Types of Materials

Semiconductors The process through which transfer of

germanium, heat or electricity takes place inside a
silicon and gallium arsenide material due to the potential difference
between the end points is
called Conduction
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 Types of Materials – Semiconductor
Materials
Semiconductors are classified as
I. Intrinsic (pure), A missing electron in the
II. Extrinsic (impure) valence band leaves a
a) N-type vacant space there,
b) P-type. which is known as a hole

Intrinsic Semiconductor:

❑ A pure semiconductor is called an intrinsic semiconductor.

❑ Even at room temperature, some of the valence electrons may acquire sufficient
energy to enter the conduction band to form free electrons.
❑ A missing electron in the valence band leaves a vacant space there, which is known as
a hole, it also contribute to electric current.
❑ Due to the poor conduction at room temperature, the intrinsic semiconductor as such,
is not useful in the electronic devices.
❑ The current conduction capability of the intrinsic semiconductor should be increased
by adding a small amount of impurity to the intrinsic semiconductor

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 Extrinsic Semiconductor Materials
Extrinsic semiconductor if formed by adding a small amount of impurity to the
intrinsic semiconductor. This process of adding impurity is known as doping.
N-type Semiconductor P-type Semiconductor
A small amount of pentavalent A small amount of trivalent impurity
impurity such as arsenic, antimony, such as aluminium or boron is added to
or phosphorus is added to the pure the pure semiconductor germanium to
semiconductor (germanium or get the P-type semiconductor. The
silicon crystal) to get an N-type germanium (Ge) atom has four valence
semiconductor. electrons and boron has three valence
electrons
one bond incomplete
which gives rise to a
hole

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 Difference between N type and P type
N-type materials are type of materials P-type materials are a type of materials
formed by adding group 5 elements formed when group 3 elements (trivalent
(pentavalent impurity atoms) to the impurity atoms) are added to the solid
semiconductor crystals and conduct the crystal. In these semiconductors the
electric current by movement of current flow is mainly due to the holes.
electrons.
In N-type Semiconductors In P-type Semiconductors

❑ The impurity atoms are pentavalent ❑ The impurity atoms are trivalent
elements. elements.
❑ Impurity elements with solid crystal ❑ Trivalent elements results in excess
give a large number of free electrons. number of holes which always accepts
❑ Pentavalent impurities are also called electrons. Hence trivalent impurities
as donors. are called as acceptors.
❑ Doping gives the less number of ❑ Doping gives the less number of free
holes in relation to the number of free electrons in relation to the number of
electrons. holes.

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 PN Junction
p-type and n-type semiconductor materials have been shown side by side, a junction
called p–n junction is formed

Electrons are negative charge carriers while holes are positive charge carriers.

❑ when an electron moves out of an ❑ addition of an electron in a hole makes

atom, the atom becomes a positively an atom a negatively charged immobile
charged ion which is immobile, i.e., ion.
unable to move.

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 What happens to the electrons and holes at
a p–n junction.
1. At the p–n junction there will be a 3. Negative ions created on the p-side close
tendency of the free electrons from the n- to the junction will acquire a negative
type material to diffuse into the p-side and voltage and the positive ions created on the
combine with a hole nearest to the n-side close to the junction will acquire a
junction. positive voltage.
2. The free electron crossing over from the 4. The negative voltage on the p-side will
n-side to the p-side will leave behind repel further diffusion of electrons from
positive immobile ions on the n-side of the n-side. The positive voltage on the n-
the junction. The electrons crossing over side will repel diffusion of holes from the
the junction will occupy the holes in the p- p-side.
type material making the atoms negatively
5. When a p–n junction is made there is an
charged immobile ions.
initial diffusion of electrons and holes
which creates a barrier voltage at the
junction, which stops any further diffusion
of charge carriers.
6. This initial diffusion of charge carriers at
the junction, and the development resultant
barrier voltage take place when a p–n
junction is formed
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 Definitions
The shaded portion on both sides of the p–n junction is having only immobile ions of
opposite polarities which creates a potential difference, i.e., barrier voltage. This
portion is devoid of any electron or hole, i.e., any charge carriers. This region
is depleted of any charge carrier, and hence is called the depletion region.

Diffusion current is a current in a semiconductor caused by the diffusion of charge

carriers (holes and/or electrons).

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 Biasing PN Junction Diode
Application of some external voltage across the two sides of the p–n junction
❑ When the p-side is connected to the positive terminal of a battery and the
n-side is connected to the negative terminal, the p–n junction is said to be a
forward-biased junction
❑ The positive terminal of the battery is connected to the n-side and the
negative terminal on the p-side, the p–n junction is said to be a reverse-biased
junction.

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 Forward biased P-N junction
1) The holes on the p-side are positively charged
and the electrons on the n- side are negatively
charged.
2) When forward biased, the positive terminal
of the battery will repel the holes from the
terminal. Similarly, electrons on the n-side
will be repelled from the negative terminal of
the battery.
3) Holes travel through P material towards
negative charge at PN junction , thus
neutralize partly the negative charge.
4) Similarly the electrons in N layer neutralize
the positive charge.
5) The width of the depletion layer will be
reduced. The potential barrier will also get
reduced.
6) If the applied voltage is gradually increased,
the depletion region and barrier potential will
disappear

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 V–I Characteristics of a Diode under
Forward Bias In forward-biased p–n
1) When the voltage is gradually
junction, the potential
increased from zero voltage to 0.3 V,
barrier is neutralized
for the germanium semiconductor
allowing current flow.
the barrier voltage is there. When the
barrier voltage is overcome, the
depletion layer disappears.

2) Electrons from the n-side are

attracted by the positive terminal A
of the p-side and the holes from the
p-side get attracted by the negative
terminal B of the n-side.

3) The potential barrier is overcome at

0.3 V for germanium and at 0.7 V for
silicon, the majority charge carriers
start moving across the p–n junction
establishing a forward current, If to
flow
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 Reverse-biased p–n junction
1. The negative terminal of the battery is
connected to the p-side and the positive
terminal is connected to the n-side of the p–
n junction
2. Electrons from the n-side are attracted to
the positive terminal of the battery and the
holes from the p-side get attracted to the
negative terminal of the battery.
3. The depletion layer gets widened as the
applied voltage is gradually increased.
4. The barrier voltage also gets gradually
increased
5. Due to minority charge carriers a negligibly
small current of the order of micro amperes
will flow
6. Reverse-biased p–n junction offers very high
resistance to current flow.
7. Reverse current is referred to as reverse
saturation current.
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 Summary
❑ A p–n junction is forward biased if its p-side is connected to the positive terminal of
the supply and n-side is connected to the negative terminal of the supply.
❑ The depletion layer width gets narrowed down on application of forward voltage.
❑ The majority charge carriers current is established in a forward-biased p–n junction.
❑ Barrier potential for germanium is 0.3 V and for silicon is 0.7 V at room temperature.
❑ When p-side is connected to the negative terminal and n-side is connected to the
positive terminal of the supply, the p–n junction is said to be reverse biased.
❑ The width of the depletion layer, and hence the barrier potential increases when the
junction is reverse biased.
❑ small current flows through a reverse-biased p–n junction due to the minority charge
carriers.
❑ A forward-biased p–n junction offers very small resistance to current flow while a
reverse-biased p–n junction offers very high resistance to current flow.

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 Semiconductor Diodes

❑ A P-N junction diode (known as a

semiconductor diode) consists of a P-N
junction, formed either in germanium or
silicon crystal. The diode has two
terminals namely anode and cathode.

❑ The anode refers to the P-type region

and cathode refers to the N-type region
as shown

❑ The arrow head, shown in the circuit

symbol, points the direction of current
flow, when it is “forward biased”

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 V-I Characteristics of a P-N Junction
Diode

A diode is specified in terms of certain

parameters.
(i) Forward Voltage drop, VF
(ii) Reverse Breakdown Voltage, VRB
(iii) Reverse saturation current, IR
(iv) Dynamic resistance, rd
(v) Maximum forward current, IFM
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 Problems
A silicon diode is connected across a 3 V supply with a series resistance of 20 Ω.
Neglecting diode resistance, calculate the diode current.

silicon diode has VF = 0.7 V.

Applying Kirchhoff ’s voltage law,

+ 3 V – 20 I – 0.7 V = 0

I = 0.115A

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