Post on 06-Apr-2018
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Physics ofSemiconductor Devices
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Formation of PN - Junction
When a P-type Semiconductor is joined together
with an N-type Semiconductor a PN junction is
formed. And it is also known as a Semiconductor
Diode.
Semiconductor diodes are widely used in Rectifiers
which converts input AC signal into DC outputsignal.
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P N
----
---- +
+
+
+
+
+
+
+
Space charge region
(OR)
Depletion region
JunctioJunctio
nn
Ionized donorsIonized donors
Potential barrier(VPotential barrier(V00))
Potential barrier widthPotential barrier width
(W)(W)
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Depletion Region & Space Charge
The diffusing majority carriers from the two regionsrecombine near the junction and disappear.
The uncompensatedAcceptor and Donor ions set up anElectric field which halts majority carrier Diffusion and
causes minority carrier Drift.
The two kinds of majority carriers diffusing across thejunction meet each other near the junction and undergo
recombination's, leavingnegative ions on the P-side andpositive ions on the N-side of the junction.
This distribution of Positive and Negative Charges is calledSpace charge.
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N P
Cathode Anode
_+
Diode Symbol
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Fermi level
Depletion region
PN - junction
EFn
Valence band
Conduction band
NP
EFp
E
eVB
eVB
Ev
Ec
Ev
Ec
Energy level diagram
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V - I Characteristics of PN Junction
The diode can be operated in two different ways, as Forward and
Reverse bias.
When positive terminal of the battery is connected to the P-type& negative terminal is to the N-type of the PN-junction diode,
known the diode is kept in forward bias.
When negative terminal of the battery is connected to the P-type& positive terminal is to the N-type of the PN-junction diode,
known the diode is kept in reverse bias.
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Forward Bias
Reverse Bias V
I
Current
Reversebreak down
current
Forward Current
Knee Voltage
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The region between knee voltage & breakdown
voltage is known as non-ohmic region.
Above the knee & breakdown voltage the currentincreases.
Breakdown voltage is due to thermally broken
covalent bonds.Diode is conducting in forward bias &
non-conducting in reverse bias.
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A Rectifier is an electronic circuit which
converts alternating current to direct current
(OR) unidirectional current.
Rectifiers are mainly three types1.Half wave rectifiers
2.Full wave rectifiers
3.Bridge rectifiers
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An electronic circuit which converts
alternating voltage (OR) current for
half the period of input cycle hence
it is named as half-wave rectifier.
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Half Wave Rectifier
A.C Input
Pulsated
D .C Output
transformer
B
RL
rf
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The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
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&
Therefore,
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since,
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An electronic circuit which converts
alternating voltage (OR) current into
pulsating voltage (OR) current duringboth half cycle of input is known as
full-wave rectifier.
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Full Wave Rectifier
D .C Output
A
B
Center tapped
transformer
rf
rf
RL
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The ratio of D.C power output to applied A.Cpower input is known as rectifier efficiency.
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&
Therefore,
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2
3
2
2
3
2
)2(2Where
)1().........exp(
)exp()2(2
h
kTmN
kT
EENn
kT
EE
h
kTmn
eC
cFC
cFe
!
!
!
T
T
2
3
2
2
3
2
)2(2Where
)2).......(exp(
)exp()2(2
h
kTmN
kT
EENp
kT
EE
h
kTmp
hv
FVv
FVe
!
!
!
T
T
Consider Intrinsic Semiconductor
Electron Concentration Holes Concentration
Equation 1 & 2 holds good for both intrinsic and extrinsic semiconductors
under Thermal equilibrium condition.
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)exp(
,,
kT
EENn
EEEEnn
cnFnCn
cncFnFn
!
!!!
For N type Material
)exp(
,,
kT
EENn
EEEEnn
cpFp
Cp
cpcFpFp
!
!!!
For P type Material
Electron Concentration
)3.().........exp(
)exp(
)exp(
)exp(
)exp(
kT
eVnn
kT
eV
n
n
kT
EE
kT
EEkT
EE
n
n
Bnp
B
p
n
cncp
cpFp
cnFn
p
n
!
!
p
!
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)exp(
,,
kT
EENp
EEEEpp
Fpvp
vp
vpvFpFp
!
!!!
For N type Material
)exp(
,,
kT
EENp
EEEEpp
Fnvnvn
vnvFnFn
!
!!!
For P type Material
Hole Concentration
)4().........exp(
)exp(
)exp(
)exp(
)exp(
kT
eVpp
kT
eV
p
p
kT
EE
kT
EEkT
EE
p
p
Bpn
B
p
n
vpvn
Fpvp
Fnvn
p
n
!
!
p
!
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}1){exp(
)exp(
)exp().exp(
))(
exp(
!(
!(
!(
!(
kT
eVnn
kT
eV
nnn
kT
eV
kT
eVnnn
kT
VVennn
pp
ppp
Bnpp
Bnpp
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}1{11 !(!TK
eV
ppeBencnci
}1{22 !(!TK
eV
nnhBepcpci
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}1){( 21 !! TK
eV
npheBepcnciiI
}1){( 21 !!
TK
eV
npheBepcnciiI
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I0 is called reverse saturation current
021 )( IpcncI np !!
1
KT
eV
e
1)exp(0
!
KT
eVII
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1)exp(0
!
KT
eV
II F
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LEDs are typically made of compound
semiconductors (OR) direct band gap
semiconductors like gallium arsenide.
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LED is a highly doped
diode
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substrate
VF
N
POhmic
Contacts(Al)
Sio2
Photons
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+ -
CathodeAnode
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V
Current
I(MA)
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+ -
CathodeAnode
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P
N
i
Photons
Electron hole
pair
VR
Figure shows thereverse bias of
p-i-n diode.
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N+
Layer 1
Layer 2
Layer 3
Layer 4
P
i
P+
VRPhotons
Electron holepair
N+ - heavily doped N-region
P+ - heavily doped P-region
P - lightly doped P-region
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