L3 electronics pn junction

Instructor: Muhammad Bilal
The pn junction diode
When a pn junction is formed, electrons in the n-material
diffuse across the junction and recombine with holes in
the p-material. This action continues until the voltage of
the barrier repels further diffusion. Further diffusion
across the barrier requires the application of a voltage.
The pn junction is basically a diode,
which is a device that allows current
in only one direction. A few typical
diodes are shown.

Forward bias
When a pn junction is forward-biased, current is
permitted. The bias voltage pushes conduction-band
electrons in the n-region and holes in the p-region toward
the junction where they combine.
The barrier potential in the depletion
region must be overcome in order
for the external source to cause
current. For a silicon diode, this is
about 0.7 V.
p-region n-region
p n
+ −
R
VBIAS
The forward-bias causes the depletion region to be narrow.

Reverse bias
When a pn junction is reverse-biased, the bias voltage
moves conduction-band electrons and holes away from the
junction, so current is prevented.
The diode effectively acts as an
insulator. A relatively few electrons
manage to diffuse across the
junction, creating only a tiny reverse
current.
p-region n-region
p n
+−
VBIAS
R
The reverse-bias causes the depletion region to widen.

Diode characteristics
The forward and reverse characteristics are shown on
a V-I characteristic curve.
In the forward bias region, current
increases dramatically after the
barrier potential (0.7 V for Si) is
reached. The voltage across the
diode remains approximately
equal to the barrier potential.
VR VF
IF
IR
Reverse
bias
Forward
bias
0.7 V
Barrier
potential
The reverse-biased diode
effectively acts as an insulator
until breakdown is reached.
VBR (breakdown)

Diode models
The characteristic curve for a diode can be approximated
by various models of diode behavior. The model you will
use depends on your requirements.
The ideal model assumes the diode is
either an open or closed switch.
VR VF
IF
IR
Reverse
bias
Forward
bias
The complete model includes the
forward resistance of the diode.
The practical model includes the
barrier voltage in the approximation.
0.7 V

Half-wave Rectifier
Rectifiers are circuits that convert ac to dc. Special
diodes, called rectifier diodes, are designed to handle the
higher current requirements in these circuits.
The half-wave rectifier
converts ac to pulsating
dc by acting as a closed
switch during the
positive alteration.
The diode acts as an
open switch during the
negative alteration.
D
D
RL
RL
+ −
− +

Full-wave Rectifier
The full-wave rectifier allows unidirectional current on
both alterations of the input. The center-tapped full-wave
rectifier uses two diodes and a center-tapped transformer.
F D1
D2
RL
Vsec
2
Vsec
2
The ac on each side of the center-tap is ½ of the total secondary
voltage. Only one diode will be biased on at a time.

Bridge Rectifier
The bridge rectifier is a type of full-wave circuit that uses
four diodes. The bridge rectifier does not require a
center-tapped transformer.
F
D1
D2
RL
At any instant, two of the diodes are conducting and two are off.
D3
D4

Peak inverse voltage
Diodes must be able to withstand a reverse voltage when
they are reverse biased. This is called the peak inverse
voltage (PIV). The PIV depends on the type of rectifier
circuit and the maximum secondary voltage.
For example, in a full-wave circuit, if one diode is conducting
(assuming 0 V drop), the other diode has the secondary voltage
across it as you can see from applying KVL around the green path.
0 V
Vsec
Notice that Vp(sec) = 2Vp(out) for
the full-wave circuit because
the output is referenced to the
center tap.

Peak inverse voltage
For the bridge rectifier, KVL can be applied to a loop
that includes two of the diodes. Assume the top diode is
conducting (ideally, 0 V) and the lower diode is off. The
secondary voltage will appear across the non-conducting
diode in the loop.
0 V
Notice that Vp(sec) = Vp(out) for the bridge because the output is
across the entire secondary.
Vsec

Power supplies
By adding a filter and regulator to the basic rectifier, a
basic power supply is formed.
7805
F
D1
D2
C1
D3
D4
C2
Typically, a large electrolytic capacitor is used as a filter before
the regulator, with a smaller one following the regulator to
complete filtering action.
1000 µF 1 µF
IC regulator

Special-purpose diodes
Special purpose diodes include
Zener diodes – used for establishing a reference voltage
Varactor diodes – used as variable capacitors
Light-emitting diodes – used in displays
Photodiodes – used as light sensors

Troubleshooting power supplies
Begin troubleshooting by analyzing the symptoms and how
it failed. Try to focus on the most likely causes of failure.
7805
F
D1
D2
C1
D3
D4
C2
1000 µF 1 µF
IC regulator
A power supply has no output, but was working until a
newly manufactured PC board was connected to it. (a) Analyze
possible failures. (b) Form a plan for troubleshooting.

7805
F
D1
D2
C1
D3
D4
C2
1000 µF 1 µF
IC regulator
The supply had been working, so the problem is not
likely to be an incorrect part or wiring problem. The failure was
linked to the fact that a new PC board was connected to it,
which points to a possible overloading problem. If the load was
too much for the supply, it is likely a fuse would have blown,
or a part would likely have overheated, accounting for the lack
of output.

1. Disconnect power and check the fuse. If it is bad, replace it.
Before reapplying power, remove the load, open the power
supply case, and look for evidence of overheating (such as
discolored parts or boards). If no evidence of overheating
proceed.2. Check the new pc board (the load) for a short or overloading of
the power supply that would cause the fuse to blow. Look for
evidence of overheating.
3. Verify operation of the supply with measurements (see next
slide).
Based on the analysis, a sample plan is as
follows. (It can be modified as circumstances warrant.)

Reapply power to the supply but with no load. If the output is
okay, put a resistive test load on the power supply and measure
the output to verify it is operational. If the output is correct, the
problem is probably with the new pc board. If not, you will need
to further refine the analysis and plan, looking for an internal
problem.
The analysis showed that a
likely cause of failure was due to an overload. For the
measurement step, it may be as simple as replacing the
fuse and confirming that the supply works. After
replacing the fuse:

Majority carrier
Minority carrier
PN junction
Diode
The most numerous charge carrier in a doped
semiconductor material (either free electrons
or holes.
Selected Key Terms
The boundary between n-type and p-type
semiconductive materials.
An electronic device that permits current in
only one direction.
The least numerous charge carrier in a doped
semiconductor material (either free electrons
or holes.

Barrier
potential
Forward bias
Reverse bias
Full-wave
rectifier
A circuit that converts an alternating sine-
wave into a pulsating dc consisting of both
halves of a sine wave for each input cycle.
The condition in which a diode conducts
current.
The inherent voltage across the depletion
region of a pn junction diode.
Selected Key Terms
The condition in which a diode prevents
current.

Bridge rectifier
Zener diode
Varactor
Photodiode A diode whose reverse resistance changes
with incident light.
A type of diode that operates in reverse
breakdown (called zener breakdown) to
provide a voltage reference.
A type of full-wave rectifier consisting of
diodes arranged in a four corner configuration.
Selected Key Terms
A diode used as a voltage-variable capacitor.

L3 electronics pn junction

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