introduction to semiconductors and diodes

1. INTRODUCTION

2. What is a semiconductor
 Semiconductors are materials whose conductivity is
between conductors and insulators
 They have conductivity better than insulators and
lower than that of conductors at room temperature
 Some common semiconductors are
 Elemental (Si-Silicon, Ge –Germanium)
 Compound (Ga As-Gallium arsenide, GaP - Gallium
phosphide, Aluminum arsenide , Indium Phosphide)

3. Intrinsic and Extrinsic
 Intrinsic semiconductors are pure semiconductors
without any added impurities
 Extrinsic semiconductors are semiconductors with
added elements called impurities
 The process of adding impurities to an intrinsic
semiconductor to change its electrical characterisitcs
is called Doping

4.  In an atom electrons occupy discrete energy levels called
shells. Each shell has a definite quantum of energy
 The K shell has energy level n=1, L shell has n=2 so on. Not
all energy leves are permitted for an electron.
 When an electron acquire energy by radiatin from external
sourses they are excited to higher energy levels. They
cannot stay in this state for long. After some time they
return to their original energy level by radiating their excess
energy.
 The allowable energies are represented by horizontal lines
in a diagram, called the energy level diagram of the atom.
Fig. 1.1 shows such a diagram
 If an electron have sufficiently high energy so that the
electron can overcome the attraction of the nucleus and
gets detached from the atom. The energy required for this
to occur is known as the ionization potential.
ENERGY LEVELS INSIDE ATOMS

5. Energy bands in solids
 A crystal is a solid consisting of a regular and
repetitive arrangement of atoms or molecules
(strictly speaking, ions) in space. If the positions of
the atoms in the crystal are represented by points,
called lattice points, we get a crystal lattice.
 When atoms combine to form a solid crystal each
energy level of the isolated atoms combine to form
energy bands

6. Energy band diagram
 When atoms combine to form a crystal the allowed
energy levels of a single atom expand into energy
bands .
 The energy bands are separated by gaps of forbidden
energy.
 Each energy band has millions of electrons but
electrons in lower energy bands are more attracted
towards the parent atoms
 The outer most shell electrons who is more able to
move because of higher energy and lower attractive
force from the nucleus.

7. Band Theory of Solids
 In a single isolated atom, the electrons in each orbit
have definite energy associated with it. But in case of
solids all the atoms are close to each other, so the
energy levels of outermost orbit electrons are affected
by the neighboring atoms.
 The electrons in same orbit exhibits slightly different
energy levels. The grouping of this different energy
levels is called energy band.
 However, the energy levels of inner orbit electrons are
not much affected by the presence of neighboring
atoms.

8. Important energy bands in solids
 There are number of energy
bands in solids but three of them
are very important. These three
energy bands are important to
understand the behavior of
solids. These energy bands are
 Valence band
 Conduction band
 Forbidden band or forbidden gap

9. Valence band
 There are millions of electrons in the valence
band of different atoms in a solid they differ
slightly in energies. These energy bands which
differ slightly combine to form a Valence band
 The energy band which is formed by grouping the
energy levels of the valence electrons or outermost
orbit electrons is called as valence band.
 Valence band is present below the conduction band as
shown in figure. Electrons in the valence band have
lower energy than the electrons in conduction band.

10. Conduction band
 It represent the next higher group of permissible
energy levels above the valence band
 Electrons in conduction band are free to move so they
contribute to the conduction
 Generally, the conduction band is empty but when
external energy is applied the electrons in the valence
band jumps in to the conduction band and becomes
free electrons. Electrons in the conduction band have
higher energy than the electrons in valence band.

11. Forbidden gap
 The energy gap which is present between the valence
band and conduction is called as forbidden band or
forbidden gap. Denoted by Eg
 In solids, electrons cannot stay in forbidden gap
because there is no allowed energy state in this region.
 Forbidden gap is the major factor for determining the
electrical conductivity of a solid.
 The classification of materials as insulators,
conductors and semiconductors is mainly depends on
forbidden gap.

13. METALS
 In metals the conduction
band and valence band are
over lapped ie there is no
energy gap between these
bands
 Electrons in conduction
band are free to move
towards conduction band
 So a metal have a large
number of free electrons in
conduction band without
supply of external energy

14. Insulators
 In an insulator the Energy band diagram shows that
there is an energy gap of 5eV or more
 So it is practically impossible for an electron to jump
this gap to reach the conduction band
 Insulators conduct only at high temperatures or if a
high voltage is applied across it. This is termed as
break down of the insulator

15. Semiconductor
 In the case of a semiconductor the forbidden energy
gap is small it is of the order of 1eV
 (For Ge Eg=0.72eV for Si Eg=1.12eV
 The energy provided by heat at room temperature is
sufficient to lift electrons from valence band to
conduction band
 Some electrons do jump the gap at room temperature
so semiconductors conduct slightly in room
temperature

16. Crystal structure of semiconductors
 Semiconductors have four electrons in their outer
most shell
 Each atom tries to acquire 8 electrons in its outermost
shell for a stable state
 Each of these atoms forms covalent bond with four
adjacent atoms and forms a crystal lattice
 Covalent bonds are formed by sharing of atoms from
neighbouring atoms

17. Representation of a
Semiconductor atom In a
crystal
 The core represent the nucleus and all electrons except
valence electrons. So core has a +4 charge due to
uncompensated protons there. The four valence
electrons are shown external to the core so atom as a
whole is neutral
 Each of the four electron take part in the formation of a
covalent bond with neighbouring atoms
 A covalent bond consist of two electrons one from each
adjascent atoms

18. Electron hole pair
 At room temperature the valence electrons in the crystal
may move away from the parent atom .
 Thus a covalent bond is broken
 when this happens electrons become free to move in the
crystal
 When an electron breaks a covalent bond and moves away
a vacancy is created in the broken covalent bond. This
vacancy of electron is called hole
 Whenever an electron is generated a hole is also created
simultaneously. Ie electrons and holes are generated in
pairs.
 The type of generation of free electron hole pair is referred
to as thermal generation

19. Random movement of carriers
 Electrons possess negative charge and holes possess
positive charge
 The amount of energy required to break a covalent
bond is 1.12 eV in Ge and 1.12 eV in Si.
 The conductivity of silicon will be less than that of
germanium at room temperature
 Both electrons an hole move randomly in the crystal
due to thermal generation and recombination of
electrons and holes inside the crystal

20. Drift current and Diffusion current
 When we connect a power supply across an intrinsic
semiconductor. The electrons experience an attractive force
towards the positive terminal and holes towards the
negative terminal.
 So the random movement gets modified . There is a
current in the external circuit due to the flow of electrons
and hole . When the flow of carriers is due to the applied
voltage the resultant current is called drift current.
 The flow of carriers (holes and electrons) due to difference
in carrier concentation from one region to the other is
called diffusion current which take place near the PN
junction.

21. N TYPE SEMICONDUCTOR
 When we add a pentavalent impurity such as
phosphorous, Arsenic, Antimony etc. in small amount
to an intrinsic semiconductor, the impurity atom
replaces a silicon atom in the crystal structure
 The phosphorous atom has five valence electrons .
Four of them form covalent bond with four
neighbouring silicon atoms the fifth electron does not
take part in covalent bond and is free to move.
 A small energy of the order of 0.01 for Ge and 0.05 for
Si is require to move this atom to conduction band
 At room temperature impurity atom donates one
electron to the conduction band so it is called donor
type impurity

22. N type semiconductor

23. Majority and minority carriers
 In N-tye material the amout of thermally generated
electron hole pairs are less when compared to electrons
donated by impurity atom.
 Since the concentation of electrons are high they combine
with thermally generted holes so hole concentration is very
less in N-type material
 So N type material has electrons as majority carriers and
holes as minority carriers
 The core of the impurity atom acquries an excess +ve
charge because the 5th electon leaves the atom and it’s gets
+5 charge and become an immobile ion since it is tightly
bound in the crystal

24. Representation of Ntype
semiconductor
 The N type semiconductor is electrically neutral
 In N type semiconductor there are a large number of
electrons denoted by black dots, a few holes as small
circles, and a large number of immobile positive ions
which are encircled with + sign.
 The charge of free electrons and holes which are
generated in pairs due to thermal energy are
compensated by each other
 The charge of electrons generated by donor atoms are
balanced by positive charge of immobile ions.

25. N TYPE SEMICONDUCTOR
 Semiconductor which are obtained by introducing a
pentavalent impurity atoms(ie atoms containing 5 valence
electrons) are known as N-type semiconductors
 Pentavalent impurity is from Vth group of periodic
table(Phosphorous, Antimony, Arsenic, Bismith)
 When a pentavlent impurity is added to a pure semiconductor
it diplaces some atoms. Out of 5 valence electrons 4 will form
covalent bonds with neighbouring atoms by sharing of
electrons from neighbouring atoms
 The 5th electron is loosely boud with the impurity atom and
may move away form impurity atom. Then it becomes a
immobile positive ion. It is also known as donor ion. The ion is
bound in the crystal by four semiconductor atoms and are not
able to move
 Majority carriers are electrons

26. P-TYPE SEMICONDUCTOR
 The semiconductor which are obtained by introducing a
trivalent impurity atom are known as P-type
semiconductors (Gallium, Indium, Aluminium, Boron etc)
ie III rd group elements in periodic table
 When a trivalent impurity is added to a pure
semiconductor it displaces some of its atoms
 The three electrons are shared by three nearby
semiconductor atoms and forms covalent bonds. The
fourth bond is incomplete. A vacancy exist in the
incomplete covalent bond constitute a hole
 The impurity atom captures electron from the surrounding
atom and becomes an immobile ion with one excess –ve
charge . It is also called as acceptor ion
 Majority carriers are holes

27. PN JUNCTON
 If we join a piece of P-type and N-type semiconductor
such that the crystal remains continuous at the
boundary as in figure a PN juncion is formed. Such a
device is called a Diode

28. Formation of depletion
layer
 We knows that electrons and holes are mobile charge carriers where as
ions are immobile
 The holes from P region diffuse to N region where they combine with
free electrons
 The electrons from N region diffuse to P region where they combine
with holes
 The diffusion of holes and electrons take place due to the difference in
concentration in the two regions. Ie P region has more no of holes and
N region has more no of electrons. The difference of concentration
creaes a concentration gradient across the junction. this result in
diffusion of charge carriers across the junction
 This diffusion of holes and electrons across the junction takes place for
a short time. The hole reaching the N region combine with electrons
leaving behind unbalanced +ve ions. The holes which reach the P
region near the junction combine with electrons there and leaving
behind un balanced –ve ions . So a depletion region is formed near the
junction containing no mobile charge carriers.
Formation of Depletion layer

29.  Further movement of charge carriers are repelled by the
unbalance ions in the depletion region
 Positive Donor ions in the depletion region of N type
material repel the holes from P region and Negative
acceptor ions in the depletion region of P type material
will repel electrons from N region.
 Additional holes trying to diffuse to N region are repelled
by positive charge of donor ions near the junction in the
depletion region
 Similarly addition electrons trying to diffuse to P region are
repelled by negative charge of acceptor ions
 The depletion region is also called as space charge region

30. BARRIER POTENTIAL
 The depletion layer of a PN junction has no mobile
charge carriers and it contains oppositely charged ions
on its two sides. Because of this charge separation an
electric potential is established across the junction
even when the junction is not connected to external
voltage source. This electric potential is called Barrier
potential VB=0.7 for Si and 0.3 for Ge

31. PN junction forward
biased
 When we connect the positive terminal of the battery to P side
and Negative terminal to N side of a PN junction. the junction is
said to be forward biased.This reduces the barrier potential
 When the applied voltage is grater than the barrier potential ie
>0.7v for Si and >0.3v for Ge the current start to flow through the
junction
 When junction is forward biased holes are repelled by positive
terminal of the battery and moves towards the junction. similary
electrons are repelled from the negative terminal of the battery
and drift towards the junction. this reduces barrier potential and
majoriy carriers diffuse across the junction.these carriers
recombine and cause movement of carriers in depletion region
 Cont….

32.  For each recombination of free electron hole that
occures an electron from the negative terminal of the
battery enters the N-type material and drift towards
the junction
 Similarly in P type material near the positive terminal
of the battery an electron breaks a bond in the crystal
and enters the positive terminal of the battery.for each
electron that breaks a bond a hole is created. the hole
drift towards the junction and recombine. Note that
there is a continuous flow of current in the external
circuit
 How Diode works

33. PN Junction reverse biased
 When the positive terminal of the battery is connected to N side and
negative terminal is connected to P side of the PN junction the
junction is said to be reverse biased
 The holes in P region moves towards negative terminal of battery and
electrons in N region moves towards positive terminal creating an
increse in potential barrier. The increased potential barrier makes it
more difficult for the majority carriers to diffuse across the junction
 The incresed barrer potential is helpful to minority carries in both P
and N region. The rate of generation of minority carriers are
temperature dependant so if the temperature is constant the rate of
generation of minority carrier is constant
 The current due to the flow of minority carrier is constant and is not
depend on applies voltage.this current is called reverse saturation
current. It is very small of the order of nano amperes in silicon and
microamperes in germanium

34. Reverse break down
 In a reverse biased PN junction the current is due to
movement of minority carriers
 If the reverse bias is made too high the current
through the PN junction increases suddenly. The
voltage at which this happens is called break down
voltage. At this voltage the crystal structure breaks
down. It will return to its previous state when excess
bias is removed provided that overheating does not
damage the junction

35. AVALANCHE AND ZENER BREAKDOWN
 When the reverse breakdown increases the electric field at
the junction also increases. High electric field causes
covalent bonds to break. Thus a large no of carriers are
generated causes a large current to flow. This breakdown is
called Zener breakdown
 Avalanche breakdown: in this case the increased reverse
voltage increases the amount of energy imparted to
minority carriers as they diffuse across the junction. this
inceases the velocities of minority carriers and they collide
with the semiconductor atom(Si) within the crystal
structure and break covalent bonds generating additional
charge carriers. These additional charge carriers pick up
energy from applied voltage and generate still more carriers
as a result reverse current increases rapidly. This
cumulative process of carrier generation is known as
avalanche beakdown.

36. Forward and Reverse chara of
Diode

37. Forward and Reverse chara of
Diode
 There are two operating regions and three possible
“biasing” conditions for the standard Junction Diode and
these are:
 1. Zero Bias – No external voltage potential is applied to the
PN junction diode.
 2. Reverse Bias – The voltage potential is connected
negative, (-ve) to the P-type material and positive, (+ve) to
the N-type material across the diode which has the effect
of Increasing the PN junction diode’s width.
 3. Forward Bias – The voltage potential is connected
positive, (+ve) to the P-type material and negative, (-ve) to
the N-type material across the diode which has the effect
of Decreasing the PN junction diodes width.

38. Forward chara of diode
 When a diode is connected in a Forward
Bias condition, a negative voltage is applied to the N-
type material and a positive voltage is applied to the P-
type material.
 If this external voltage becomes greater than the value
of the potential barrier, approx. 0.7 volts for silicon and
0.3 volts for germanium, the potential barriers
opposition will be overcome and current will start to
flow.

39. Reverse bias chara of PN diode

40. Rectifier Circuits
Rectifiers are the circuits used to convert alternating current (AC) into direct
current (DC).
There are two main types Half wave and Full wave rectifiers. In full wave rectifier
two types are there Centre taped and Bridge rectifier.
Half wave Rectifier

41. Half wave circuit operation
 The input we give here is an alternating current. This
input voltage is stepped down using a transformer. The
reduced voltage is fed to the diode ‘D’ and load
resistance RL.
 During the positive half cycles of the input wave, the
diode ‘D’ will be forward biased and during the
negative half cycles of input wave, the diode ‘D’ will be
reverse biased.
 We take the output across load resistor RL. Since the
diode passes current only during one half cycle of the
input wave, we get an output as shown in diagram. The
output is positive and significant during the positive.

42. Rectifier parameters
 Different parameters associated with the half wave rectifiers are
 Peak Inverse Voltage (PIV): This is the maximum voltage
which should be withstood by the diode under reverse biased
condition and is equal to the peak of the input voltage, Vm.
 Average Voltage: This is the DC content of the voltage across
the load and is given by Vm/π. Similarly DC current is given as
Im/π, where Im is the maximum value of the current.
 Ripple Factor (r): It is the ratio of root mean square (rms) value
of AC component to the DC component in the output and is
given by
 Vrms= Vm/2 and Vdc= Vm/П
 It is equal to 1.21
 Efficiency: It is the ratio of DC output power to the AC input
power and is equal to 40.6 %. (η=Pdc / Pac)

43. Full wave Centre
taped rectifier
Figure shows such a rectifier designed using a multiple
winding transformer whose secondary winding is equally
divided into two parts with a provision for the connection
at its central point (and thus referred to as the center-
tapped transformer),Two diodes (D1 and D2) and a
load resistor (RL).
Here the AC input is fed to the primary winding of the
transformer while an arrangement of diodes and the load
resistor which yields the DC output, is made across its
secondary terminal

44. •Figure 2a shows the case where the AC pulse is positive in nature i.e. the
polarity at the top of the primary winding is positive while its bottom will be
negative in polarity. This causes the top part of the secondary winding to
acquire a positive charge while the common center-tap terminal of
the transformerwill become negative.This causes the diode D1 to be forward
biased which inturn causes the flow of current through RL along the direction
shown in Figure 2a. However at the same time, diode D2 will be reverse biased
and hence acts like an open circuit. This causes the appearance of positive
pulse across the RL, which will be the DC output.
.

45.  Next, if the input pulse becomes negative in nature, then
the top and the bottom of the primary winding will acquire
the negative and the positive polarities respectively. This
causes the bottom of the secondary winding to become
positive while its center-tapped terminal will become
negative. Thus the diode D2gets forward biased while the
D1 will get reverse biased which allows the flow of current
as shown in the Figure 2b.
 Here the most important thing to note is the fact that the
direction in which the current flows via RL will be identical
in either case (both for positive as well as for negative input
pulses). Thus we get the positive output pulse even for the
case of negative input pulse (Figure 3), which indicates that
both the half cycles of the input AC are rectified.

46. BRIDGE RECTIFIER
 Bridge Rectifiers are the circuits which convert
alternating current (AC) into direct current (DC) using
the diodes arranged in the bridge circuit configuration.
They usually comprise of four diodes which cause the
output generated to be of the same polarity
irrespective of the polarity at the input.

47.  Now consider the case wherein the positive pulse
appears at the AC input i.e. the terminal A is positive
while the terminal B is negative. This causes the
diodes D1 and D3 to get forward biased and at the same
time, the diodes D2 and D4 will be reverse biased.
 As a result, the current flows along the short-circuited
path created by the diodes D1 and D3 (considering the
diodes to be ideal), as shown by Figure 2a. Thus
the voltage developed across the load resistor RL will
be positive towards the end connected to terminal D
and negative at the end connected to the terminal C.

48.  Next if the negative pulse appears at the AC input, then the
terminals A and B are negative and positive respectively.
This forward biases the diodes D2 and D4, while reverse
biasing D1 and D3 which causes the current to flow in the
direction shown by Figure 2b.
 At this instant, one has to note that the polarity of the
voltage developed across RL is identical to that produced
when the incoming AC pulse was positive in nature. This
means that for both positive and negative pulse, the output
of the bridge rectifier will be identical in polarity as shown
by the wave forms in Figure 3.

49. Performance of Full wave rectifier
 PIV = Vm
 Ripple factor=
 Vrms= Vm/√2
 Vdc=(2Vm)/П
 Ripple factor =0.48
 Rectification efficiency Pac/Pdc =81.2%