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Presentation Report on:
Intrinsic semiconductors and
Extrinsic semiconductors
N-type and P-type semiconductors
Forward and Reverse Biasing
Submitted by:
• Hamid Raza (BSP01183003)
• Hafiz Muhammad Sajjad Hussain (BSP01183067)
• Zulfiqar Mushtaq (BSP01183013)
• Obaid ur Rehman (BSP01183010)
Department of Physics
University of Lahore

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Brief history of Semiconductor:
Semiconductor any of a class of crystalline solids intermediate in electrical conductivity
between a conductor and can be insulator.
The development of studies in semiconductors materials is traced from its beginnings with
Michael Faraday in 1833.Faraday was the first person who noticed the behavior of silver sulfide
that the conducting power increases with increasing temperature on the other hand conductivity of
metallic conductors decrease as their temperature rises.
Semiconductor Material:
Solid state materials are commonly grouped into three class insulators, semiconductors and
conductors (At low temperature some conductors, semi-conductors and insulators many becomes
superconductors from figure.
Shows the conductivities and the corresponding resistivity’s that are associated with some
important materials in each of the three classes.
Insulators such as fused quartz and glass, have very low conductivities on the order of 10-18
to 1010
(Siemens per centimeter) and conductors such as aluminum have high conductivities
typically from 104
to106
(semen’s per centimeter).
The conductivities of semiconductors are between these extremes and are generally sensitive
to temperature, illumination, magnetic field, and minute amounts of impurity atoms. For instance
the addition of about 10 atoms of boron known as a (dopant) per million atoms of silicon can
increase its electrical conductivity.
Intrinsic Semiconductors:
A semiconductor which does not have any kind of impurities behave as an Insulators at 0K
and behave as a Conductors at higher temperature is known as Intrinsic Semiconductor.
This type of semiconductors is not doped. Its valance band completely filled and the
conduction band is completely empty. When the temperature is raised and some heat energy is
supplied to it, some of the valence electron are lifted to conduction band leaving behind holes in
then valence. The electron reaching at the conduction band randomly. This means that with the
increase in temperature, the resistivity of the materials decreases and the conductivity increase.
In Graph Forward biasing means putting a voltages across a diode that allows current to flow
easily, while reverse biasing means putting a voltage across a diode in the opposite direction.
The voltages with reverse biasing does not cause any appreciable current to flow. So, this is
useful for changing AC current to DC current.

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Extrinsic Semiconductors:
Extrinsic semiconductors are those in which impurities of large quantity are present.
Usually, the impurities can be either 3rd group elements or 5th group elements.
The phenomenon of adding impurities in semiconductors is called doping and materials those are
adding are called dopants. After doping, the materials becomes impure or extrinsic
semiconductors. Depending upon which type of impurities are doping
Extrinsic Semiconductors are divided into two classes are given below:
1. N-type Semiconductors
2. P-type Semiconductors
N-type Semiconductors:
When a small amount of pentavalent impurities, such as arsenic, antimony and bismuth, is
added to a pure semiconductors crystal, the resulting crystal is called the N-type semiconductors.
When a pentavalent impurities is added to silicon, the impurity atoms from covalent bonds
with the silicon atoms. Pentavalent impurities is called the donor type impurity as it donates one
electron to the conduction band of a semiconductor. Though each impurity provides enough
atoms to supply millions of free electrons. As shown in the figure:
The energy required to detach this fifth electron from the atom is of the order 0.05 eV for silicon
(0.01 eV for germanium). This energy is so small that at room temperature practically all such
electrons become free. In other words, at room temperature each impurity atom donates one
electron to the conduction band.

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These donated electron are called excess electron, because they are excess to the electrons which
are thermally generated by covalent bonds. An n-type semiconductors has electrons as majority
carriers, and the holes as minority carries.
P-type Semiconductors:
When a small amount of trivalent impurities such as boron, gallium, aluminum, is added to
a pure semiconductors, the result is known as p-type semiconductors.
When a trivalent impurity is added to silicon, these impurity atoms from covalent bonds
with four semiconductors atoms but one is left incomplete and gives rise to a hole, as shown in
figure. Such impurities make available positive carriers because they create holes which can
accept electrons. These impurities are also known as Accepter.
Since holes can be said to have a positive charge, accepter-doped semiconductor materials
is referred to as P-type semiconductors.
Conduction in p-type semiconductors:
In p-type semiconductors, the population of hole in valence band is more, whereas the
population of free electron in conduction band is less. So, current is mainly because of holes in
valence band. Free electrons in conduction band constitute little current. Hence in p-type
semiconductors, holes are called majority carriers and free electrons are called minority carriers.

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ForwardBias:
When an external field, with P-region connected to the positive terminal and N-region is
connected to negative terminal of battery, is applied across the junction. The junction is said to
be forward biased.
The holes on P side being positively charged particles are repelled from the positive bias
terminal and driven toward the junction. Similarly, the electros on N-side are repelled from the
negative bias terminal and driven toward the junction. The result is the depletion region is
reduced in width, and barrier potential is also reduced.
If the applied voltage is increased from zero, the barrier potential gets progressively smaller
and smaller until it effectively disappears and charge carriers can easily flow across the junction.
Electron from N-side are then attracted across to the positive bias terminal on P-side the
holes from P-side flow across to the negative bias terminal on N-Side. Thus a majority carrier
current flows. Since barrier potential is very small, a small forward voltage is sufficient to
eliminate the barrier completely.
Once the barrier is eliminated by application of forward voltage, junction resistance becomes
almost zero and a low resistance path is established in the entire circuit. The current called the
forward current, flows in the circuit.
The mechanism of current flow in a forward biased P-N junction is summed up as follows:
I. The electrons from negative bias terminal continue to arrive into N-region while the free
electrons in N-region move toward P-N region.
II. The electrons travel through N-region as free electrons.
III. These free electrons on reaching P-N region junction, combine with holes and become
valance electron. Since a hole is in covalent bond and therefore, when a free electron
combines with a hole, it become a valance electron.
IV. The electron travel through P-region as valance electron and current in this P-region.
V. These valance electron on reaching the left end of the crystal, flow into positive terminal
of battery.
We see that the current in N-type region is due to movement whereas in P-type region it is
carried by holes. However, in the external circuit the current is carried by electron.
Reverse Bias:
If the external voltage is applied with positive terminal to N-side and negative terminal to P-
side of a P-N junction, the junction is said to be reverse biased.
The electrons from N-side are attracted to the positive bias terminal and holes from P-side
are attracted to the negative bias terminal. Holes from the impurity atoms on P-side of the

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junction are attracted away from the junction, and the electrons are attracted from the atom on
the N-side of the junction.
Thus the depletion region is widened, and the barrier potential is increased by the magnitude of
the applied bias. With the increased barrier potential, there is no possibility of majority carrier
current flow across the junction. Thus P-N junction is in non-conductive state.
I. The barrier potential is increased and width of depletion is also increased.
II. The junction offers high resistance called the reverse resistance rr , to flow of
current.
III. Owing to establishment of high resistance path, and very small current flow called
leakage current or reverse current, flows through the circuit.
IV. The reverse or leakage current is not affected by barrier and depend on temperature,
material and construction of junction.
Thus the P-N junction allow of flow of large current only when forward biased and only small
revere current when reverse biased.
SpeciallyDesignedP-N junction
Semiconductor diodes are not only used for rectification but they are used for some other special
purposes. The most commonly used diodes are as follow:
Light emitting diodes (L.E.D):
Definition:
“The light emitting diode is a specially designed junction diode which radiate light energy when it is
forward biased.”
Construction:
LEDs are made of special semiconductors e.g.,gallium arsenide phosphide (GaAsP) and gallium
arsenide (GaAs) etc. Which radiate different colors of light e.g., red, green, orange, etc. When they are
forward biased.
Working:
We know that when a diode is forward biased electrons move from N side to P side therefore
recombination of electrons and holes take place.
As, the electrons move from higher energy to lower energy so each combination of electron hole results in
the form of photon.
The wavelength of light emitted by a photon depends upon the following factors.
a) Energy level of electron
b) Nature of semiconductor material

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Uses of LED:
 LEDs are used as small source of light or indicators
 LEDs are used in optical fiber transmitter, to send electrical signals.
 A special designed seven segmented LED is used for displaying many electronic calculators’
clocks and measuring instruments etc.
Advantages of LED:
The advantages of LED over filament lamp are:
i. Small size
ii. Reliability
iii. Long life
iv. High operating speed
v. Economical
Photodiode:
“A device which is used to convert high energy into electrical energy and is used to detect the
intensity of light is known as photodiode”
Construction:
A photodiode is a semiconductor diode with a built in lens to focus a parallel beam of incident light.
It is connected in series with a low resistance R and a source of emf. a photodiode is operated reverse
biased commonly.
Working:
The working principle of photodiode is based on photoelectric effect. When light is made incident
on a reverse biased diode then reverse current passes through the external circuit, when no light is
incident on the diode then reverse current is zero.
When light falls on the junction the electron hole pairs, are generated the number of generated
electron-hole pairs is directly proportional to the intensity of incident light.
Application of photodiode:
i. It is used as signal detector in optical fiber receivers.
ii. It can detect both visible and invisible light.
iii. It is used in logic circuits.
iv. It is used in automatic switching circuits.
v. It is used as very fast counter (which generates a pulse of current every time a beam of light is
interrupted).

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Photovoltaic cellor solarcell:
Definition:
“A semiconductor which converts the light energy into electrical energy when it is unbiased
is known as photovoltaic cell or solar cell.”
Construction:
Photovoltaic cell is actually a semiconductor diode, which consists of a thick N-type region
covered by a thin P-type layer.
Working:
When light is made incident on such a P-n junction having no external biasing, then the
potential barrier between p and n reigns is used to drive a current in the external circuit.
As, the incident light produces minority carriers in the form of electron hole pairs in p type
and n type regions. The current is directly proportional to the intensity of light. A simple silicone
photovoltaic cell produces about 0.6V and few milli-amperes current. In order to get a greater
voltage, series array of such cells are used in the form of solar panels, in satellite and space
vehicles.

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