Electronic Devices

Diodes
1. Solids are classified into two categories.
a) Crystalline solids
2) Amorphous solids
2. The conductivity of solids can be explained by Band Theory of solids.
3. Band Theory of solids: A solid is a periodic arrangement of atoms. In case of
isolated atom the energy levels are discrete or well separated. For a particular
atom in the solid, neighbouring atoms influence the energies of the outer electrons.
The discrete energy levels spread into band of energy levels. The highest filled
band is called valance band. The next higher unfilled band is called conduction
band. The valance band and conduction band are separated by certain forbidden
energy region called forbidden energy gap (E
.g.
).
4. The energy bands which are completely filled at 0 K are called valance bands. The
bands with higher energies are called conduction bands.
5. Based on band theory of solids, solids are classified as conductors, semiconductors
and insulators.





6. Semi-conductors
a) If the forbidden energy gap between the conduction band and valence band is
small (about 1 eV) then such substances are called semiconductors.
E.g.: Silicon & Germanium.
b) For silicon forbidden energy gap is 1.1 eV and for germanium 0.72 eV.
c) At absolute zero, semiconductors behave as perfect insulators.
CB
CB
CB
VB
VB
VB
Conductor
E >>kT
g
E ~kT
g
Insulator Semiconductor
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d) Semiconductors are of two types.
a) Intrinsic 2) Extrinsic
7. Hole
a) A hole is an unfilled covalent bond (or) A vacant energy state in the valance
band of a semiconductor is called hole.
b) The existence of energy level of a hole can be observed in valence band of a
semiconductor.
c) Hole acts like a positive charge but not a particle. Hole drifts in opposite
direction to electrons with lesser speed.
8. Fermi energy: The highest energy level which an electron can occupy at 0 k is
called Fermi level. For intrinsic semiconductors this level lies in the middle of the
forbidden gap. It can also take as average energy of charge carriers.
9. Intrinsic semiconductors
a) Pure form of Si or Ge crystals is called intrinsic semiconductors (tetravalent).
b) The responsible charge carriers for conduction are both the
free electrons and holes.
c) The number of holes and the number of free electrons are
equal (n
e
=n
h
) and increase with increase of temperature.
d) Even though the responsible charge carriers are both the free electrons and
holes the current contributed by the electrons is more than that of holes because
of their higher mobility.
e) Mobility of electrons is nearly twice to that of holes in Germanium and 4 times
in silicon.
f) Fermi-energy level lies exactly at the midpoint of the forbidden gap.
10. Extrinsic semiconductors
a) The conductivity of intrinsic semiconductor is relatively less. To increase their
conductivity pure semiconductors are doped with trivalent or pentavalent
substances.
b) Doping: Adding of selected impurities to a semiconductor to increase its
conductivity is called doping.
CONDUCTIONBAND
VALENCEBAND
FermiLevel
Intrinsicsemiconductor
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c) The doped semiconductors are called as extrinsic semiconductors. They are of
two types. i) p-type, ii) n-type.
11. p-type semiconductor
a) When a trivalent substance (III group elements) like Boron,
Aluminium, Gallium, Indium etc., are added in sufficient
quantities (1 in 10
6
or less) to the pure form of Si or Ge crystal
then it is said to be p-type.
b) In p-type semiconductor, holes are majority carriers and electrons are minority
carriers (n
h
>n
e
)
c) The Fermi-energy level lies nearer to the valence band.
d) It is electrically neutral.
e) The energy level formed slightly above (about 0.01 eV) the
valence band due to acceptor impurities is called acceptor energy
level.
12. n-type semiconductor
a) When a pentavalent substance (V group elements) like Phosphorous, Arsenic,
Antimony etc., are added in sufficient quantities to the pure form of Si or Ge
crystal then it is said to be n-type.
b) In n-type semiconductor, electrons are majority carriers and holes are minority
carriers. (n
e
>n
h
).
c) The Fermi-energy level in nearer to the conduction band.
d) It is electrically neutral.
e) The energy level formed slightly below (about 0.01 eV) the conduction band
due to donor impurities is called donor energy level.
1) Conductivity of n-type >p-type >intrinsic.
2) With the increase of temperature Fermi level moves down.
13. p-n junction: When a semi conducting material such as silicon or germanium is
doped with impurity in such a way that one side has a large number of acceptor
impurities and the other side has a large number of donor impurities. The resulting
semiconductor is called p-n junction.
CONDUCTIONBAND
A
c
c
e
p
t
o
r

L
e
v
e
l

ptypesemiconductor
VALENCEBAND
FermiLevel
CONDUCTIONBAND

D
o
n
o
r

l
e
v
e
l

VALENCEBAND
FermiLevel
ntypesemiconductor
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14. p-n junction diode
a) A p-n junction diode cannot be obtained by simple contact of p-type and n-type
semiconductor.
b) Near the junction, the free electrons from n-region migrate towards p-region
and the holes in p-region migrate towards n-region. This process is known as
diffusion. This diffusion is due to concentration gradient.
c) Due to diffusion, positive ions are left over in n-region and negative ions are
left over in p-region, near the junction. These ions are immobile.
d) Due to the immobile ions on either side of the junction an internal electric field
is formed at the junction, which is directed from n to p.
e) The no charge carrier region formed at p-n junction due to the combination of
electrons and holes is called depletion layer.
f) The thickness of the depletion layer is of the order of 10
6
m.
g) When the depletion layer is sufficient by built up, it prevents the electrons
diffusion from n to p side and hole diffusion from p to n side i.e., it acts as a
barrier.
h) The potential difference across the barrier which is set up to prevent diffusion
of charge carriers through the junction is called potential barrier or contact
potential.
i) The potential barrier for silicon is 0.7 volts and for germanium is 0.3 volts.
j) The potential barrier value lies in between 0.1 to 0.7 volts, which depends on
the nature of semiconductor, doping concentration and temperature of the
junction.
k) It can be presumed to be equivalent to a condenser in which the depletion layer
acts as a dielectric.
l) p-n junction diode can be used as rectifiers, detectors.
m) In a circuit p-n junction diode is represented as. Here arrow mark represents
the direction of current in forward bias. It represents p side.


p
n
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Drift Current
n) Due to thermal collisions, electron-hole pairs are created in the depletion
region. The electron is quickly pushed by the electric field towards the n-side
and the hole towards the p-side. As electron-hole pairs are continuously created
in the depletion region, there is a regular flow of electrons towards the n-side
and of holes towards the p-side. This makes a current from the n-side to the
p-side. This current is called the drift current.
a) Diffusion current: When a p-n junction is formed, because of the
concentration difference, holes try to diffuse from the p-side to n-side.
Similarly, electrons try to diffuse from n-side to p-side. This diffusion results in
an electric current from the p-side to the n-side known as diffusion current.
v) The drift current and the diffusion current are in opposite directions. In
unbiased junction, in steady state, the diffusion current equals the drift current
in magnitude there is no net transfer of charge at any cross-section.
15. Forward Bias
a) In a p-n junction diode, if p-region is connected to +ve terminal (relatively
higher potential) of the battery and n-region is connected to ve terminal
(relatively lower potential) of the battery then it is said to be forward biased.
b) In forward biased condition, the width of depletion layer and barrier potential
decrease.
c) It is a low resistance connection.
d) The resistance of an ideal diode in forward biased condition is zero.
e) In forward biased condition, the flow of current is mainly due to the diffusion
of electrons.
f) The direction of current is from p to n.
16. Reverse Bias
a) In a p-n junction diode, if p-region is connected to ve terminal (relatively
low potential) of the battery and n-region is connected to the +ve terminal
(relatively high potential) of the battery then it is said to be reverse biased.
p n
+
p n
+
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b) In reverse biased condition, the width of the depletion layer and barrier
potential increase.
c) It is a high resistance connection.
d) The resistance of an ideal diode in reverse bias condition is infinity.
e) In reverse biased condition, the flow of current is mainly due to the drift of
charges.
f) The direction of current in it is from n to p.
i-v relation of diode is ] 1 e [ i i
KT
qv
0
= where i
0
is reverse saturation current, KT is
thermal energy and q is charge of electron. In forward bias
KT
qv
0
e i i =

in reverse
bias
0
i i =
I-V characteristics









Diode is unidirectional. It allows current in forward bias when applied potential
is greater than the barrier potential.

17. Rectifier
a) The conversion of A.C. voltage to D.C. voltage is called rectification.
b) A p-n junction diode is used as a rectifier.
c) When a single diode is used as a rectifier, the rectification of only one half of
the A.C. wave form takes place. Such a rectification is called half wave
rectification.
V
I
V
b
mA
V
0
A
V -break down voltage
V -barrier potential
b
0
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18. Efficiency of half wave rectifier
a) The ratio of D.C. power output to the applied input A.C. power is known as
rectifier efficiency.
b) Rectifier efficiency
f L
L
ac
dc
r R
R 406 . 0
P
P
+
= = where R
L
- load resistance, r
f
- diode
resistance.
19. In half wave rectification, a maximum of 40.6% of A.C. power is converted into
D.C. power.
In a half wave rectifier, if input frequency is n Hz A.C., then the output pulse
frequency is n Hz D.C.
a) Maximum current i
m
=
L f
m
R r
V
+
where V
m
= maximum voltage, r
f
=internal
resistance of the diode, R
L
=load resistance.
b) Average current I
dc
=

m
I


c) rms current i
rms
=
2
i
m
.
d) a.c power input = ) R (r x i
L f
2
rms
+
e) d.c power output P
dc
=(I
d3)
2
xR
L


20. When two diodes are used, then the rectification of both halves of the alternating
voltage or current can be obtained. Such a rectification is called full wave
rectification.





a) In a full wave rectifier, if input frequency is n Hz A.C., then the output pulse
frequency is 2n Hz pulsated D.C
~
A
B
A.C.
input
R
L
o
u
t
p
u
t
D
1
p
n
D
2
p
n
p t
t
V
o
V
i
~ R
L
A
B
D
A.C.
input
o
u
t
p
u
t
p
n
t
t
V
i
V
o
+ +
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21. Filter circuit: The output current either from half wave rectifier or full wave
rectifier, though unidirectional is not steady. It also contains A.C. components
which are undesirable and are to be removed by using filter circuit. Filter circuit
is a device which removes the A.C. component of rectifier output and allows the
D.C. component to reach the load.
22. Efficiency of full wave rectifier
a) The ratio of D.C. power output to the applied input A.C. power is known as
rectifier efficiency.
b) Full wave rectifier efficiency
L f
L
ac
dc
R r
R 812 . 0
P
P
+
= = where r
f
- diode resistance,
R
L
- load resistance.
23. In full wave rectifier a maximum of 81.2% of A.C. power is converted into D.C.
power.
a) Maximum current I
m
=
L f
m
R r
V
+
where V
m
=maximum voltage, r
f
=internal
resistance of the diode, R
L
=load resistance.
b) Average current i
av
=

m
I 2
where I
m
=maximum current
c) rms current i
rms
=
2
m
I
.
d) A.C. power input = ) r R ( x i
f L
2
rms
+ ;
e) D.C. power output P
dc
=(I
d3)
2
xR
L

24. Advantages of Semiconductor diodes
a) p-n junction diodes are minute (very small in size). Therefore these are used in
microcircuits.
b) As they are solid state devices, no evacuation is needed as in vacuum tubes.
c) They are also quite strong and sturdy.
d) Usually they have long life.
e) There is no filament heating and consequent power loss.
f) These can be prepared to function over wide voltage ranges and to give very
large rectified currents.
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25. Zener diode
a) It is a heavily doped p-n junction diode which is operated in
the breakdown region in reverse bias mode.
b) Zener diode has a sharp breakdown voltage in the reverse
bias because of heavy doping. This voltage is called Zener
Voltage (V
Z
).
c) Because of heavy doping width of the depletion layer
decreases, the electric intensity in the depletion layer increases Zener
breakdown is pulling the electrons from valence bonds by the action of this
strong electric field.
d) In forward bias, Zener diode act like an ordinary p-n junction diode.
e) Zener diode is used as a voltage regulator.
f) Output voltage (V
O
)=Zener voltage (V
Z
)
g) Current through load resistance (I
L
) =
L
Z
R
V
.
h) Voltage across series resistance (V) =input voltage zener voltage. V=V
i
V
Z

i) Current through series resistance (R) is I=
R
V V
R
V
Z i

= .
j) Current through Zener diode (I
Z
) =II
L
.
k) Series resistance absorbs voltage fluctuations and Zener diode absorbs current
fluctuations.
l) The maximum reverse bias potential that can be applied before commencement
of Zener region is called the Peak Inverse Voltage [PIV].







p n
V -break down voltage
b
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