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Friday, March 1, 2013
Semiconductor and Electronic Devices Theory | Notes
Energy
band diagrams
- In metals, the conduction band and valence band partly
overlap each other and there is no forbidden energy gap or there is partially filled conduction band.
 |
| Energy band diagram of metals |
- In insulators, the conduction band is empty and valence
band is completely filled and forbidden gap is quite large ~ 6 eV. No
electron from valence band can cross over to conduction band at room
temperature, even if electric field is applied. Hence there is no
conductivity of the insulators.
 |
| Energy band diagram of insulators |
 |
| Energy band diagram of semiconductors |
Differences
Distinction between Intrinsic and Extrinsic Semiconductor
|
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Intrinsic
|
Extrinsic
|
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1
|
It is pure semiconducting material and no impurity atoms are added to
it
|
1
|
It is prepared by doping a small quantity of impurity atoms to the pure
semiconducting material.
|
|
2
|
Examples are crystalline forms of pure silicon and germanium.
|
2
|
Examples are silicon and germanium crystals with impurity atoms
of arsenic, antimony, phosphorous etc.
or indium, boron, aluminum etc.
|
|
3
|
The number of free electron in conduction band and the number of
holes in valence band is exactly equal and very small indeed.
|
3
|
The number of free electrons and holes is never equal. There is
excess of electrons in n-type semiconductors and excess of holes in p-type
semiconductors.
|
|
4
|
Its electrical conductivity is low
|
4
|
Its electrical conductivity is high.
|
|
5
|
Its electrical conductivity is a function of temperature alone.
|
5
|
Its electrical conductivity depends upon the temperature as well as
on the quantity of impurity atoms doped in the structure.
|
|
Distinction between n-type and p-type
semiconductors
|
||||
n-type semiconductors
|
p-type semiconductors
|
|||
1
|
It is an extrinsic semiconductors which is obtained by doping the
impurity atoms of Vth group of periodic table to the pure germanium or
silicon semiconductor.
|
1
|
It is an intrinsic semiconductors which is obtained by doping the
impurity atoms of III group of periodic table to the pure germanium or
silicon semiconductor.
|
|
2
|
The impurity atoms added, provide extra electrons in the structure,
and are called donor atoms.
|
2
|
The impurity atoms added, create vacancies of electrons (i.e. holes)
in the structure and are called acceptor atoms.
|
|
3
|
The electrons are majority carriers and holes are minority carriers.
|
3
|
The holes are majority carriers and electrons are minority carriers.
|
|
4
|
The electron density (ne) is much greater than the hole
density (nh)i.e. ne>>(nh)
|
4
|
The hole density (ne) is much greater than the electron
density (nh)i.e. nh>> ne
|
|
5
|
The donor energy level is close to the conduction band and far away
from valence band.
|
5
|
The acceptor energy level is close to valence band and is far away
from the conduction band.
|
|
6
|
The Fermi energy level lies in between the donor energy level and
conduction band.
|
6
|
The Fermi energy level lies in between the acceptor energy level and
valence band.
|
|
P-n junction diode
- Two important processes occur during the formation of p-n junction, diffusion and drift.
- The motion of majority charge carriers give rise to diffusion current.
- Due to the positive space charge on n-side junction and negative space charge region on p-side the electric field is set up and potential barrier develops at the junction Due to electric field electrons on p-side moves to n and holes from n-side to p-side which is called drift current.
- In equilibrium state, there is no current across p-n junction and potential barrier across p-n junction has maximum value .
- The width of the depletion region and magnitude of barrier potential depends on the nature of semiconductor and doping concentration on two sides of p-n junction –
Forward Bias
- P-n junction is forward biased when p-type is connected to the positive terminal of battery and n-type connected to negative terminal of battery.
- Potential barrier height is reduced and width of depletion layer decreases.
- Resistance of p-n junction is low to the flow of current in this condition.
Reverse Bias
- P-n junction is reverse biased when p-type connected to the negative terminal of battery and n-type connected to positive terminal of battery.
- Resistance of p-n junction is high to the flow of current in this condition.
- Potential barrier height is increased and width of depletion layer increases
Diode Characteristics:
 |
| Diode Characteristics circuit diagram under forward bias |
 |
| Diode Characteristics circuit diagram under reverse bias |
 |
| Diode Characteristics graph under reverse bias |
 |
| Diode Characteristics graph under forward bias |
Rectification  |
| p-n junction diode as half wave rectifier |
 |
| p-n junction diode as full wave rectifier |
 |
| Zener diode as a voltage regulator |
Photonic(Optoelectronic) Devices
LED
|
PHOTODIODE
|
SOLARCELL
|
||||||
Forward biased
|
Reverse biased
|
No external baising,It generates emf when solar radiation falls on
it.
|
||||||
Recombination of electrons and holes
take place at the junction and emits electromagnetic radiations
|
Energy is supplied by light to take an electron from valence band to
conduction band.
|
Generation of emf by solar cells is due to three basic process
generation of electron-hole pair,separation and collection
|
||||||
It is used in Burglar alarm, remote control
|
It is used in photo detectors in communication
|
It is used in satellites,space vechicles calculators.
|
||||||
Transistor
 |
| npn transistor and pnp transistor |
• There
are two types of transistor – NPN & PNP
• Applications
of transistor
- Transistor as a switch
- Transistor as an amplifier
- Transistor as an oscillator
Transistor-
Switch
When a transistor is used in cut off or saturated
state, it behaves as a switch.
 |
| Transistor as switch, Cutoff region, Active Region, Saturation Region |
Transistor as an Amplifier
 |
| Current gain, Voltage gain and Power gain |
An amplifier is a device which is used for increasing the amplitude of variation of alternating voltage or current or power,thus it produces an enlarged version of the input signal.
For Circuit diagram refer Ncert diagram
Transistor-Oscillator-
•
In
an oscillator, we get ac output without any external input signal. In other
words, the output in an oscillator is self- sustained. Oscillator converts D.C
into A.C
Digital Electronics –Logic
Gates
• The
three basic Logic Gates are
(1) OR
Gate
 |
| OR gate, AND gate & NOT gate |
OUTPUT
Y= A + B
(2) AND Gate
OUTPUT
Y=A.B
(3) NOT GATE
OUTPUT Y=Y’
COMBINATION OF
GATES
__
(1)
NOR GATE--OUT PUT Y = A+B
__
(2) NAND GATE--OUT PUT Y= A .B
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