Semiconductor Devices and its types

1. SEMICONDUCTOR DEVICES
A SOLID STATE ELECTRONICS

2. INTRODUCTION Solids
Conductors Semiconductors Insulators
 Conductors: Solids With high conductivity or low resistivity. Conductivity range – 102 to 108 S m-1 and Resistivity
range – 10-2 to 10-8Ω m. example: Aluminum, Copper, Gold, Silver etc..,
 Insulators: Solids with low conductivity or high resistivity Conductivity range – 10-11 to 10-19 S m-1 and Resistivity
range – 1011 to 1019 Ω m. example: Rubber, Plastic, Mica, Glass etc..,
 Semiconductors: Solids whose conductivity lies between metals and insulators. Conductivity range – 10-6 S m-1 and
Resistivity range – 10-5 to 106 Ω m . example: Silicon, Germanium[elemental semiconductor], Cds,
GaAs[inorganic-Compound semiconductors], Anthracene, doped pthalocyanines [organic]
Conductors Semiconductors Insulators
Energy
band
Diagrams

3. Semiconductors
Intrinsic
Semiconductor
Extrinsic
semiconductor
Intrinsic Semiconductors
 An intrinsic semiconductor is a pure semiconductor and possesses poor conductivity. At absolute zero temperature,
an intrinsic semiconductor behaves like an insulator. Its conductivity can be varied by varying the temperature
only.
Example: germanium, silicon.
 In a crystal of germanium or silicon, each atom forms four covalent bonds (valence bonds) by sharing its four
valence electrons with neighboring four atoms. At 0K all the covalent bonds are complete and there are no free charge
carriers in the crystal. Hence an intrinsic Ge or Si at 0K has a completely filled valence band and a completely empty
conduction band.
Transforma
tion

4.  As temperature increases, thermal energy becomes available to the electron and some of the
electrons may break away from the covalent bond, becoming free.
 The electron which leaves the covalent bond is known free electron. The vacancy created in the
covalent bond due to the release of the electron is known as a hole. Hole behaves as an apparent
free particle with effective positive charge.
 Both holes and electrons play an important role in electrical conduction in semiconductors.
 In intrinsic semiconductors, the number of free electrons ne is
equal to the number of holes nh. ne = nh = ni
ni is called intrinsic carrier concentration.

5. EXTRINSIC SEMICONDUCTORS
A semiconductor to which an impurity is added to increase its conductivity is known as an extrinsic
semiconductor or impurity semiconductor.
The process of adding impurities to an intrinsic semiconductor is known as doping. The impurity
atoms are called dopants.
There are two types of dopants used in doping the tetravalent Si or Ge:
(i) Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.
(ii) Trivalent (valency 3); like Indium (In), Boron (B), Aluminum (Al), etc.
Extrinsic
Semiconductor
N-type
semiconductor
P-type
semiconductor
Depending on Impurity

6. o n-type semiconductor
When a small amount of pentavalent impurity is added to pure semiconductor it becomes n-type
semiconductor.
o Consider the case of germanium (Ge) doped with phosphorus (P). The impurity atom gets surrounded by
four Ge atoms. Four of the five valence electrons of P are involved in covalent bonds. The fifth valence
electron becomes free and can move to the conduction band easily. This leaves behind an immobile positive
ion in the crystal. Since an impurity atom donates a free electron to the semiconductor, impurity atom is
called donor.
o At room temperature, electron- hole pairs are created due to the breaking of covalent bond. But number of
electrons are greater than number of holes. Thus, in n-type semiconductor electrons are the majority charge
carriers and holes are minority charge carriers.

7. p-type semiconductor
o When a small amount of trivalent impurity is added to pure semiconductor it becomes p-type
semiconductor.
o Consider the case of germanium (Ge) doped with boron(B) The impurity atom gets surrounded by four
Ge atoms. But it provides only three valence electrons to make three covalent bonds. The vacancy that
exist in the fourth bond constitutes a hole. This hole accepts electron from the neighborhood, creating a
vacancy or hole at its own site. Thus, the hole is available for conduction.
o When impurity atom accepts an electron from adjoining Ge atom, it becomes an immobile negative
ion. Since an impurity atom accepts an electron from the semiconductor, an impurity atom is called
acceptor.
o At room temperature, electron- hole pairs are created due to the breaking of covalent bond. But
number of holes are greater than number of electrons. Thus, in p-type semiconductor holes are the
majority charge carriers and electrons are minority charge carriers.

8. p-n Junction
When a semiconductor crystal ( Si or Ge) is doped in such a manner that one-half is p-type and the other half is n-type, the contact
surface dividing the two halves is called p-n junction.
p-n Junction formation
 During the formation of p-n junction due to the concentration gradient across p-side and n-side, holes diffuse from p-
side to n-side (p─›n) and electrons diffuse from n-side to p-side (n─›p). This motion of charge carriers gives rise to
diffusion current across the junction.
When an electron diffuses from n─›p, it leaves behind an ionised immobile donor (positive charge) on n-side. As the
electrons continue to diffuse from n─›p, a layer of positive charge on n-side of the junction is developed. Similarly,
when a hole diffuses from p─›n due to the concentration gradient, it leaves behind an ionised immobile acceptor
(negative charge) on p-side. As the holes continue to diffuse, a layer of negative charge on the p-side of the junction is
developed.
 The space charge region on either side of the p-n junction having no mobile charge carries known as depletion
region or depletion layer.

9.  Because of positive ion layer on n-side and negative ion layer on p-side, an electric filed is set up across the junction.
This electric field sets a barrier at the junction which opposes the further diffusion of majority charge carries into
opposite regions. This is called potential barrier.
It facilitates the flow of minority charge carriers across the junction. The motion of these charge carriers due to electric
field is called drift. The current due to minority charge carriers facilitated by the electric filed is called drift current,
which is opposite in direction to the diffusion current.
Initially the diffusion current is large and drift current is small. As the process of formation of junction builds up the
diffusion current decreases, until the two becomes equal and an equilibrium is established.
Semiconductor diode
Semiconductor diode consists of a p-n junction with metallic
contacts provided at the ends for the application of an external
voltage. It is a two-terminal device. The barrier potential can be
altered by applying an external voltage across the diode.
Representation of p-n
junction diode under
equilibrium (without bias)

10. APPLICATIONS OF SEMICONDUCTORS
• Semiconductors are also used in the design of transistors, which are
used both for fast switching and for current amplification.
• The MOSFET (metal-oxide-semiconductor field-effect transistor) is
the most common semiconductor device in the world.
• Consumer electronics: Mobile phones, laptops, games consoles,
microwaves and refrigerators all operate with the use of
semiconductor components such as integrated chips, diodes and
transistors.
• Embedded systems: Embedded systems are small computers
that form part of a larger machine. They can control the
device and allow user interaction. Embedded systems that we
commonly use include central heating systems, digital
watches, GPS systems, fitness trackers, televisions and
engine management systems in vehicles.

11. • Lighting and LED displays: Some semiconductors, usually
those available in liquid or amorphous form as a thin-coated
film, can produce light and are used in LEDs and OLEDs.
• Thermal conductivity: Some semiconductors have high
thermal conductivity, so can be used as a cooling agent in
certain thermoelectric applications.
Solar cells: Silicon is also the most
commonly used semiconductor in
the production of solar panel cells
Pictorial
Representation of
how Solar works,
using principle of
p-n junction

12. Thank You