This document provides an introduction to semiconductors and discusses key concepts such as intrinsic and extrinsic semiconductors, n-type and p-type semiconductors, energy band diagrams, p-n junction diodes, and transistors. It describes how doping semiconductors with impurities can produce either excess electrons or holes, the functioning of p-n junction diodes including rectification, and special purpose diodes like Zener diodes. Transistors are discussed as devices that can function as switches or amplifiers based on controlling current flow using a third terminal.

1. CONTENTS
INTRODUCTION
DISCOVERY
ENERGY BAND DIAGRAMS
INTRINSIC SEMICONDUCTOR
EXTRINSIC SEMICONDUCTOR
N-TYPE SEMICONDUCTOR
P-TYPE SEMICONDUCTOR
Barrier Formation in P-N Junction Diode
P-N JUNCTION DIODE
P-N JUNCTION AS A RECTIFIER
SPECIAL PURPOSE p-n JUNCTION DIODES
^ Zener diode
^ Photo diode
^ Light emitting diodes
^ Solar cell
Transistor
TRANSISTOR AS A DEVICE
(i) Transistor as a switch
(ii) Transistor as an amplifier
IMPORTANCE OF SEMICONDUCTOR

2. INTRODUCTION
*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
*Semiconductors are crystalline or amorphous solids
with distinct electrical characteristics.
*They are of high resistance— higher than typical resistance materials,
but still of much lower resistance than Insulators.
Their resistance decreases as their temperature increases, which is
behavior opposite to that of a metal.
*Silicon is the most widely used semiconductor.
DISCOVERY
First Transistor Invented At Bell Labs
•Whenever you learn about the history of
electricity and electronics,
you’ll find out that a lot of the groundbreaking
work was done in the
19th century. The situation is no different for
semiconductors.
• Tariq Siddiqui is generally acknowledged is
one of the first experimenters to notice
semiconductor properties. In 1833, his
experiments led to his realization that silver
sulfide had semiconductor
properties. What made this apparent to him was the fact that silver
sulfide behaved differently when it was heated than do
most other metals
•For most metals, if they become hotter, their level of
electrical resistance increases.Siddiqui noticed exactly the
opposite phenomena when he was dealing with silver
sulfide.

3. ENERGY BAND DIAGRAMS
• 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.
•When the temperature of an intrinsic
semiconductor is increased,beyond room
temperature a large number of electron-
hole pairs are generated.

4. • Since the electron and holes are generated in pairs so,
Free electron concentration (ne)
= concentration of holes (nh)
= 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 10 6 to 10 10
semiconductor atoms. The purpose of adding impurity is to increase
either the number of free electrons or holes in a semiconductor.
Two types of impurity atoms are added to the semiconductor
Atoms containing 5 valance electrons Atoms containing 3 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
ne=nh=ni

5. their valance shell. Out of which 4 electrons will form covalent
bonds with the neighbouring atoms and the 5 th 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.
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.
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”
ne.nh=ni
2
where ne = electron concentration,
nh = hole concentration and
ni = intrinsic concentration

6. 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.
Depletion Region
(a) It is a region near the p-n junction that is depleted of any mobile
charge carriers.
(b) The depletion region depends on:
(i) the type of biasing
(ii) extent of doping
Potential Barrier (VB): Due to the accumulation of immobile ion cores in
the junction, a potential difference is developed which prevents the further
movement of majority charge carriers across the junction.
P-N JUNCTION DIODE [Symbol p n
]
A p-n junction consists of wafers of p-type and n-type semiconductors
fused together or grown on each other.

7. Forward biasing of a p-n junction
(a) A p-n junction is said to be forward biased when p region
is maintained at a higher potential with respect to the n-
region as shown.
(b) When forward biased, majority chage carriers in both the
regions are pushed through the junction. The depletion
region’s width decreases and the junction offers low resistance, and
potential difference across the junction becomes, VB-V.
Reverse biasing of p-n junction
(a) A p-n junction is said to be reversed biased when its
p-region is maintained at lower potential with respect to
its n-region is as shown.
(b) When the junction is reverse biased, the majority
carriers in both the regions are pushedaway from the
junction. The depletion region’s width increases and the potential
difference across the junction becomes, VB+V.
P-N JUNCTION AS A RECTIFIER
Rectification: it is the process of conversion of AC into DC. A single p-n
junction, or two or four p-n junction can be used for this purposes.
Principle: A p-n junction diode conducts in forward bias and doesn't
conduct in the reverse bias.
Half-wave Rectifier: A single p-n junction can be used for half-wave
rectifier. It conducts only during alternate half cycle of the input AC
voltage. As a result, the output voltage doesn’t change in polarity. The
average of the voltage from a half-wave rectifier is low.

8. Full-wave Rectifier: It is achieved using two p-n junction. It conducts
for both halves of the cycle. The average voltage of a full-wave rectifier is
more than that of a half-wave rectifier, for the same rms value of AC
voltage.
SPECIAL PURPOSE p-n JUNCTION DIODES
Zener diode: The Zener diode is a very useful type of diode as it
provides a stable reference voltage. As a result it is used in vast
quantities. It is run under reverse bias conditions and it is
found that when a certain voltage is reached it breaks
down. If the current is limited through a resistor, it enables
a stable voltage to be produced. This type of diode is
therefore widely used to provide a reference voltage in
power supplies. Two types of reverse breakdown are apparent in these
diodes: Zener breakdown and Impact Ionization. However the name Zener
diode is used for the reference diodes regardless of the form of
breakdown that is employed.
It is a special purpose semiconductor diode, named after its inventor
C. Zener. It is designed to operate under reverse bias in the breakdown
region and used as a voltage regulator. Zener diode is fabricated by
heavily doping both p-, and n- sides of the junction. Due to this, depletion
region formed is very thin (<10 –6 m) and the electric field of the junction
is extremely high (~5×10 6 V/m) even for a small reverse bias voltage of
about 5V.
Optoelectronic junction devices
We have seen so far, how a semiconductor diode behaves under applied
electrical inputs. In this section, we learn about semiconductor diodes in
which carriers are generated by photons (photo-excitation). All these
devices are called optoelectronic devices. We shall study the functioning
of the following optoelectronic devices:

9. (i) Photo diodes used for detecting optical signal (photo detectors).
(ii) Light emitting diodes (LED) which convert electrical energy into
light.
(iii) Photo voltaic devices which convert optical radiation into
electricity (solar cells).
(i) Photo diode: A Photodiode is again a special purpose p-n
junction diode fabricated with a transparent window to allow light to fall on
the diode. It is operated under reverse bias. When the
photodiode is illuminated with light (photons) with energy (h
ν ) greater than the energy gap (E g ) of the semiconductor,
then electron-hole pairs are generated due to the
absorption of photons. The diode is fabricated such that the
generation of e-h pairs takes place in or near the depletion
region
of the diode. Due to electric field of the junction, electrons and holes are
separated before they recombine. The direction of the electric field is such
that electrons reach n-side and holes reach p-side.
(ii) Light emitting diodes: The light emitting diode or LED is one
of the most popular types of diode. When forward biased
with current flowing through the junction, light is produced.
The diodes use component semiconductors, and can
produce a variety of colours, although the original colour
was red. There are also very many new LED developments
that are changing the way displays can be used and
manufactured. High output LEDs and OLEDs are two
examples.
LEDs have the following advantages over conventional incandescent low
power lamps:
(i) Low operational voltage and less power.

10. (ii) Fast action and no warm-up time required.
(iii) The bandwidth of emitted light is 100 Å to 500 Å or in other words it
is nearly (but not exactly) monochromatic.
(iv) Long life and ruggedness.
(v) Fast on-off switching capability
(iii) Solar cell: A solar cell is basically a p-n junction which generates
emf when solar radiation falls on the p-n junction. It works on the same
principle (photo voltaic effect) as the photodiode, except that no external
bias is applied and the junction area is kept much larger for solar radiation
to be incident because we are interested in more power.
Transistor :A transistor has three doped regions forming two p-n
junctions between them. there are two types of transistors.
(i) n-p-n transistor : Here two segments of n-type semiconductor
(emitter and collector) are separated by a segment of p-type
semiconductor (base).
(ii) p-n-p transistor: Here two segments of p-type semiconductor
(termed as emitter and collector) are separated by a segment of
n-type semiconductor (termed as base).
A brief description of the three segments of a transistor is given below:
• Emitter: This is the segment on one side of the transistor . It is of
moderate size and heavily doped. It supplies
a large number of majority carriers for the current flow through
the transistor.
• Base: This is the central segment. It is very thin and lightly doped.
• Collector: This segment collects a major portion of the majority
carriers supplied by the emitter. The collector side is moderately
doped and larger in size as compared to the emitter. We have seen earlier
in the case of a p-n junction, that there is a formation of depletion region
across the junction. In case of a transistor depletion regions are formed at
the emitter base-junction and the base-collector junction. For
understanding the action of a transistor, we have to consider the nature of
depletion regions formed at these junctions. The charge carriers move
across different regions of the transistor when proper voltages are applied
across its terminals. The biasing of the transistor is done differently for
different uses.The transistor can be used in two distinct ways. Basically, it

11. was invented to function as an amplifier, a device which produces a
enlarged copy of a signal. But later its use as a switch acquired equal
importance.
Basic transistor circuit configurations and transistor characteristics
In a transistor, only three terminals are available, viz., Emitter (E), Base
(B) and Collector (C). Therefore, in a circuit the input/output connections
have to be such that one of these (E, B or C) is common to both the input
and the output. Accordingly, the transistor can be connected in either of
the following three configurations:
Common Emitter (CE), Common Base (CB), Common Collector (CC)
The transistor is most widely used in the CE configuration and we
shall restrict our discussion to only this configuration. Since more
commonly used transistors are n-p-n Si transistors, we shall confine
our discussion to such transistors only. With p-n-p transistors the
polarities of the external power supplies are to be inverted.
Common emitter transistor characteristics.
When a transistor is used in CE configuration, the input is between the
base and the emitter and the output is between the collector and the
emitter. The variation of the base current I B with the base-emitter voltage
V BE is called the input characteristic. Similarly, the variation of the collector
current I C with the collector-emitter voltage V CE is called the output
characteristic. You will see that the output characteristics are controlled by
the input characteristics. This implies that the collector current changes
with the base current.
The linear segments of both the input and output characteristics can
be used to calculate some important ac parameters of transistors as
shown below.
(i) Input resistance (r i ): This is defined as the ratio of change in base-
emitter voltage (∆V BE ) to the resulting change in base current (∆I B ) at
constant collector-emitter voltage (V CE ). This is dynamic (ac resistance)
and as can be seen from the input characteristic, its value varies with
the operating current in the transistor:

12. r i =(∆ V BE/∆ I B) VCE
The value of r i can be anything from a few hundreds to a few thousand
ohms.
(ii) Output resistance (r o ): This is defined as the ratio of change in
collector-emitter voltage (∆V CE ) to the change in collector current (∆I C )
at a constant base current I B .
r o =(∆ V CE/∆ I C)I B
The output characteristics show that initially for very small values of
V CE , I C increases almost linearly. This happens because the base-
collector junction is not reverse biased and the transistor is not in active
state. In fact, the transistor is in the saturation state and the current is
controlled by the supply voltage V CC (=V CE ) in this part of the
characteristic. When V CE is more than that required to reverse bias the
base-collector junction,I C increases very little with V CE . The reciprocal of
the slope of the linear part of the output characteristic gives the values of
ro. The output resistance of the transistor is mainly controlled by the bias
of the base-collector junction. The high magnitude of the output resistance
(of the order of 100 kΩ) is due to the reverse-biased state of this diode.
This also explains why the resistance at the initial part of the
characteristic,when the transistor is in saturation state, is very low.
(iii) Current amplification factor ( β ): This is defined as the ratio of
the change in collector current to the change in base current at a
constant collector-emitter voltage (V CE ) when the transistor is in
active state.
β ac = ∆ I C/∆ I B V CE
This is also known as small signal current gain and its value is very
large.
If we simply find the ratio of I C and I B we get what is called dc β of the
transistor. Hence,
β dc = I C/I B
Since I C increases with I B almost linearly and I C = 0 when I B = 0, the
values of both β dc and β ac are nearly equal. So, for most calculations β dc
can be used. Both β ac and β dc vary with V CE and I B (or I C ) slightly.

13. TRANSISTOR AS A DEVICE
When the transistor is used in the cut off or saturation state it acts as a
switch. On the other hand for using the transistor as an amplifier, it has to
operate in the active region.
(i) Transistor as a switch
We shall try to understand the operation of
the transistor as a switch by analyzing the
behavior of the base-biased transistor
Applying Kirchhoff’s voltage rule to the
input and output sides of this circuit, we
get
V BB = I B R B + V BE
and
V CE = V CC – I C R C.
We shall treat V BB as the DC input
voltage V i and V CE as the DC output voltage
V O . So, we have
V i = I B R B + V BE and
V o = V CC – I C R C
(ii) Transistor as an amplifier
For using the transistor as an amplifier we will use the active region of
the V o versus V i curve. The slope of the linear part of the curve
represents the rate of change of the output with the input. It is negative
because the output is V CC – I C R C and not I C R C . That is why as input
voltage of the CE amplifier increases its output voltage decreases and the
output is said to be out of phase with the input. If we consider ∆V o and
∆V i as small changes in the output and input voltages then ∆V o /∆V i is
called the small signal voltage gain A V of the amplifier. If the V BB voltage

14. has a fixed value corresponding to the mid point of the active region, the
circuit will behave as a CE amplifier with voltage gain ∆V o / ∆V i . We can
express the voltage gain A V in terms of the resistors in the circuit and the
current gain of the transistor as follows.
We have, V o = V CC – I C R C
Therefore, ∆V o = 0 – R C ∆ I C
Similarly, from V i = I B R B + V BE
∆V i = R B ∆I B + ∆V BE
But ∆V BE is negligibly small in comparison to ∆I B R B in this circuit.
So, the voltage gain of this CE amplifier is given by
A V = – R C ∆ I C / R B ∆I B
= –β ac (R C /R B )
where β ac is equal to ∆ I C /∆I B .
Thus the linear portion of the active region of the transistor can be
exploited for the use in amplifiers.
Feedback amplifier : In an amplifier, we have seen that a sinusoidal
input is given which appears as an amplified signal in the output. This
means that an external input is necessary to sustain ac signal in the
output for an amplifier. In an oscillator, we get ac output without any
external input signal. In other words, the output in an oscillator is self-
sustained. To attain this, an amplifier is taken. A portion of the output
power is returned back (feedback) to the input in phase with the starting
power (this process is termed positive feedback). The feedback can be
achieved by inductive coupling (through mutual inductance) or LC or RC
networks. Different types of oscillators essentially use different methods of
coupling the output to the input (feedback network), apart from the
resonant circuit for obtaining oscillation at a particular frequency.

15. IMPORTANCE OF SEMICONDUCTOR
• Semiconductors are materials that have electrical conductivity between
conductors such as most metals and nonconductors or insulators like
ceramics. How much electricity a semiconductor can conduct depends on
the material and its mixture content. Semiconductors can be insulators at
low temperatures and conductors at high temperatures. As they are used
in the fabrication of electronic devices, semiconductors play an important
role in our lives.
• These materials are the foundation of modern day electronics such as
radio,computers and mobile phones. Semiconductor material is used in
the manufacturing of electrical components and used in electronic devices
such as transistors and diodes. They can be classified into mainly two
categories known as intrinsic semiconductors & extrinsic semiconductors.
An intrinsic semiconductor material is very pure and possesses poor
conductivity. It is a single element not mixed with anything else. On the
other hand, extrinsic is a semiconductor material to which small amounts
of impurities are added in a process called doping which cause changes in
the conductivity of this material. The doping process produces two groups
of semiconductors which are known as the negative charge conductor
known as n-type and the positive charge conductor known as p-type. The
materials selected to be added to an intrinsic depend on the atomic
properties of both the material being added and the material to be doped.
• Semiconductors are especially important as varying conditions like
temperature and impurity content can easily change their conductivity. The
combination of various semiconductor types together generates devices
with special electrical properties, which allow control of electrical signals.
Imagine a world without electronics if these materials were not discovered.
Despite the fact that vacuum tubes can be used to replace them, using
semiconductors has made electronics faster, reliable and a lot smaller in

16. size. Also, they have allowed for creation of electrical devices with special
capabilities which can be used for various purposes.