = 0 i.e. Concentration of acceptor atoms
And n>>p, then
ND + 0 = 0 + n

ND = n

i.e. in N-type, concentration of donor atoms is equal to the

concentration of free electrons.

According to Mass Action Law

2
n. p  n i

2 2
p  n / n  n / ND
i i
For P-Type semiconductor
ND = 0 i.e. Concentration of donor atoms
And p>>n, then
NA + 0 = 0 + p
NA = p

i.e. in P-type, concentration of acceptor atoms is equal to the

concentration of holes.

According to Mass Action Law

2
n. p  n i

2 2
n  n / p  n / NA
i i
 Important Information

 A 1 cm3 specimen of a metal or semiconductor has

of the order of 1022 atoms. In a metal, every atom
donates at least one free electron for conduction,
thus 1 cm3 of metal contains on the order of
1022 free electrons,

 whereas a 1 cm3 sample of pure germanium at

20 °C contains about 4.2×1022 atoms, but
only 2.5×1013 free electrons and 2.5×1013 holes.

 The addition of 0.001% of arsenic (an impurity)

donates an extra 1017 free electrons in the same
volume and the electrical conductivity is increased
by a factor of 10,000.
 E-k Diagrams
An extension of the simple band energy diagram with only the vertical axis
labeled as energy, with the horizontal axis unlabeled, is to plot the energy
vertically against wave vector, k.

From de Broglie's relationship p = k where p is momentum and  is Planck's

constant, h, divided by 2π. Such plots therefore relate energy to momentum.

The energy of a classical, non-quantum, particle is proportional to the square of

its momentum. This is also true for a free electron, as in the most simple picture
possible of valence electrons in metals where the electrostatic potential from the
nuclei is ignored. (E=p2/2m)

However, in a real crystalline solid the periodicity of the lattice and the
electrostatic potential from the nuclei together mean that in the quantum world in
a crystalline material the electron energy, E, is not simply proportional to the
square of the momentum, and so is not proportional to the square of the wave
vector, k.
When plotted against the wave vector, k, the bands of allowed energy are not really
flat. This means that bands can overlap in energy, as the maximum value in one
band may be higher then the minimum value in another band. In this case the
relevant maximum and minimum will occur for different values of k because energy
bands never cross over each other.

This is one way in which metals can have partially filled energy bands. The
available energy states are filled with electrons starting with those lowest in energy.
Such overlapping of bands as a function of k does not occur in semiconductors.
 Direct and indirect-band gap materials :
Direct-band gap s/c’s (e.g. GaAs, InP, AlGaAs)

• For a direct-band gap material, the

E minimum of the conduction band and
CB
maximum of the valance band lies at the
same momentum, k, values.

e- • When an electron sitting at the bottom of

the CB recombines with a hole sitting at
k the top of the VB, there will be no change
+ in momentum values.

• Energy is conserved by means of emitting

a photon, such transitions are called as
VB radiative transitions.
 • For an indirect-band gap material; the
Indirect-band gap s/c’s (e.g. Si and Ge) minimum of the CB and maximum of
the VB lie at different k-values.
• When an e- and hole recombine in an
indirect-band gap s/c, phonons must
E be involved to conserve momentum.
CB
Phonon
e-  Atoms vibrate about their mean
position at a finite temperature.These
Eg vibrations produce vibrational waves
k inside the crystal.
+  Phonons are the quanta of these
vibrational waves. Phonons travel with
a velocity of sound .
 Their wavelength is determined by the
VB crystal lattice constant. Phonons can
only exist inside the crystal.
• The transition that involves phonons without producing photons are
called nonradiative (radiationless) transitions.

• These transitions are observed in an indirect band gap s/c and

result in inefficient photon producing.

• So in order to have efficient LED’s and LASER’s, one should

choose materials having direct band gaps such as compound s/c’s
of GaAs, AlGaAs, etc…
 Conduction in Metals
If an electric field is applied to a metal, than due to electrostatic
force, the electrons gets accelerated and their velocity would
increase indefinitely.

Due to collisions with ions the electrons loses energy and attains a
finite velocity called drift velocity.

Let E = Strength of applied electric field

F = Force experienced by electrons due to applied field
a = acceleration
t = time between the collisions
v = Drift velocity
q = charge of an electron
m = mass of an electron
 Electric current in a conductor
Consider a piece of conductor in which electrons are uniformly
distributed.

Let N = No. of free electrons distributed in the conductor

L = Length of the conductor
A = Cross-sectional area of the conductor

Average velocity of electrons L


T
Where T is the time taken by the electrons through a distance

The number of electrons passing through any area per second

N

T
 Electric current in a conductor
Total charge passing through any area per second (current)

N N L q.N .v
 q  q  
T T L L
Current per unit area (Current density)

I q.N .v
J 
A L. A
J  q.n.v
Where n is the number of electrons per unit volume
 Conductivity of a conductor

J  q.n.v

J  q.n.( .E )

J   .E
where  ( q.n. ) is called the conductivity of a metal.

Resistivity of a metal is reciprocal of conductivity

1 1
 
 q.n.
 Conductivity in semiconductors

J p ,drift  qp p E J n ,drift  qn(  n E )

J tot ,drift  J p ,drift  J n ,drift  qp p E  qn n E

J tot ,drift  q( p p  n n ) E  E
 Conductivity of N and P-type
semiconductor
For intrinsic semiconductor

 i  q.(n. n  p. p )

For N-Type semiconductor (n>>p)

  q.n. n
For P-type semiconductor (p>>n)

  q. p. p
 Diffusion Current
• Due to thermally induced random motion, mobile particles
tend to move from a region of high concentration to a region
of low concentration.
• Current flow due to mobile charge diffusion is proportional to
the carrier concentration gradient.
• Diffusion current within a semiconductor consists of hole and
electron components:

dp dn
J p ,diff  qDp J n ,diff  qDn
dx dx
dn dp
J tot ,diff  q( Dn  Dp )
dx dx
 Diffusion Current
The total current flowing in a semiconductor is the sum of drift
current and diffusion current:

J tot  J p ,drift  J n,drift  J p ,diff  J n,diff

• The characteristic constants for drift and diffusion are
related by the Einstein’s Relation:
D kT

 q
kT
•  26mV at room temperature (300K)
q
This is often referred to as the “thermal voltage”.
 Barrier Formation in P-N Junction Diode

The holes from p-side diffuses to the n-side while the free
electrons from n-side diffuses to the p-side.

This movement occurs because of charge density gradient.

This leaves the negative acceptor ions on the p-side and
positive donor ions on the n-side uncovered in the vicinity of the
junction.
 Barrier Formation in P-N Junction Diode
Thus there is negative charge on p-side and positive on n-side.
This sets up a potential difference across the junction and hence
an internal Electric field directed from n-side to p-side..

Equilibrium is established when the field becomes large enough to

stop further diffusion of the majority charge carriers.

The region which becomes depleted (free) of the mobile charge

carriers is called the depletion region. The potential barrier
across the depletion region is called the potential barrier.

Width of depletion region depends upon the doping level. The

higher the doping level, thinner will be the depletion region.
 Forward Bias P-N Junction

When an external voltage is applied to

the P-N junction making the P side
positive with respect to the N side the
diode is said to be forward biased.

The barrier potential difference is decreased by the external

applied voltage. The depletion band narrows which urges
majority carriers to flow across the junction.

A Forward biased diode has a very low resistance.

 Reverse Bias P-N Junction

When an external voltage is applied

to the P-N junction making the P side
negative with respect to the N side
the diode is said to be Reverse Biased.

The barrier potential difference increases. The depletion band

widens preventing the movement of majority carriers across the
junction.

A Reverse Bias diode has a very high resistance.

 Breakdown in P-N junction diode
In Electronics, the term “breakdown” stands for release of
electron-hole pairs in excess.

The critical value of the voltage, at which the breakdown of a

P-N junction diode occurs is called the breakdown voltage.
The breakdown voltage depends on the width of the depletion
region, which, in turn, depends on the doping level.

There are two mechanisms by which breakdown can occur at a

reverse biased P-N junction:

1. Avlanche Breakdown (uncontrolled)

2. Zener Breakdown (controlled)
 Avalanche breakdown
If the reverse bias is made very high, the thermally generated
electrons and holes get sufficient K.E from applied voltage to
break the covalent bonds near the junction and a large no. of
electron-hole pairs are released. These new carriers, in turn,
produce additional carrier again by breaking bonds. Thus
reverse current then increase abruptly and may damage the
junction by the excessive heat generated.

The avalanche breakdown occurs in lightly doped junctions,

which produce wide depletion region.

The avalanche breakdown voltage increases as the temp. of

the junction increases due to the increased probability of
collisions of electron and holes with crystal atoms.
 Zener breakdown (controlled)
Zener Breakdown occurs at low voltage in heavily doped
reverse biased p-n junction.

Strong electric field directly (without impact of electron) pull out

the electrons from the covalent bond.

Zener breakdown voltage decreases as the temp. of the

junction increases. Since an increase in temp. increase the
energy of valence electron. So escape from covalent bond
become easier for these electrons. Thus a smaller reverse
voltage Is sufficient to pull the valence electron out of the
covalent bonds.
 The Hall Effect
If a current (I) carrying semiconductor is placed in a transverse
magnetic field (B), then electric field (force) is induced in the
perpendicular direction of I and B

Application:-
Nature of semiconductor (p-type or n-type)
Carrier concentration
Conductivity
Mobility
Experimental Determination of Carrier concentration and
Mobility

Consider a semiconductor (P-Type or N-types) in which

current I and Magnetic field B is applied, a force is act on
the charge carriers. This force pushing the charge carriers
towards the back of the semiconductor.

When the mobile carriers (i.e. electrons or holes) are

pushed towards the back, the front becomes depleted and
the semiconductor loss it neutrality.

Now there is an excess of mobile charge carriers at the

back and an excess of opposite charge due to impurity
atom at the front.
If the semiconductor is N-type,
The electron will be in excess at the back surface and the surface
becomes negatively charged with respect to front. This gives rise
to a potential difference called Hall voltage between front and
back
If the semiconductor is P-type,
The hole will be in excess at the back surface and the surface
becomes positively charged with respect to front. The polarity of
Hall voltage is in reverse direction
At equilibrium, the force exerted on electrons due to electric
field and magnetic field must balance each other.

F(electric)  F( magnetic)  0
F  q.E
B.J
n.q.E  B.J  0 E - J  q.n.v
n.q
The Hall coefficient
1
RH  -
n.q

E  B.J.R H
 V
E H
d
VH  Hall Voltage, d = Distance between front & back surface

Current density can be written as

I I
J 
A d .t
Where t is the thickness of the semiconductor
VH I 1
 B 
d d .t n.q
B.I B.I
VH  or n
n.q.t VH .q.t
The concentration of holes in a P-type semiconductor can be
given by
B.I
p
VH .q.t
The Hall coefficient for a P-type semiconductor is given by

1
RH  
n.q
The electrical conductivity for an extrinsic semiconductor is

  n.q. n For N-type semiconductor

  p.q. p For P-type semiconductor

If the conductivity and Hall coefficient are given


n    .RH For N-type semiconductor
n.q

p    .RH For P-type semiconductor
p.q
 Applications:
• Position sensing:
• Sensing the presence of magnetic object is the most common industrial
application of Hall effect sensors The Hall effect sensors are also used in
the brushless DC motor to sense the position of the rotor and to switch
the transistor in the right sequence.
• DC current transformers:
• Hall effect sensors may be utilized for contactless measurements of DC
current in current transformers. In such a case Hall effect sensor is
mounted in the gap in magnetic core arround the current conductor. As a
result, the DC magnetic flux can be measured, and the DC current in the
conductor can be calculated.
• Automotive fuel level indicator:
• The Hall sensor is used in some automotive fuel level Indicators. The
main principle of operation of such indicator is position sensing of
floating element.
This can either be done by using a vertical float magnet or a rotating
lever sensor.
 Zener Diode
Zener diodes are semiconductor diodes which have been
manufactured to have their reverse breakdown occur at a
specific, well-defined voltage (its “Zener voltage”), and that
are designed such that they can be operated continuously in
that breakdown mode.
Commonly available Zener diodes are available with
breakdown voltages (“Zener voltages”) anywhere from 1.8 to
200 V
 Zener Diode as voltage regulator
The function of a regulator is to provide a
constant output voltage to a load connected in
parallel with it in spite of the ripples in the
supply voltage
The variation in the load current and the zener
diode will continue to regulate the voltage until
the diodes current falls below the
minimum IZ(min)value in the reverse breakdown
region.
As the input voltage Vin varies (Vin > Vz ) the
zener diode maintains an almost constant
voltage across the output terminal.
Resistor R is used for current limiting.
Minimum current used for current limiting is
The resistor is selected so that when the input voltage is at
VIN(min) and the load current is at IL(max) that the current through
the Zener diode is at least Iz(min). Then for all other combinations
of input voltage and load current the Zener diode conducts the
excess current thus maintaining a constant voltage across the
load.

The Zener conducts the least current when the load current is
the highest and it conducts the most current when the load
current is the lowest.
THE LIGHT EMITTING
DIODE
• Thus, for a direct band gap material, the excess
energy of the electron-hole recombination can
either be taken away as heat, or more likely, as
a photon of light.
• This radiative transition then
conserves energy and momentum
by giving off light whenever an
electron and hole recombine. CB

This gives rise to

(for us) a new type
of device; VB
the light emitting diode (LED).
 Construction of Typical LED

Al
Light output
SiO2

n
Electrical
contacts

Substrate
 LED Construction

 Efficient light emitter is also an efficient absorbers of

radiation therefore, a shallow p-n junction required.
 Active materials (n and p) will be grown on a lattice
matched substrate.
 The p-n junction will be forward biased with contacts
made by metallisation to the upper and lower
surfaces.
 Ought to leave the upper part ‘clear’ so photon can
escape.
 The silica provides passivation/device isolation and
carrier confinement
 Efficient LED
• Need a p-n junction (preferably the same
semiconductor material only different dopants)
• Recombination must occur  Radiative
transmission to give out the ‘right coloured LED’
• ‘Right coloured LED’  hc/ = Ec-Ev = Eg
 so choose material with the right Eg
• Direct band gap semiconductors to allow
efficient recombination
• All photons created must be able to leave the
semiconductor
• Little or no reabsorption of photons
 Light Emitting Diodes
A light emitting diode is essentially a pn junction diode made
from a direct band gap semiconductor
  UV-ED  ~0.5-400nm
Direct band gap
Eg > 3.25eV
materials
 LED -  ~450-650nm
e.g. GaAs not Si Eg = 3.1eV to 1.6eV
 IR-ED-  ~750nm- 1nm
Eg = 1.65eV

Candidate Materials

Materials with refractive Readily doped n or p-types

index that could allow light
to ‘get out’
• When we talk about light ,it is conventional to
specify its wavelength, λ, instead of its
frequency.
• Visible light has a wavelength on the order of
nanometers.
hc 1242
 (nm)   (nm) 
E (eV ) E (eV )
• Thus, a semiconductor with a 2 eV band-gap
should give a light at about 620 nm (in the red). A
3 eV band-gap material would emit at 414 nm, in
the violet.
• The human eye, of course, is not equally
responsive to all colors.
Relative response of the human eye to various
colors Relative eye response

100

10-1

GaAs.14p86

GaAs.35p65

GaAs.6p4
GaP:N
ZnSe
10-2
GaN

10-3

violet blue green yellow orange red

10-4

350 400 450 500 550 600 650 700 750

Wavelength in nanometers

The materials which are used for important light emitting

diodes (LEDs) for each of the different spectral regions.
 Visible LED
Definition:
LED which could emit visible light, the band gap of the materials that we use
must be in the region of visible wavelength = 390- 770nm. This coincides with
the energy value of 3.18eV- 1.61eV which corresponds to colours as stated
below:

Violet ~ 3.17eV The band gap, Eg

that the
Blue ~ 2.73eV semiconductor
Green ~ 2.52eV must posses to
Colour of an Yellow ~ 2.15eV emit each light
LED should
emits
Orange ~ 2.08eV
Red ~ 1.62eV
Principle of Diode
LASER
 What is the word LASER
stands for?
• Light amplification by Stimulated
Emission of Radiation

What is Population
Inversion?
 Stimulated Emission
E2 E2 E2
h h
h h  In
Out
h

E1 E1 E1

(a) Absorption (b) Spontaneous emission (c) Stimulated emission

Absorption,
In stimulatedspontaneous (random photon
emission, an incoming photon)withemission and
energy h stimulated
stimulates the
emission.
emission process by inducing electrons in E2 to transit down to E1.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
While moving down to E1, photon of the same energy h will be emitted
Resulting in 2 photons coming out of the system
Photons are amplified – one incoming photon resulting in two photons
coming out.
Population Inversion N2> N1 More electrons
here
• Non equilibrium distribution of
electrons among the various N2 E2
energy level atomic system
• To induce more electrons in E2,
N1 E1
i.e. to create population
inversion, a large amount of
There level
energy is required to excite E3 system
atoms to E2
E2
• The excitation process of atoms
so N2 > N2 is called pumping
• It is difficult to attain pumping E1
when using two-level-system.
• Require 3-level system instead
 Principles of Laser
hu 32
E E E E
3 3 3 3

hu 13
E E E E
2 2 2 2
Metastable IN OUT
state
hu 2 1 hu 21

E Coherent photons
E 1 E E
1 1 1

( a) (b ) (c) (d )

• In actual case, excite atoms from E1 to E3.

• Exciting atoms from E1 to E3 optical pumping .
• Atoms from E3 decays rapidly to E2 emitting h3
• If E2 is a long lived state, atoms from E2 will not decay to E1 rapidly
• Condition where there are a lot of atoms in E2 population inversion
achieved! i.e. between E2 and E1.
 Coherent Photons Production
(explanation of (d))

• When one atom in E2 decays

spontaneously, a random photon
resulted which will induce stimulated
photon from the neighbouring atoms
• The photons from the neighbouring
atoms will stimulate their neighbours
and form avalanche of photons.
• Large collection of coherent photons
resulted.
Laser Diode Principle

Consider a p-n junction

In order to design a laser diode, the p-n
junction must be heavily doped.
In other word, the p and n materials must
be degenerately doped
By degenerated doping, the Fermi level of
the n-side will lies in the conduction band
whereas the Fermi level in the p-region will
lie in the valance band.
 Diode Laser Operation
p+ Junction n+
E
c
p+ n+
E
g eV E E
o c Fn
Inversion E
E region c
E
Ev Holes in VB E g eV
Fp Electrons Electrons in CB E Fn
c
E
Fp

E
(a) v (b)

• P-n junction must be degenerately doped. V

• Fermi level in valance band (p) and • Forward bias, eV> Eg
conduction band (n). • Built in potential diminished to zero
• No bias, built n potential; eVo barrier to • Electrons and holes can diffuse to the space
stop electron and holes movement charge layer
Application of Forward Bias

Suppose that the degenerately doped p-n

junction is forward biased by a voltage
greater than the band gap; eV > Eg
The separation between EFn and EFp is now
the applied potential energy
The applied voltage diminished the built-in
potential barrier, eVo to almost zero.
Electrons can now flow to the p-side
Holes can now flow to the n-side
 Population Inversion in Diode
LaserMore electrons in
the conduction
band near EC
CB
EFn
Electrons in CB

eV Eg
Than electrons in
Holes in VB the valance band
EFp near EV
VB

EFn-EfP = eV
There is therefore a population inversion between
eV > Eg
energies near EC and near EV around the junction.
eV = forward bias voltage
Fwd Diode current pumping  This only achieved when degenerately doped p-n
injection pumping junction is forward bias with energy > Egap
 Pumping Mechanism in Laser
Diode

• It is obvious that the population

inversion between energies near EC
and those near EV occurs by injection
of large charge carrier across the
junction by forward biasing the
junction.
• Therefore the pumping mechanism is
FORWARD DIODE CURRENT 
Injection pumping
 The Lasing Action
• The population inversion region is a layer along the
junction  also call inversion layer or active region
• Now consider a photon with E = Eg
• Obviously this photon can not excite electrons from
EV since there is NO electrons there
• However the photon CAN STIMULATE electron to
fall down from CB to VB.
• Therefore, the incoming photon stimulates emission
than absorption
• The active region is then said to have ‘optical gain’
since the incoming photon has the ability to cause
emission rather than being absorbed.
 For Successful Lasing Action:
1. Optical Gain (not absorb)
Achieved by population inversion
2. Optical Feedback
Achieved by device configuration
Needed to increase the total optical amplification by
making photons pass through the gain region multiple
times
Insert 2 mirrors at each end of laser
This is term an oscillator cavity or Fabry Perot cavity
Mirrors are partly transmitted and party reflected
Typical Exam Questions!!
• What is laser diode?
• Describe the principle of a laser diode
• Give some examples of applications of
laser diode.
• What is the pumping mechanism in a laser
diode and explain in term of the p-n
junction.
 Photodiodes
• Photodiodes are semiconductor light sensors that
generates current or voltage when PN junction is
illuminated by light.
• Photodiode working as photo-detector is essentially a
reverse biased PN junction diode which is designed to
respond to photon absorption.
• When photodiode is kept under dark condition and
sufficient reverse voltage is applied then almost
constant current, independent of magnitude of reverse
bias is obtained. Which is called dark current.
 Photodiode fundamentals
• Based on PN or PIN junction diode
– photon absorption in the P
+
hole
depletion region induces current h
flow I IL RL

– Depletion layer must be exposed electron

-
optically to source light and thick N

enough to interact with the light

• Spectral sensitivity
Material Band gap Spectral sensitivity
(eV)
silicon (Si) 1.12 250 to 1100 nm

indium arsenide (InGaAs) ~0.35 1000 to 2200 nm

Germanium (Ge) .67 900 to 1600 nm

93
 Photodiode structure
Incident light Front
Contact

Absorbtion in the
Insulation depletion layer
p+ Active Area causses current to
flow across the
Depletion region photodiode and if
the diode is
n- region reverse biased
considerable
n+ Back Diffusion current flow will be
Back Metalization induced

Rear
Contact

96
 Photovoltaic effect
The photovoltaic effect is the creation of voltage or electric current in a material upon
exposure to light.
The photovoltaic effect was first observed by French physicist A. E. Becquerel in 1839

The voltage potential created by a typical silicon PV cell is about 0.5 to 0.6 volts dc
under open-circuit, no-load conditions. The power of a PV cell depends on the
intensity of the solar radiation, the surface area of the PV cell, and its overall
efficiency
 Solar cell
Power can be delivered to an external circuit by an illuminated junction thus
Converting Solar energy to electrical energy

Solar cell is a photovoltaic device that converts the light

energy into electrical energy based on the principles of
photovoltaic effect
• The illumination is through thin
n side
• The depletion region extends
to p side
• A thin anti reflection coating on
the surface reduces reflection
loss.
• As the n-side is very narrow,
most of the photons are
absorbed within the depletion
region (W) and within the
neutral p-side (lp) and photo
generate EHPs in these
regions.
• An open circuit voltage develops between the
terminals of the device with the p-side positive with
respect to the n-side.
•Materials for Solar cell

•Solar cells are composed of various semiconducting

materials

1. Crystalline silicon
2. Cadmium telluride
3. Copper indium diselenide
4. Gallium arsenide
5. Indium phosphide
6. Zinc sulphide
Solar cell characteristics
FILL FACTOR: The fill factor is the ratio of the actual maximum
obtainable power to the product of the open circuit voltage and short
circuit current. This is a key parameter in evaluating performance.
IMPP - MPP current (A),
VMPP - MPP voltage (V)
Isc - short circuit current (A),
Voc - open cirquit voltage (V)