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29P
Electron
Isolated copper Atom
Conductor
Valence orbithas only oneElectron and isloosely bound to core
Core
Isolated silicon atom
Electron
Semiconductor
Valence orbithas four electrons
14P
r1r2
r3
Center of core
r1
r2
r3
Energy
Energy levels in a single atomElectrons in the same orbit has same energy
A silicon crystal is formed by zillions of silicon atoms
14P
14P
14P 14P
14P
Covalent Bond
Electron
Silicon crystal
An electronshared by twoneighboring atomsto form a covalentbond.This way an atomcan have a stablestructure witheight valence bandelectrons.
14P
Energy bands
Electron(in conduction band)Hole (in valence band)
In a crystal, electrons in the same orbit do not have the same energy and thus form energy bands
1st band
2nd band
Valence band
Conduction band
Hig
her
band
hig
her
ener
gy
14P
14P
14P 14P
14P
Electron
Hole
Thermal energy produces free electron and hole pair
(in valence band)
(in conduction band)
14P
14P
14P 14P
14P
Electron
Hole
Recombination of free electron and hole
(in valence band)
(in conduction band)
A
B
C D
E
F
Free Electron (in conduction band)++++++
+
+
+
+
------
-
-
-
-
Hole/electron flow through a semiconductor
The hole moves A-B-C-D-E-F (pseudo movement)
The electron moves F-E-D-C-B-A
Hole
14P 14P 14P
14P 14P 14P
(in valence band)
Intrinsic and extrinsic semiconductor
Intrinsic = pure
Extrinsic = impure or doped
Doping• Doping means mixing a pure semiconductor with impurities to increase its electrical conductivity
Can be done in two ways:
• Increasing the number of electrons by mixing pentavalent elements such as phosphorous, arsenic, antimony (means adding donor impurities)
• Increasing the number of holes by mixing trivalent elements such as aluminum, boron, gallium (means adding acceptor impurities)
15P
14P
14P 14P
14P
Free Electron
N-type semiconductor
Phosphorous atom
Has many free electrons in conduction band and few holesIn valence band
13P
14P
14P 14P
14P
P-type semiconductor
Aluminum atom Hole
Has few free electrons in conduction band and many holesIn valence band
Majority and minority carriers
Electrons are• Majority carriers in N-type semiconductor• Minority carriers in P-type semiconductor
Holes are• Majority carriers in P-type semiconductor• Minority carriers in N-type semiconductor
A diode is formed by putting a N-type and P-type of semiconductor together
N typeP type
Note: Both N and P-type of materials are electrically neutral
Anode Cathode
P-N Junction
P type N type++++
----
Migration of holes from P to NAnd electrons from N to P causes
a formation of depletion layer
This gives rise to barrier potential(Eγ) preventing further migration of
holes and electrons
Anode Cathode----
++++
Energy
PN
Depletion layer
Energy bands in a unbiased diode
Conduction band
Valence band
P type N type
++++
----
+ -
Forward Biased diode
R
VB
Anode Cathode
+ - Vγ
Energy
PN
Smaller depletion layer
Energy bands of a forward biased diode
Conduction band
Valence band
Forward Biased diode
•The diode behaves like a ‘ON’ switch in this mode
• Resistance R and diode’s body resistance limits the current through the diode
• VB has to overcome Vγ in order for the diode to conduct
P type N type
+-
++++
++++
++++
----
----
----
Reverse biased diodeLarger depletion layer
Anode Cathode
VB
Energy
PN
Larger Depletion layer
Valence band
Conduction band
Energy bands in a reverse biased diode
Reverse Biased diode
•The diode behaves like a ‘OFF’ switch in this mode
• If we continue to increase reverse voltage VB
breakdown voltage of the diode is reached
• Once breakdown voltage is reached diode conducts heavily causing its destruction
Breakdown • Diode breakdown is caused by thermally generated electrons in the depletion region
•When the reverse voltage across diode reaches breakdown voltage these electrons will get sufficient energy to collide and dislodge other electrons
• The number of high energy electrons increases in geometric progression leading to an avalanche effect causing heavy current and ultimately destruction of diode