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: SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES
AND SIMPLE CIRCUITS
Vacuum Tubes
- Has two electrodes; cathode (plate) and anode
- Electrons are supplied by heated cathode
- Vacuum is required in the inter-electrode space so that the moving electrons don’t lose their energy upon collision
with the air molecules.
- Electrons only flow in one direction i.e.; from cathode to anode. Therefore, referred to as valves
- Bulky, consume high power, operate generally at high voltages (~100 V) and have limited life and low reliability.
Note: Vacuum tube- 2 electrodes
Triode- 3 electrodes (cathode plate and grid)
Tetrode- 4 electrodes
Pentode- 5 electrodes
Solid-state Semiconductors
- Supply and flow of charge carriers in the semiconductor devices are within the solid itself
- Are small in size, consume low power, operate at low voltages and have long life and high reliability
Classification of Metals, Conductors and Semiconductors
On the basis of conductivity
On the basis of the relative values of electrical conductivity (σ) or resistivity (ρ = 1/σ), the solids are broadly classified
as:
1. Metals
They possess very low resistivity (or high conductivity).
ρ ~ 10-2 – 10-8 Ω m
σ ~ 102 – 108 Sm-1
2. Semiconductors
They have resistivity or conductivity intermediate to metals and insulators.
ρ ~ 10-5 – 106 Ω m
σ ~ 105 – 10-6 S m-1
3. Insulators
They have high resistivity (or low conductivity).
ρ ~ 1011 – 1019 Ω m
σ ~ 10-11 – 10-19 S m-1
Types of semiconductors
1. Elemental semiconductors- Si and Ge
2. Compound semiconductors-
a. Inorganic- CdS, GaAs, CdSe, InP, etc.
b. Organic- anthracene, doped phthalocyanines, etc.
c. Organic polymers- polypyrrole, polyaniline, polythiophene, etc.
Band theory in Solids
- According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in
which it revolves.
- But when the atoms come together to form a solid, they are close to each other. So, the outer orbits of electrons
from neighboring atoms would come very close or could even overlap.
, - Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of
surrounding charges. Because of this, each electron will have a different energy level.
- These different energy levels of electrons (when present in bulk/solid) with continuous energy variation form energy
bands.
- The energy band which includes the energy levels of the valence electrons is called the valence band.
- The energy band above the valence band is called the conduction band.
- With no external energy, all the valence electrons will reside in the valence band.
- If the lowest level in the conduction band happens to be lower than the highest level of the valence band, the
electrons from the valence band can easily move into the conduction band. Normally the conduction band is empty.
But when it overlaps on the valence band electrons can move freely into it.
- If there is some gap between the conduction band and the valence band, electrons in the valence band all remain
bound and no free electrons are available in the conduction band.
Conduction in solids on the basis of Band Theory
Let us consider what happens in the case of Si or Ge crystal containing N atoms. For Si, the outermost orbit is the
third orbit (n = 3), while for Ge it is the fourth orbit (n = 4). The number of electrons in the outermost orbit is 4 (2s
and 2p electrons). Hence, the total number of outer electrons in the crystal is 4N. The maximum possible number of
electrons in the outer orbit is 8 (2s + 6p electrons). So, for the 4N valence electrons there are 8N available energy
states.
At the distance between the atoms in the crystal lattices of Si and Ge, the energy band of these 8N states is split
apart into two which are separated by an energy gap Eg.
- The lower band which is completely occupied by the 4N valence electrons at temperature of absolute zero is the
valence band.
- The other band consisting of 4N energy states, called the conduction band, is completely empty at absolute zero.
The lowest energy level in the conduction band is shown as EC and highest
energy level in the valence band is shown as EV. Above EC and below EV
there are a large number of closely spaced energy levels. The gap between
the top of the valence band and bottom of the conduction band is called
the energy band gap (Energy gap Eg). It may be large, small, or zero,
depending upon the material.
Case1
When the conduction band is partially filled and the balanced band is partially empty or when the conduction and
valance bands overlap, the solid is said to be conductor. When there is overlap electrons from valence band can
easily move into the conduction band. This situation makes a large number of electrons available for electrical
conduction. When the valence band is partially empty, electrons from its lower level can move to higher level
making conduction possible. Therefore, the resistance of such materials is low or the conductivity is high.
: SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES
AND SIMPLE CIRCUITS
Vacuum Tubes
- Has two electrodes; cathode (plate) and anode
- Electrons are supplied by heated cathode
- Vacuum is required in the inter-electrode space so that the moving electrons don’t lose their energy upon collision
with the air molecules.
- Electrons only flow in one direction i.e.; from cathode to anode. Therefore, referred to as valves
- Bulky, consume high power, operate generally at high voltages (~100 V) and have limited life and low reliability.
Note: Vacuum tube- 2 electrodes
Triode- 3 electrodes (cathode plate and grid)
Tetrode- 4 electrodes
Pentode- 5 electrodes
Solid-state Semiconductors
- Supply and flow of charge carriers in the semiconductor devices are within the solid itself
- Are small in size, consume low power, operate at low voltages and have long life and high reliability
Classification of Metals, Conductors and Semiconductors
On the basis of conductivity
On the basis of the relative values of electrical conductivity (σ) or resistivity (ρ = 1/σ), the solids are broadly classified
as:
1. Metals
They possess very low resistivity (or high conductivity).
ρ ~ 10-2 – 10-8 Ω m
σ ~ 102 – 108 Sm-1
2. Semiconductors
They have resistivity or conductivity intermediate to metals and insulators.
ρ ~ 10-5 – 106 Ω m
σ ~ 105 – 10-6 S m-1
3. Insulators
They have high resistivity (or low conductivity).
ρ ~ 1011 – 1019 Ω m
σ ~ 10-11 – 10-19 S m-1
Types of semiconductors
1. Elemental semiconductors- Si and Ge
2. Compound semiconductors-
a. Inorganic- CdS, GaAs, CdSe, InP, etc.
b. Organic- anthracene, doped phthalocyanines, etc.
c. Organic polymers- polypyrrole, polyaniline, polythiophene, etc.
Band theory in Solids
- According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in
which it revolves.
- But when the atoms come together to form a solid, they are close to each other. So, the outer orbits of electrons
from neighboring atoms would come very close or could even overlap.
, - Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of
surrounding charges. Because of this, each electron will have a different energy level.
- These different energy levels of electrons (when present in bulk/solid) with continuous energy variation form energy
bands.
- The energy band which includes the energy levels of the valence electrons is called the valence band.
- The energy band above the valence band is called the conduction band.
- With no external energy, all the valence electrons will reside in the valence band.
- If the lowest level in the conduction band happens to be lower than the highest level of the valence band, the
electrons from the valence band can easily move into the conduction band. Normally the conduction band is empty.
But when it overlaps on the valence band electrons can move freely into it.
- If there is some gap between the conduction band and the valence band, electrons in the valence band all remain
bound and no free electrons are available in the conduction band.
Conduction in solids on the basis of Band Theory
Let us consider what happens in the case of Si or Ge crystal containing N atoms. For Si, the outermost orbit is the
third orbit (n = 3), while for Ge it is the fourth orbit (n = 4). The number of electrons in the outermost orbit is 4 (2s
and 2p electrons). Hence, the total number of outer electrons in the crystal is 4N. The maximum possible number of
electrons in the outer orbit is 8 (2s + 6p electrons). So, for the 4N valence electrons there are 8N available energy
states.
At the distance between the atoms in the crystal lattices of Si and Ge, the energy band of these 8N states is split
apart into two which are separated by an energy gap Eg.
- The lower band which is completely occupied by the 4N valence electrons at temperature of absolute zero is the
valence band.
- The other band consisting of 4N energy states, called the conduction band, is completely empty at absolute zero.
The lowest energy level in the conduction band is shown as EC and highest
energy level in the valence band is shown as EV. Above EC and below EV
there are a large number of closely spaced energy levels. The gap between
the top of the valence band and bottom of the conduction band is called
the energy band gap (Energy gap Eg). It may be large, small, or zero,
depending upon the material.
Case1
When the conduction band is partially filled and the balanced band is partially empty or when the conduction and
valance bands overlap, the solid is said to be conductor. When there is overlap electrons from valence band can
easily move into the conduction band. This situation makes a large number of electrons available for electrical
conduction. When the valence band is partially empty, electrons from its lower level can move to higher level
making conduction possible. Therefore, the resistance of such materials is low or the conductivity is high.
