UNIT  II. PHYSICS OF SEMICONDUCTOR DEVICES. P type semiconductors. Electron (minority carriers). Hole (majority carriers). . . . . . . . . Hole (mobile charge). Acceptor ions (immobile charge). . (p ≈ N A ). . ≈. N type semiconductors.
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Ionized acceptors
Ionized donors
Junction
P
N
+
+
+
+
+
+
+
+
Space charge region








(OR)
Depletion region
Potential barrier height(V0)
Potential barrier width
(W)
From figure the following points to be noted:
The conduction band edge Ecp in the Ptype
Therefore, the total shift in the energy level
E0 = E1 + E2 = Ecp – Ecn = Evp  Evn
Where,
V0= contact potential (OR) barrier potential
( exists across an open circuited PN junction)
When positive terminal of the battery is connected
to the Ptype & negative terminal is to the Ntype
of the PNjunction diode, known the diode is kept
in forward bias.
When the diode is in forward bias the following
points are noted.
When negative terminal of the battery is connected
to the Ptype & positive terminal is to the Ntype
of the PNjunction diode, known the diode is kept
in reverse bias.
P
N
Space charge region
P
N
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+




















Reverse bias
VR
When the diode is in reverse bias the following
points are noted.
(VI curves)
The graph is plotting in between the voltage is taking on Xaxis & current is on Yaxis, is known
as VI characteristics of a PN junction.
Note:
These curves are drawn on the basis of diode
is connected in forward & reverse bias.
From the graph the following points are noted.
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 halfwave rectifier.
During the +ve half cycle of input,end A
The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
D.C. power output
η =
A.C. power input
D.C. power output = ( Im /π ) 2 x RL
&
A.C. power input = (Im /2 ) 2 x (rf + RL )
Therefore,
( Im /π ) 2 x RL
(Im /2 ) 2 x (rf + RL )
η =
=
=
Conclusion:
0.406 x 100%
40.6%
rf + RL
since,
rf « RL
η =
The maximum efficiency of a halfwave rectifier
is 40.6% of A.C power is converted into D.C power.
An electronic circuit which converts
alternating voltage (OR) current into
pulsating voltage (OR) current during
both half cycle of input is known as
fullwave rectifier.
During the +ve half cycle of input,end A
The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.
D.C. power output
η =
A.C. power input
D.C. power output = ( 2Im /π ) 2 x RL
A.C. power input = (Im / √2 ) 2 x (rf + RL )
Therefore,
(2 Im /π ) 2 x RL
(Im / √2 ) 2 x (rf + RL )
η =
=
=
Conclusion:
0.812 x 100%
81.2%
rf + RL
But,
rf « RL
The maximum efficiency of a fullwave rectifier
is 81.2% of A.C power is converted into D.C power.
Therefore, the fullwave rectifier efficiency is twice
of a halfwave rectifier.
η =
3/2
3/2
( )
( )
(EF – EC ) / KBT
(EF – EC ) / KBT
n =
nc = Nc
2πme*KBT
2πme*KBT
e
e
2
2
(Diode current equation)
h2
h2
The concentration of electrons in the conduction band of a semiconductor is given by
(OR)
 (1)
Where,
= Nc
( )
(EV – EF ) / KBT
nv = Nv
2πmh*KBT
e
2
Similarly the concentration of holes in the valence band of a semiconductor is given by
h2
 (2)
Where,
Eq(1) & (2) are applicable for intrinsic semiconductors .
= Nv
For Ntype semiconductor electron density is given by
(EFn – Ecn ) / KBT
(EFp – Ecp ) / KBT
nn = Nc
np = Nc
e
e
Where,
Where,
In eq(1), nc = nn , EF = EFn , Ec = Ecn
In eq(1), nc = np , EF = EFp , Ec = Ecp
Similarly, for Ptype semiconductor electron density is
given by
 (3)
 (4)
For Ntype semiconductor hole density is given by
(Evn – EFn ) / KBT
(Evp – EFp ) / KBT
pn = Nv
pp = Nv
e
e
Where,
Where,
In eq(2), nv = pn , EF = EFn , Ev = Evn
In eq(2), nv = pp , EF = EFp , Ev = Evp
Similarly, for Ptype semiconductor hole density is
given by
 (5)
 (6)
Dividing the eq(3) with (4), we get,
(EFn – Ecn )
(EFp – Ecp ) / KBT
e
e
(Ecp – Ecn ) / KBT
e
nn
nn
=
=
np
np
Since the Fermi levels are equal , hence EFn = EFp
Therefore, the above equation becomes as
 (7)
eVB / KBT
 eVB / KBT
Where,
e
e
nn
VB = the barrier potential
=
=
np
nn
np
Therefore, the eq(7) becomes as
(OR)
 (8)
Similarly, dividing the eq(5) with (6) & simplifying, we get,
 eVB / KBT
 eVB / KBT
e
e
pn
=
=
pn
pp
pp
(OR)
When the junction is in forward biased with voltage V, then the electron density in the Pregion becomes as
 e(VB V) / KBT
 (9)
np + ∆np = nn e
eV / KBT
e
np + ∆np = np
e
 eVB / KBT
e
=
np
nn
Since,
Therefore, increase in electron density in the pregion
is given by
eV / KBT
e
∆np = np
 1
 (10)
 eVB / KBT
(From eq(8)
np + ∆np = nn e
When the diode is in forward bias more electrons are
move from Nregion to Pregion hence, increase the
Electron density in Pregion by ∆np.
Therefore, diffusion current of electrons is given by
Where,
C = constant (depends on the semiconductor)
eV / KBT
e
ie= C1 ∆np = C1 np
 1
 (11)
Similarly diffusion current of holes is given by
Therefore, total current is given by
I = ie + ih
eV / KBT
eV / KBT
e
e
ih = C2 ∆pn = C2 pn
I = ie + ih = (C1 np+C2 pn)
 1
 1
When the junction is in reverse bias V =  V ,
then the reverse current is given by
 (12)
 (13)
 eV / KBT
e
« 1
Therefore,
I = (C1 np+ C2 pn) = I0 (14)
 eV / KBT
e
I = ie + ih = (C1 np+C2 pn)
 1
 (13)
since, I0 is the steady reverse current.
Substituting eq(14) in eq(13), we get,
Eq(15) is known as the diode equation.
But, practically the diode equation is written as.
NOTE:
 eV / KBT
 eV / β KBT
e
e
I = I0
I = I0
 1
 1
 (15)
 (16)
βis a constant depends on the material
of the diode. For Ge = 1 & for Si = 2
Definition:
LED is a semiconductor PNjunction diode
which converts electrical energy to light
under forward biasing. It emits light in
both visible & IR region.
NOTE:
LEDs are typically made of compound
semiconductors (OR) direct band gap
semiconductors like gallium arsenide.
Injection Luminescence
When LED is forward biased, the majority charge carrier
moves from P to N & similarly from N to P region and
becomes excess minority charge carriers. Then these excess
minority charge carriers diffuse through the junction and
recombines with the majority charge carriers
in N & P region respectively to produce light.
LED is a highly doped diode
The PNjunction is made by doping
In order to increase the probability
Definition:
A PNjunction diode which converts the
photonic energy into its equivalent
Electrical energy under in reversed bias
is called photo diode.
Its operation is quite reverse from LED
& used in optical communication.
Photo diode are two types.
+

Anode
Cathode
1. pin photo diode(PIN Diode)
Principle:
Under reverse bias when light is made to fall
on the neutral (or) intrinsic region ‘i’ electron
hole pairs are generated. These electrons and
holes are accelerated by the external electric
field, which results in photocurrent.
Thus light is converted into electrical signal.
The mobile charges are accelerated by
Principle:
Under reverse bias when light is made to fall
on the neutral (or) intrinsic region ‘i’ electron
hole pairs are generated. By avalanche effect
more number of electronhole pairs are
Created, which results in large photo current
than of the PIN diode.
Thus light is converted into electrical signal.
pair
N+
N+  heavily doped Nregion
P+  heavily doped Pregion
P  lightly doped Pregion
Layer 1
P
Layer 2
VR
Photons
i
Layer 3
Figure shows
the reverse bias
of avalanche photo diode.
Layer 4
P+
Here, they collide with free electrons in the