A brief introduction to semiconductor devices

1. INTRODUCTION TO SEMICONDUCTOR DEVICES
BY
DR N. SIVASANKARA REDDY
ASSISTANT PROFESSOR
DEPARTMENT OF PHYSICS
PRESIDENCY UNIVERSITY
BANGALORE - 560064
ICAMSE 2020

2. Band Theory – a preview
 Kroning and Penney included this assumption in Quantum Free electron theory and
formed the basis of the band theory (Self study)
 Band A set of closely packed energy levels is called as band
 Valence band A band which is occupied by the valence electrons is called as valence band. The
valence band may be partially or completely filled up depending on the nature of the material.
 Conduction band The lowest unfilled energy band is called as conduction band. This band may be
empty of partially filled. In conduction band the electrons can move freely.
 Forbidden gap The energy gap between valence band and conduction band is called forbidden
energy gap or forbidden gap or band gap.
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Module 1: Electrical Properties – Band Theory

3. Semiconductors - Introduction
 At 0K the valence band is completely filled and the conduction band
is totally empty. When a small amount of energy is supplied, the
electrons can easily jump from valence band to conduction band. The
charge carriers in semiconductors are both electrons and holes.
 Examples: Germanium (0.7eV), Silicon (1.1 eV)
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Module 1: Semiconductors – Introduction

4. Mechanism of conduction
 Electrons absorb energy to
jump from valance band to
conduction band.
 This creates both conduction
band electrons and same
number of holes to conduct
electricity called carriers.
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Module 1: Semiconductors – Mechanism

5. Intrinsic and Extrinsic Semiconductors
 Intrinsic semiconductors are pure semiconductors without any added
impurities (dopants) to change their carrier concentration.
 Each electron in the conduction band is associated with a hole in the valence
band
 Extrinsic semiconductor is a semiconductor doped by specific
impurities which is able to modify its electrical properties, making it
suitable for electronic applications (diodes, transistors, LEDs, etc.)
 Further classified as p-type and n-type semiconductors
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Module 1: Semiconductors – Classification

6. p-type Semiconductor
 When Si, Ge or other semiconducting crystal is doped with trivalent
impurity we get p-type semiconductor.
 Ex: Boron, Aluminium, Gallium, etc.
 p-type semiconductors have ‘holes’ as majority charge carriers.
 The Fermi level is closer to the valance band
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Module 1: Semiconductors – Types

7. n-type semiconductor
 When Si, Ge or other semiconducting crystal is doped with
pentavalent impurity we get n type semiconductor.
 Ex: Antimony, Arsenic, Phosphorus, etc.
 Electrons are the majority charge carriers.
 The Fermi level is closer to the conduction band
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Module 1: Semiconductors – Types

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Module 1: Semiconductors – Explanation

9. p-n junctions
 When p-type and n-type materials are placed in contact with each
other, the junction behaves very differently than either type of
material alone. Specifically, current will flow readily in one direction
(forward biased) but not in the other (reverse biased), creating the
basic diode. This non-reversing behavior arises from the nature of the
charge transport process in the two types of materials and the
formation of a depletion zone.
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Module 1: Semiconductors – p-n junction

10. Depletion Zone
 Some of the free electrons in the n-region diffuse across the junction
and combine with holes to form negative ions. In so doing they leave
behind positive ions at the donor impurity sites.
 This coulomb force creates a source of barrier as these ions repel
further diffusion of electrons
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Module 1: Semiconductors – p-n junction

11. Forward and Reverse Bias
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Module 1: Semiconductors – p-n junction
 the p side is made more positive, so
an electron can move across the
junction and fill a vacancy or "hole"
near the junction. It can then move
from vacancy to vacancy leftward
toward the positive terminal, which
could be described as the hole
moving right.
• the p side is made more negative,
making it "uphill" for electrons
moving across the junction. The
conduction direction for electrons
in the diagram is right to left, and
the upward direction represents
increasing electron energy.

12. Applications of Semiconductors
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1. Solar Cells
2. Light emitting diodes
3. Semiconducting laser diodes

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Solar Cell is a semiconductor device which converts light energy into the electrical energy. A solar
cell is basically a p-n junction diode. It works on the principle of photovoltaic effect to convert light
energy into electrical energy.
Construction of Solar Cell
Solar cell is basically a junction diode, but construction is little bit different form conventional p-n
junction diode. A very thin layer of n-type semiconductor is grown on a relatively thicker p-type
semiconductor. Few finer electrodes are provided on the top of the n-type semiconductor layer.
These electrodes do not obstruct light to reach the thin n-type layer. Just below the n-type layer
there is a p-n junction. A current collecting electrode is provided at the bottom of the p-type layer.
The entire assembly is enclosed by thin glass to protect the solar cell from any mechanical shock.
Solar Cell

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When light passes through very thin n-type layer reaches the p-n junction, the light photons can
enter easily into the junction. These light photons, carry sufficient energy to create a number of
electron-hole pairs. The incident light breaks the thermal equilibrium condition of the junction.
The free electrons in the depletion region can quickly come to the n-type side of the junction.
Similarly, the holes can quickly come to the p-type side of the junction.
These newly created free electrons and holes cannot further cross the junction because of
barrier potential of the junction. As the concentration of electrons becomes higher in n-type
side of the junction and concentration of holes becomes higher in p-type side of the junction.
Now the p-n junction will behave like a small battery cell. A voltage is set up which is known
as photo voltage. If we connect a small load across the junction, there will be a meaningful
current flowing through it.
Working

16. Light Emitting Diode (LED)
Inside a Light Emitting Diode
1. Transparent Plastic Case
2. Terminal Pins
3. Diode

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When current flows across a diode. Negative electrons move one way and positive holes move the other
way.
The holes exist at a lower energy level than the free electrons. Therefore when a free electrons falls it
losses energy
This energy is emitted in a form of a photon, which causes light. The color of the light is determined by
energy gap of the light emitting diode.
Working

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Different color Light Emitting Diodes

19. Semiconductor diode LASER:
A semiconductor laser or diode laser is a pn junction device that emits laser
light when it is forward biased. First diode laser fabricated by R. N Hall and his
co-workers.
Construction
It is operated at low temperature and emits light in near IR region.
 They are of very small size, each side is in the order of 1 mm.
p and n regions are made from same semiconductor material (GaAs).
The p type region is formed by doping with zinc atoms and n type by doping
with tellurium.

22. Working
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 When the junction is forward biased, at low voltage the electron and hole recombine and cause
spontaneous emission.
 When the forward – biased voltage is increased, more and more light photons are emitted and the
light production instantly becomes stronger. These photons will trigger a chain of stimulated
recombination resulting in the release of photons in phase.
 The photons moving at the plane of the junction travels back and forth by reflection between two
sides placed parallel and opposite to each other and grow in strength to produce laser light.
 The wavelength of laser lightdepends on band gap of the semiconductor diode.