BS Physics (Foundation) Series

1. 1
Module # 25
Doping, Diode & Transistor
Majority and Minority Charge Carriers
An extrinsic semiconductor either p or n-type has a large number
of charge carriers. Due to the effect of impurity, p-type
semiconductor has a large number of holes whereas n-type
semiconductor has a large number of free electrons. In a p-type
semiconductor, the number of holes are much larger than the
number of electrons. Therefore, in such type of semiconductor,
holes are called majority carriers and electrons as minority
carriers.
Similarly, in an n-type semiconductor, the number of electrons are
much larger than the number of holes. Therefore, electrons are
called majority carriers and holes as minority carriers in an n-type
semiconductor.
In order to obtain the desired conduction properties, the pure
Silicon or Germanium is doped with minute amount of selected
elements as controlled impurities. The impurities are added at the
rates of one atom of doping element to 105
to 108
atoms of semi-
conductor.

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By addition of pentavalent impurities such as Antimony,
Phosphorus or Arsenic, we have created a material having only
electrons as free and mobile charges. Such a semi-conductor is
said to be n-type material. The pentavalent impurities which have
donated the electrons are called donor impurities.
In an n-type semiconductor, the conduction will be predominantly
by electrons and the electrons are called the majority carriers.
There will be a few holes present in n-type material due to
electron hole pair generation at usual ambient temperatures. The
holes present in an n-type material are known as the minority
carriers.
The use of trivalent impurity elements such as Boron, Indium, or
Gallium has created a material with free holes, so conduction
occurs by hole transfer and impurity semiconductor is said to be
p-type material. The trivalent impurity atoms accept electrons to
fill their bond vacancies and are called acceptor atoms and the
impurities as acceptor impurities.
The conduction in a p-type material will be primarily due to holes
and they are known as majority carriers. There will be electrons
present in a p-type material due to thermal pair-generation. These
electrons in a p-type material are known as minority carriers.

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If a silicon crystal is doped with a pentavalent element such as
arsenic, then, four out of five valence electrons of arsenic atom
form four covalent bonds with four neighboring silicon atoms. The
fifth electron is free to move about, making the doped silicon
crystal a better conductor. This material is called an n-type
semiconductor because the fifth valence electron of each
pentavalent atom is available to form an electric current.
Germanium can also be made an n-type semiconductor when
doped with pentavalent element.
Perfectly, pure silicon and germanium crystals are almost
complete insulators at low temperatures. This is because all the
four valence electrons of each atom form covalent bonds with
their neighboring atoms. Since these electrons are tightly held in
covalent bonds, there are no free electrons and, therefore, no
current flows on the application of a potential difference.
The germanium or silicon crystal formed after adding a
pentavalent impurity (i.e. atoms containing five valence electrons)
such as phosphorus, antimony, arsenic, bismuth etc. is called n-
type semiconductor.
N-type semiconductor is formed after adding a pentavalent
impurity such as phosphorus, antimony, arsenic, bismuth etc. in a
germanium or silicon crystal.

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If a silicon crystal is doped with a trivalent element such as
indium, all three available valence electrons of an indium atom
form three covalent bonds with three neighboring silicon atoms. A
gap (space) called a hole is left in the silicon crystal due to the
shortage of an electron. This hole behaves like a positive charge
and can move from place to place in the crystal on the application
of potential difference. Such a material is known as a p-type
semiconductor because of an excess of positive charge
(deficiency of negative charge). It is worthy to note that holes flow
(move) in directions opposite to that of electrons.
In the same way, the germanium or silicon crystal formed after
adding a trivalent impurity (i.e. atoms containing three valence
electrons) such as indium, gallium, aluminum etc. is called p-type
semiconductor.
Acceptor Impurities
Group III impurities are called acceptor impurities.
Donor Impurities
Group V impurities are called donor impurities.
Doping
The electrical conductivity of silicon or germanium can be
increased by adding (to them) a small amount of an element

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which has either three or five valence electrons in its atoms. The
process is known as doping.
In other words, the process of adding impurity in a pure
semiconductor material is called doping.
Generally, the impurities are added at the rate of only one atom of
impurity per 106 to 1010 semiconductor atoms. For germanium,
the impurity level is less than 1 part in 108
parts of germanium
and for silicon; it is less than 1 part in 1012
parts of silicon.
Forward Biased
When p-end of the semiconductor diode is connected to the
positive terminal of a battery or D.C. power supply and its n-end is
connected to negative terminal, then, the p-type material will be at
a positive potential and the n-type material will be at a negative
potential. The positive holes will now drift from the p-type material
to the n-type material across the junction while free negative
electrons will move from the n-type to the p-type material. This is
because the positive terminal repels positive holes in p-type
material towards the junction, while, the negative terminal repels
negative electrons in n-type material towards the junction. Under
this condition, an electric current can easily pass through the
junction. The diode, under this condition, has a very low electrical
resistance and is said to be forward biased or in a condition of

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forward bias.
Thus, forward biased is an external voltage which is applied to a
PN junction to reduce its barrier and permitting current flow in it.
Reverse Biased
If the semiconductor diode is connected to a battery or D.C power
supply in such a way that p-type and n-type materials are
connected with the negative and positive terminals respectively,
then, the positive holes in p-type material and the negative
electrons in n-type material will be attracted toward negative and
positive terminals respectively and will, therefore, move away
from the junction. There will be no flow of current across the
junction. The region around the junction will lose its charge and
will become an insulator. This condition, in contrast with the
forward biased condition, widens the depletion layer and also
increases the potential barrier. Since, there is no electron-hole
combination, so, no current flows. The diode offers very high
electrical resistance and is said to be reverse biased.
Thus, reverse biased is an external voltage which is applied to a
PN junction to increase its barrier.

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Depletion Layer & Depletion Region
At the instant the PN junction is formed, some of holes in P-region
and the free electrons in the N-region diffuse in each other and
disappear due to recombination. During the process, the electrons
where they exit as minority carriers in the p-region and holes in
the N-region where they also exist as minority carriers are
uncompensated in the vicinity of junction. The additional holes
trying to diffuse into the N-region are repelled by the
uncompensated positive charge. Similarly, the electrons trying to
diffuse into the p-region are repelled by the uncompensated
negative charge. Hence, the further diffusion of free electrons and
holes across the junction is terminated.
The region containing positive charge on N-side and negative
charge on P-side is called depletion region. It is called so because
the mobile charge carriers i.e. free electrons and holes have been
depleted (emptied) in this region. As the uncompensated charges
within the depletion region exist in the form of parallel plates of
opposite charges, therefore, it is known as depletion layer.
Width of depletion layer depends upon the doping level of the
impurity in p-type or n-type semiconductor. The higher the doping
level, the thinner will be the depletion layer and vice-versa. The
reason is that a highly doped PN junction contains a large number

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of electrons and holes; hence a diffusing charge carrier has not to
travel across the junction for recombination. The emerge of
electron in hole is called recombination.
Behavior of Depletion layer
The depletion layer behaves like an insulator.
Integrated Circuits
An integrated circuit is one in which circuit components such as
transistor, diodes, resistors and capacitors etc. are fabricated on a
small semiconductor chip.
Basically, there are two general types of integrated circuits analog
and digital.
An integrated circuit is a device that integrates (combines) both
active components such as transistors, diodes etc. and passive
components such as resistors, capacitors etc. of a complete
electronic circuit in a single chip.
These integrated circuits are being used in all types of
instruments and equipment such as toys, washing machines,
industrial control circuits, radio, TV, VCR, robots, and computers
etc. This is due to the integrated circuits that we have very small
radios and TVs etc.

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An integrated circuit, abbreviated as IC is an electronic circuit in
which both active components (transistor, diode, etc.) and passive
components (resistors, capacitors, etc.) are fabricated on a single
small silicon chip (a tiny slice or wafer of semiconductor crystal).
The individual components cannot be removed or replaced within
it. The size of an IC is so small that we need a powerful
microscope called Scanning Electron Microscope (SEM) to see
the connections between the components. The normal dimension
of an integrated circuit is about 0.2 mm x 0.2 mm x 0.001 mm.
Advantages of IC
An integrated circuit has several advantages over conventionally
wire circuit of discrete components. These advantages include
1 A drastic reduction in size and weight.
2 (It has shown) A large increase in reliability.
3 (It is) Very easy to replace by a new one.
4 (Its) Reduced cost has become possible, as hundreds of
identical Integrated Circuits can be built simultaneously on a
single silicon chip.
5 (It has) Greater ability to operate at extreme values of
temperature.

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6 (Its power requirement is low) Low power requirements.
Limitations (of IC)
The integrated circuit has some limitations as given below:
The coils or inductors cannot be fabricated.
It is neither convenient nor economical to fabricate capacitances
exceeding 30 pF.
It is not possible to produce integrated circuit capable of handling
large amount of power i.e. greater than 10 watt.
Integrated circuit requires low voltages ranging from 3 to 30 volts.
Isolation between components is poor.
Range of values of passive components used in the circuit is
comparatively small.
Analog IC's
In analog type of circuit outputs are generally proportional to the
inputs.
Analog integrated circuits perform amplification and other
essential linear operation on signals. These circuits are widely
used in aircrafts, space vehicles, radars, communication system,
oscilloscopes, televisions, audio and video cassette recorders etc.
Analog ICS are also used in analog computers.

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Some of the important forms of analog integrated circuits are
1 Small signal amplifier
2 Microwave amplifiers
3 Voltage comparators multipliers
4 Power amplifier
5 RF and IF amplifier
6 Voltage regulators
Digital IC's
In digital type of (integrated) circuits both inputs and outputs can
have only two values i.e. 0 and 1. The digital integrated circuits
are widely used in computers, calculators and digital watches etc.
Some of the important forms of digital integrated circuits are
1 Logic circuit
2 Counters
3 Microprocessor chips
4 Coder and encoder
5 D/A and A/D converter chips
6 Flip flops
7 Memory chips

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8 Clock and Calculators Chips
9 Multiplexer and de-multiplexer
Potential Barrier in Semiconductors
As the depletion layer builds up, a difference of potential appears
across the junction because of the negative charges on one side
and the positive charges on the other. Eventually, this difference
of potential becomes large enough to prevent hole and free
electron movement across the junction. The difference of potential
at the junction is called potential barrier.
Potential barrier is an electrostatic field which has been created
by the joining of a region of N material with a region of P material.
Since holes and electrons must overcome this field to cross the
junction, the electrostatic field is commonly called a barrier.
Because there is a lack or depletion of free electrons and holes in
the area around the barrier, this area is called depletion region.
P-N Junction Diode
A p-n junction is known as a semiconductor diode. A p-n junction
diode is an electronic device formed from a p-type semiconductor
and an n-type semiconductor.
A small amount of indium is placed on a plate (called wafer) of n-
type germanium. Indium on being heated to about 550°C melts

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and diffuses through a small part of the n-type germanium. Indium
being the p-type impurity converts the part of the n-type natural
germanium to p-type material.
Thus, a junction is formed between a p-type and n type materials.
A brass base is used to fix the p-n junction to which leads are
attached. The whole assembly is then sealed in a metal or glass
container. Due to an excess of mobile positive charges in p-type
material and an excess of electrons in n-type material, the holes
from p-type material and electrons from n-type material move
across the junction and combine. The migration of electrons from
n-type material to p-type material forms a layer of electrons on the
p-type material. Similarly, the migration of holes from p-type
material to the n-type material forms a layer of positive charge on
the n-type material. A potential difference is developed across the
junction due to these charged layers and this potential difference
prevents further migration or flow of charge from one side to the
other side.
P-type n-type
Such a device is called a semiconductor diode and is symbolically
represented as shown in the figure above. PN Junction diode

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makes use of the rectifying properties of a PN junction to convert
alternating current into direct current by permitting current flow in
only one direction. Therefore, a p-n junction or a semiconductor
diode is also known as a rectifier.
A junction between p and n materials forms a semi-conductor
diode. It contains donor impurities on one side and accepter
impurities on the other side of a single crystal of Germanium or
Silicon during the process of its growth.
The semi-conductor diode has the property of one way
conduction, i.e., it allows the current to flow only in one direction.
The flow of current is practically zero in the opposite direction; it
acts as a diode. This can be understood by considering an
example of water flow (from higher gravitational level to a lower
one) in which the reduction in the height of obstruction or barrier
increases the water flow and vice-versa. Thus, if we want to send
the current across a p-n junction, we have to reduce the height of
the potential barrier at the junction.
A diode is an interesting device. It is a semi-conductor. The Diode
is the most basic of semiconductors. Its function is to allow
electricity to flow in one direction, but not the other. It is like a one
way valve for electricity.
There are special terms used for each side of the diode. The

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positive side of the diode is called the ANODE. The negative side
is called the CATHODE. Current always flows from the ANODE to
the CATHODE.
As a p-n junction diode always allows an electric current to pass
through it in one direction (when forward biased), so, it is used to
convert an alternating current (A.C) to direct current (D.C). A
semiconductor diode is also known as a rectifier.
The conversion of A.C. to D.C. is shown in the figure (a) given
below.
Fig: (a) Half Wave Rectification, Fig: (b) Full Wave Rectification
The input voltage V is an alternating voltage which has to be
rectified. During the positive half cycle of V, the p-side of the
diode is positive and repels the holes towards the junction and
hence resistance of the junction is reduced which causes the
current to flow through it and the potential across resistor R
becomes V0. On the other hand, during the negative half cycle,
the p-side of the diode is negative, and it attracts holes which

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results in the increase of junction resistance. Due to this increase
in resistance, it does not conduct and no current passes through
R. The voltage drop across R becomes zero i.e., V0 = 0. Hence,
we see that the current through a diode passes only during
positive half of the A.C. cycle and rectification is achieved. This is
called half-wave rectification. For full-wave rectification, we use
two diodes in the circuit as shown in Figure (b).
Rectifier
A rectifier is a device that converts an alternating current to a
direct current and the phenomenon itself is known as rectification.
A p-n junction diode is used as a rectifier because it allows the
current to flow in one direction only (when forward biased).
Rectifier is a circuit which uses one or more diodes to convert a.c
voltage into pulsating d.c. voltage.
The diode offers very small resistance (zero in ideal case) in the
forward direction but very large resistance in the reverse direction
(infinite in ideal case). Thus, the diode can act as a switch. It is
due to this important property of diode that it can be used for
rectification, i.e., to convert alternating current into pulsating direct
current. In most of the electronic devices (Radio, T.V. etc.) the
diode serves as a rectifier.

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Light Emitting Diode (LED)
LED stands for Light Emitting Diode. An LED is a diode with a
unique property that when electricity flows through it, it emits light.
These are extremely useful, require very little power to operate,
and you (we) will find them to be very useful. The ANODE and
CATHODE are the same as on a regular diode. The way you
know that it is an LED is by the little arrows pointing away from it.
That is supposed to represent light energy being transmitted.
A light emitting diode basically is a p-n junction diode. Energy is
released when electric current passes through such a junction. If
the energy released is in the form of light (photons), then, it is
called a light emitting diode (LED). The wave length of light
emitted depends upon the energy levels of electron and nature of
the semiconductor.
Photo Diode & Photovoltaic Cell (Solar Cell)
A photodiode is a semi-conductor device usually made from
silicon.
A photovoltaic cell (solar cell) is a device which converts the light
energy into electrical energy. When light is allowed to fall on this
cell, the cell generates a voltage across its terminals. This voltage
increases with increase in the light intensity. The cell is so

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designed that a large area is exposed to light which enhances the
voltage generation across the two terminals of the cell.
The photovoltaic cell is basically a junction device that is made
from semiconductor materials. The device is usually made from
either Silicon or Selenium although many other materials are in
use.
Photocells are commonly used in satellites and spacecrafts to
convert solar energy into electrical power. The electrical energy
so generated can be used to operate other electronic equipments.
Transistor
In 1948, three American scientists, Shockley, Bratain and
Bardeen invented a tiny, three terminal semiconductor device
called a transistor which produced a revolution in the world of
electronic devices. Transistors are now being used in radio and
television sets, audio and video cassette recorders, voltage
stabilizers, telephone sets, computers and numerous other
devices.
Transistor is a three element solid state device that amplifies by
controlling the flow of current carriers through its semiconductor
materials.
Construction

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A transistor is basically a semiconductor which consists of a thin
(3-5m thick) central layer of one type of semiconductor material
sandwiched between two relatively thick pieces of the other type.
There are two types of transistors: n-p-n and p-n-p. The n-p-n
transistor has a thin piece of p-type material sandwiched between
two pieces of n-type, while, the p-n-p transistor consists of a piece
of n-type in between two pieces of p-type. The central part is
known as the base (b) and the pieces at either side are called the
emitter (e) and the collector (c). In circuit diagrams, the n-p-n and
p-n-p transistors are represented by the symbols given in the
Figure below.
Fig: (a) Symbolical Representation of p-n-p Transistor
Fig: (b) Symbolical Representation of n-p-n Transistor

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The only difference in symbology between the two transistors is
the direction of the arrow on the emitter. If the arrow points in, it is
a PNP transistor and if it points outward, it is NPN transistor.
The arrow on the symbol gives the direction in which conventional
(positive) current would flow.
Operation
A transistor consists of two P-N junctions. One is the emitter-base
(E-B) junction and the second one is collector-base (C-B)
junction. For normal transistor operation, E-B junction is always
forward biased (i.e. p-type +ve and n-type -ve), whereas the C-B
junction is reversed biased (i.e. p-type -ve and n-type +ve).
Let us take the case of an N-P-N transistor.
As the base-emitter P-N junction is forward biased (i.e. p-type +ve
and n-type -ve), so its resistance is low and a current begins to
flow across this junction by injecting a stream of electrons from
the emitter to base and Ie current starts flowing in emitter circuit.
The resistance R controls this current. As the base region is very
thin and is lightly doped, therefore, practically all the electrons
injected into the base region are attracted towards the collector
due to its large positive potential. Due to small base potential,
only a few electrons enter in base circuit, Ib current flows in base

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and Ic in collector circuit. Since the emitter current is divided into
base and collector currents, therefore,
Ie = Ib + Ic
That is, the current leaving the transistor is equal to the current
entering the transistor.
Path of Current
Thus there are two current paths through a transistor. One is the
base-emitter path (input) and the other is the collector-base path
(output). The importance of a transistor is due to the fact that if
the base-emitter potential or base current Ib is changed by a small
amount, then, the collector current Ic changes by a large amount.
Base of a Transistor
The middle portion (of the transistor) which forms two PN
junctions between the emitter and collector is called the base (of
the transistor). The base of the transistor is thin as compared to
the emitter and is a lightly doped portion. The base emitter
junction is forward biased allowing low resistance in the emitter
circuit. The base collector junction is reverse biased showing high
resistance in the collector circuit.
Collector of a Transistor
The portion on the other side of the transistor (i.e. the side

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opposite to the emitter) that collects the charge carriers (i.e.
electrons or holes) [is called the collector]. The collector is always
larger than the emitter and the base of a transistor. The doping
level of the collector is in between the heavy doping of the emitter
and the light doping of the base. In both PNP and NPN transistors
the collector base junction always should be reverse biased.
Collector of PNP transistor receives hole charges that flow in the
output circuit. Similarly, the collector of NPN transistor receives
electrons.
Emitter of a Transistor
The portion on one side of transistor that supplies charge carriers
(i.e. electrons or holes) to the other two portions (is called
emitter). The emitter is a heavily doped region. The emitter is
always forward biased with respect to base so that it can supply a
large number of majority carriers. In both PNP and NPN
transistors emitter base junction always should be forward biased.
Emitter of PNP transistor supplies hole charges to its junction with
the base. Similarly, the emitter of NPN transistor supplies free
electrons to its junction with the base.

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Transistor as an Amplifier
A transistor has a very important property that it can raise the
strength of a weak signal. This property is called amplification.
In a transistor, the base current Ib plays a vital role in the collector
current Ic. A small change in the base current produces a large
change in the collector current. Due to this characteristic, a
transistor is used as an amplifier in various electronic circuits. A
block diagram of an amplifier is shown in the figure below.
Fig: An Amplifier changes a small Voltage into a large Voltage
Output
Transistors can be used as oscillators, switches, memory units
and they perform many other useful functions. Their
characteristics are compactness, reliability including ruggedness,
very long lifetimes and low power consumption. They have
caused a major revolution in computers and electronic control
circuits.
The only drawback with transistor is that its characteristics are
strongly temperature dependent. However, the effects of these
variations in transistors can be decreased by a careful circuitry of
transistor equipment.

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In a transistor the base current Ib plays a vital role in the collector
current Ic. A small change in the base current produces a large
change in the collector current. Due to this characteristic, a
transistor is used as an amplifier in various electronic circuits.
An amplifier is the device that provides amplification without
appreciably altering the original signal. The basic transistor
amplifier amplifies by producing a large change in collector
current for a small change in base current. This action results in
voltage amplification because the load resistor placed in series
with the collector reacts to these large changes in collector
current which, in turn, results in large variations in the output
voltage.
Transistor as a Switch
The transistor is also used as a switching device. Anything which
can "switch on" a small base current to a transistor will release a
large collector current to operate a lamp, loudspeaker or relay.
The relay could, in turn, operate a bell or a motor, etc. Transistors
are exceedingly small and light and they are not easily broken.
They produce little heat and they can be used with very small
potentials.