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
Junctio
n
P
-
-
+
+
+
+
N
+
+
+
+
Space charge region
(OR)
Depletion region
Potential barrier height(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 level diagram
Depletion region
E
PN - junction
eVB
Ec
Conduction band
Ec
EFp
EFn
Fermi level
Ev
Valence band
Ev
eVB
P
N
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.
P
Open circuit PN -junction
-
-
+
+
+
+
N
+
+
+
+
Space charge region
P
N
-
+
+
+
+
Forward bias
VF
P
Open circuit PN -junction
-
-
+
+
+
+
N
+
+
+
+
Space charge region
P
N
-
-
-
+
+
+
+
Reverse bias
VR
+
+
+
+
+
+
+
+
I
Current
Forward Current
Knee Voltage
Reverse Bias
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.
Rectifiers
• A Rectifier is a device which converts alternating
current to direct or unidirectional current.
• Rectifiers are mainly three types…
1.Half wave Rectifiers
2.Full wave Rectifiers
3.Bridge Rectifiers
Half – Wave Rectifier
A
transformer
A.C input
D .C output
B
Full Wave Rectifier
Center tapped
transformer
A
C
A.C input
B
D .C output
Efficiency of a Rectifier
The efficiency of a Rectifier is defined as the ratio of
D.C. out put power to the A.C. input power
supplied to the Rectifier.
op(d .c) power
efficieny 
ip(a.c) power
Light emitting diodes:
• LED’ s are the most visible type of diode, that
emits a fairly visible colored light, invisible infrared or laser type light when a forward current is
passed through them.
Principle:
Basically LED are made from a very thin layer of
fairly heavily doped semiconductor material.
When the diode is forward biased, electrons from
conduction band combine with holes from in
valence band, releasing sufficient energy to
produce photons of light.
Semicondu
Wave
ctor
length
material
Color
Vf @ 20mA
GaAs
850-940nm Infra-Red
1.2V
GaAsP
630-660nm Red
1.8v
GaAsP
605-620nm Amber
2.0v
GaAsP:N
585-595nm yellow
2.2v
GaP
550-570nm Green
3.5v
SiC
430-505nm Blue
3.6v
GAInN
450nm
4.0v
white
Anode
Cathode
+
_
Cathode
Current
I(MA)
V
Photo diode
• A photodiode is a diode optimized to produce an
electron current flow in response to irradiation by
ultraviolet, visible, or infrared light.
• Silicon, Ge & gallium arsenide is most often used to
fabricate photodiodes.
• Few electron-hole pairs in the N and P–regions, and
most in the depletion region contribute to photo
current.
Photo diode
Sun radiation
Top metal contact
+
P-diffusion
Depletion region
N-type
N+ contact region
Bottom metal contact
Photo diode Symbol
Depletion region
Heavily doped & very thin
Anode
N
N+
Lightly doped
Cathode
_
P
+