Download What is a Semiconductor?

Lecture 1
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OUTLINE
Semiconductors,
Junction,
Diode characteristics,
Bipolar Transistors:
characteristics, small signal low
frequency h-parameter model,
Hybrid-pi model,
Course Outline
Amplifiers voltage and current
amplifiers,
Oscillators,
Differentials Amplifiers,
Operational Amplifiers,
Linear application of OP-AMPs, Gain input
and Output Impedance.
What is a
Semiconductor?
A semiconductor is a
substance which has
resistivity (10 E-4 to 0.5
Ohm-m) in between
conductors and insulators
e.g. germanium, silicon,
selenium, carbon etc.
• Low resistivity => “conductor”
• High resistivity => “insulator”
• Intermediate resistivity =>
“semiconductor”
– conductivity lies between that of
conductors and insulators
– generally crystalline in structure
for IC devices
• In recent years, however, non-crystalline
semiconductors have become
commercially very important.
polycrystalline amorphous crystalline
Properties of semiconductor
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The resistivity of a semiconductor is less than an insulator
but more than a conductor.
Semiconductors have negative temperature co-efficient of
resistance i.e. the resistance
of a semiconductor decreases with the increase in
temperature and vice-versa. For example, germanium
is actually an insulator at low temperatures but it becomes
a good conductor at high temperatures.
When a suitable metallic impurity (e.g. arsenic, gallium
etc.) is added to a semiconductor its current conducting
properties changes appreciably.
Semiconductor Materials
Phosphorus
(P)
Gallium
(Ga)
Silicon
• Atomic density: 5 x 1022 atoms/cm3
• Si has four valence electrons. Therefore, it can form
covalent bonds with four of its nearest neighbors.
• When temperature goes up, electrons can become
free to move about the Si lattice.
Electronic Properties of Si
 Silicon is a semiconductor material.
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Pure Si has a relatively high electrical resistivity at room
temperature.
 There are 2 types of mobile charge-carriers in Si:
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Conduction electrons are negatively charged;
Holes are positively charged.
 The concentration
(#/cm3) of conduction electrons & holes
in a semiconductor can be modulated in several ways:
1.
by adding special impurity atoms ( dopants )
2.
3.
4.
by applying an electric field
by changing the temperature
by irradiation
Electron-Hole Pair Generation
• When a conduction electron is thermally generated,
a “hole” is also generated.
• A hole is associated with a positive charge, and is
free to move about the Si lattice as well.
Effects of Temperature on Semiconductor
(i) At absolute zero:
At absolute zero temperature,
all the electrons are tightly
held by the semiconductor
atoms. The inner orbit
electrons are bound whereas
the valence electrons are
engaged in co-valent bonding.
At this temperature, the covalent bonds are very strong
and there are no free
electrons. Therefore, the
semiconductor crystal
behaves as a perfect
insulator.
• In terms of energy band
description, the valence band is
filled and there is a large
energy gap between valence
band and conduction band.
Therefore, no valence electron
can reach the conduction band
to become free electron. It is
due to the non-availability of
free electrons that a
semiconductor behaves as an
insulator.
Effects of Temperature on Semiconductor
• Above absolute zero.
When the temperature is raised,
some of the covalent bonds in the
semiconductor break due to the
thermal energy supplied. The
breaking of bonds sets those
electrons free which are engaged
in the formation of these bonds.
The result is that a few free
electrons exist in the
semiconductor. These free
electrons can constitute a tiny
electric current if potential
difference is applied across the
semiconductor crystal .
Classification of Semiconductor
• Intrinsic Semiconductor:
A semiconductor in an
extremely pure form is
known as an intrinsic
semiconductor.
• Extrinsic Semiconductor:
The intrinsic semiconductor has
little current conduction
capability at room temperature.
To be useful in electronic devices,
the pure semiconductor must be
altered so as to significantly
increase its conducting
properties. This is achieved by
adding a small amount of suitable
impurity to a semiconductor. It is
then called impurity or extrinsic
semiconductor.
Extrinsic Semiconductor
• n-type :
• p-type:
Carrier Concentrations in Intrinsic
Si
• The “band-gap energy” Eg is the amount of energy
needed to remove an electron from a covalent bond.
• The concentration of conduction electrons in intrinsic
silicon, ni, depends exponentially on Eg and the
absolute temperature (T):
ni  5.2 10 T
15
3/ 2
exp
 Eg
2kT
electrons / cm3
ni  11010 electrons / cm3 at 300K
ni  11015 electrons / cm3 at 600K
Doping (N type)
• Si can be “doped” with other elements to change its
electrical properties.
• For example, if Si is doped with phosphorus (P), each
P atom can contribute a conduction electron, so that
the Si lattice has more electrons than holes, i.e. it
becomes “N type”:
Notation:
n = conduction electron
concentration
Doping (P type)
• If Si is doped with Boron (B), each B atom can
contribute a hole, so that the Si lattice has more
holes than electrons, i.e. it becomes “P type”:
Notation:
p = hole concentration
Summary of Charge Carriers
Electron and Hole Concentrations
• Under thermal equilibrium conditions, the product
of the conduction-electron density and the hole
density is ALWAYS equal to the square of ni:
np  ni
N-type material
n  ND
2
p
ni
ND
2
P-type material
p  NA
2
n
n i
NA
Terminology
donor: impurity atom that increases n
acceptor: impurity atom that increases p
N-type material: contains more electrons than holes
P-type material: contains more holes than electrons
majority carrier: the most abundant carrier
minority carrier: the least abundant carrier
intrinsic semiconductor: n = p = ni
extrinsic semiconductor: doped semiconductor
Summary
• The band gap energy is the energy required to free an
electron from a covalent bond.
– Eg for Si at 300K = 1.12eV
• In a pure Si crystal, conduction electrons and holes are
formed in pairs.
– Holes can be considered as positively charged mobile particles
which exist inside a semiconductor.
– Both holes and electrons can conduct current.
• Substitutional dopants in Si:
– Group-V elements (donors) contribute conduction electrons
– Group-III elements (acceptors) contribute holes
– Very low ionization energies (<50 meV)