Download + + + + + + + + Space charge region

Physics of
Semiconductor Devices
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 output
signal.
Ionized donors
Junction
P
N
-
-
+
+
+
+
+
+
+
+
Space charge region
(OR)
Depletion region
Potential barrier(V0)
Potential barrier width
(W)
Depletion Region & Space Charge
The diffusing majority carriers from the two regions
recombine near the junction and disappear.
The uncompensated Acceptor and Donor ions set up an
Electric field which halts majority carrier Diffusion and
causes minority carrier Drift.
The two kinds of majority carriers diffusing across the
junction meet each other near the junction and undergo
recombination's, leaving negative ions on the P-side and
positive ions on the N-side of the junction.
This distribution of Positive and Negative Charges is called
Space charge.
Diode Symbol
N
Cathode
_
P
Anode
+
Energy Diagram of PN diode:
(a) & (b) Energy level
diagrams of p – type
and n-type
semiconductor
respectively
(c) Energy level diagram
of PN junction
(d) Formation of potential
barrier across the
junction
(c)
Vn
VB = Vn - Vp
Vp
(d)
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 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 in reverse bias.
I
Current
Reverse Bias
Forward Current
Knee Voltage
V
Forward Bias
Reverse
break down
current
 The region between knee voltage & breakdown
voltage is known as non-ohmic region.
 Above the knee & breakdown voltage the current
increases.
 Breakdown voltage is due to thermally broken
covalent bonds.
 Diode is conducting in forward bias &
non-conducting in reverse bias.
A Rectifier is an electronic circuit which
converts alternating current to direct current
(OR) unidirectional current.
Rectifiers are mainly three types
1.Half wave rectifiers
2.Full wave rectifiers
3.Bridge rectifiers
An electronic circuit which converts
alternating voltage (OR) current for
half the period of input cycle hence
it is named as half-wave rectifier.
Half – Wave Rectifier
A.C Input
rf
RL
B
transformer
Pulsated
D .C Output
The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
&
Therefore,
since,
An electronic circuit which converts
alternating voltage (OR) current into
pulsating voltage (OR) current during
both half cycle of input is known as
full-wave rectifier.
Full Wave Rectifier
A
rf
rf
B
RL
Center tapped
transformer
D .C Output
The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
&
Therefore,
Consider Intrinsic Semiconductor
Electron Concentration
2mekT 32
EF  Ec
n2 (
) exp(
)
2
h
kT
E  Ec
n  N C exp( F
).........(1)
kT
Holes Concentration
2mekT 32
EV  EF
p2 (
) exp(
)
2
h
kT
EV  EF
p  N v exp(
).......( 2)
kT
2mhkT 32
Where N v  2 (
)
2
h
2m kT
Where N C  2 (
)
h
Equation 1 & 2 holds good for both intrinsic and extrinsic semiconductors
under Thermal equilibrium condition.

e
2
3
2
Electron Concentration
For N type Material
n  nn , EF  EFn , Ec  Ecn
nn  N C
EFn  Ecn
exp(
)
kT
nn
np
nn
np
For P type Material
n  n p , EF  EFp , Ec  Ecp
n p  N C exp(
EFp  Ecp
E Fn  Ecn
exp(
)
Ecp  Ecn
kT

 exp(
)
E Fp  Ecp
kT
exp(
)
kT
eVB
 exp(
)
kT
 eVB
n p  nn exp(
)..........(3)
kT
kT
)
Hole Concentration
For P type Material
p  p p , EF  EFp , Ev  Evp
p p  N v exp(
Evp  EFp
kT
pn
pp
pn
pp
)
For N type Material
p  pn , EF  EFn , Ev  Evn
pn  N v
Evn  EFn
exp(
)
kT
Evn  EFn
)
Evn  Evp
kT

 exp(
)
Evp  EFp
kT
exp(
)
kT
 eVB
 exp(
)
kT
exp(
pn  p p exp(
 eVB
).........(4)
kT
 e(VB  V )
n p  n p  nn exp(
)
kT
 eVB
eV
n p  n p  nn exp(
). exp(
)
kT
kT
eV
n p  n p  n p exp(
)
kT
eV
n p  n p {exp(
)  1}
kT
ie  c1n p  c1n p {e
eV
K BT
ih  c2 pn  c2 pn {e
 1}
eV
K BT
 1}
I  ie  ih  (c1n p  c2 pn ){e
eV
K BT
I  ie  ih  (c1n p  c2 pn ){e
 1}
 eV
K BT
 1}
e
 eV
KT
 1
I  (c1n p  c2 pn )   I 0
I0 is called reverse saturation current
 eV
I  I 0 exp(
) 1
KT
 eV
I  I 0 exp(
) 1
KT
LEDs are typically made of compound
semiconductors (OR) direct band gap
semiconductors like gallium arsenide.
LED is a highly doped diode
Photons
Sio2
P
Ohmic
Contacts(Al)
N
substrate
VF
-
+
Anode
Cathode
Current
I(MA)
V (volts)