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PN Junction Diode (Forward Bias)

• If a suitable positive voltage (forward bias) is
applied between the two ends of the PN
junction, it can supply free electrons and holes
with the extra energy they require to cross the
junction as the width of the depletion layer
around the PN junction is decreased.
• As more electrons flow into the depletion
region reducing the number of negative ions
and similarly more holes move in reducing the
positive ions. Key points: What
1.Width decreases (barrier happened to
reduces) minority
2.Resistance decreases carriers?
3.Act as short circuit
(ideal ,R=0) ZERO current or
4. Current conduction due KNEE current.
to ………carriers? 2
VI Characteristics of Diode for Forward Bias

Function
IF= (Vbias- of R ?
VF)/R

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 QUICK QUIZ (POLL)
A diode has forward resistance of the order of ……………
A.kΩ
B.Ω
C.MΩ
D.none of the above
 QUICK QUIZ (POLL)
The knee voltage of a crystal diode is approximately equal
to ………….
A) applied voltage
B) breakdown voltage
C) forward voltage
D) barrier potential
 PN Junction Diode (Reverse Bias)

Positive side of Vbias is connected to the n-

region whereas the negative side is connected
with p-region.
By applying a negative voltage (reverse
bias) results in the free charges being pulled
away from the junction resulting……
 Depletion region get wider with this configuration.
 Effective resistance increases (high in K ohm).
 A high potential barrier is created across the
junction thus preventing current from flowing
through the semiconductor material.
 Act as open switch (ideal , R=Infinite)
 But ,A small amount current is generated due to
the minority carriers in p and n regions called
leakage current or reverse saturation current

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VI Characteristics of Diode for Reverse Bias

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 QUICK QUIZ (POLL)
A silicon diode has an external bias voltage is 12V with an
external resistor of 150 ohm. What is the total forward current
a) 150mA
b) 100mA
c) 75mA
d) 12mA
 QUICK QUIZ (POLL)
• In a PN junction when the applied voltage overcomes the ........
potential, the diode current is large, which is known as .............

A) Depletion, negative bias

B) Reverse, reverse bias
C) Resistance, reverse bias
D) Barrier, forward bias
Avalanche breakdown
• The reverse bias increases the electrical field
across the depletion region. When the high
electric field exists across the depletion, the
velocity of minority charge carrier crossing the
depletion region increases. These carriers
collide with the atoms of the crystal. Because
of the violent collision, the charge carrier
takes out the electrons from the atom.

• The collision increases the electron-hole pair.

As the electron-hole induces in the high
electric field, they are quickly separated and
collide with the other atoms of the crystals.
The process is continuous, and the electric
field becomes so much higher then the
reverse current starts flowing in the PN
junction. The process is known as
the Avalanche breakdown. After the
breakdown, the junction cannot regain its
original position because the diode is
completely burnt off.
 Avalanche Breakdown Phenomenon:
• After certain reverse voltage across the
junction, the minority carriers get sufficient
kinetic energy due to reverse voltage and
the strong electric field.
• Free electrons with sufficient kinetic energy
collide with stationary ions of the depletion
layer and knock out more free electrons.
• These newly created free electrons also get
sufficient kinetic energy due to the same
electric field, and they create more free
electrons by collision cumulatively.
• As a result, very soon, huge number of
free electrons get created in the depletion
layer, and the entire diode will become
conductive.
• This type of breakdown of the depletion
layer is known as avalanche breakdown.
 Reverse Breakdown in the Reverse Bias

If the external bias voltage is

increased to a value call breakdown
voltage the reverse current can
increase drastically.
Free minority electrons get enough
energy to knock valance electron into
the conduction band.
The newly released electron can
further strike with other atoms.

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 QUICK QUIZ (POLL)
During reverse bias, a small current develops known as
a) Forward current
b) Reverse current
c) Reverse saturation current
d) Active current

(In pico ampere)

 Summerize Diode
Diode, semiconductor material, such as silicon, in which half is doped as p-region and half
is doped as n-region with a PN-junction in between.
The p region is called anode and n type region is called cathode.

It conducts current in one direction and offers high (ideally infinite) resistance in
other direction.

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Complete VI Characteristics of Diode

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 QUICK QUIZ (POLL)
An ideal crystal diode is one which behaves as a perfect ………..
when forward biased.
A.conductor
B.insulator
C.resistance material
D.none of the above
 Summary
• The PN junction region of a Junction Diode has the following important characteristics:
1.Semiconductors contain two types of mobile charge carriers, “Holes” and “Electrons”.
2.The holes are positively charged while the electrons negatively charged.
3.A semiconductor may be doped with donor impurities such as Antimony (N-type doping),
so that it contains mobile charges which are primarily electrons.
4.A semiconductor may be doped with acceptor impurities such as Boron (P-type doping),
so that it contains mobile charges which are mainly holes.
5.The junction region itself has no charge carriers and is known as the depletion region.
6.The junction (depletion) region has a physical thickness that varies with the applied
voltage.
7.When a diode is Zero Biased no external energy source is applied and a natural Potential
Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon
diodes and approximately 0.3 of a volt for germanium diodes.
8.When a junction diode is Forward Biased the thickness of the depletion region reduces
and the diode acts like a short circuit allowing full circuit current to flow.
9.When a junction diode is Reverse Biased the thickness of the depletion region increases
and the diode acts like an open circuit blocking any current flow, (only a very small leakage
current will flow).
 Diode Model
• Ideal Diode
• Practical Diode
 Applications of a Diode
1. Rectifiers
❑ Half Wave Rectifier
❑Full Wave Rectifier
2. Clipper
3. Clamper
 Testing of diode

https://www.youtube.com/watch?v=8LGD5LdOzEM
The knee voltage of a diode approximately is equal to the

A.Breakdown voltage
B.Barrier potential
C.Applied voltage
D.Forward voltage

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The turn-on voltage of a Ge junction diode is

A.1.0 V
B.0.3 V
C.0.7 V
D.0.1 V

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In reverse biased diode, as the reverse voltage decreases the
depletion region

A.Remains same
B.Decreases
C.Increases
D.Depends on doping

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The forward region of a semiconductor diode characteristic curve is
where diode appears as

A.An OFF switch

B.A capacitor
C.A constant current source
D.An ON switch

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Application of forward bias to a junction diode

A.Reduces forward current

B.Reduces minority carrier current
C.Reduces potential barrier height
D.All of the above

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Diode as Rectifiers
Reversing diode.

Average value of Half wave output voltage:

VAVG = VP / pi

VAVG is approx 31.8% of Vp

 PIV: Peak Inverse Voltage = Vp

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Half wave Rectifiers
As diodes conduct current in one direction and block in other.
When connected with ac voltage, diode only allows half cycle passing
through it and hence convert ac into dc.
As the half of the wave get rectified, the process called half wave
rectification.

 A diode is connected to an ac source and a load resistor

forming a half wave rectifier.
 Positive half cycle causes current through diode, that causes
voltage drop across resistor.

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Full wave rectifiers
A full wave rectifier allows unidirectional current through the load
during the entire 360 degree of input cycle.

Full Wave Rectifier

The output voltage have twice the input frequency.

VAVG = 2VP / pi

VAVG is 63.7% of Vp

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 The Center-Tapped Full wave
rectifiers
• A center-tapped transformer is used with two diodes that conduct
on alternating half-cycles.

During the positive half-

cycle, the upper diode is
forward-biased and the
lower diode is reverse-
biased.

During the negative half-

cycle, the lower diode is
forward-biased and the upper
diode is reverse-biased.

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The Bridge Full-wave rectifiers
The Bridge Full-Wave rectifier uses four diodes connected across the
entire secondary as shown.

Conduction path for the

positive half-cycle.

Conduction path for the

negative half-cycle.

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 The Bridge Full-Wave Rectifier

Determine the peak output voltage and current in the 3.3 kW load resistor if
Vsec = 24 Vrms. Use the practical diode model.

The peak output voltage is:

= 32.5 V
Applying Ohm’s law,
Ip(out) = 9.8 mA

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Application of forward bias to a junction diode

A.Reduces forward current

B.Reduces minority carrier current
C.Reduces potential barrier height
D.All of the above

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 Homework
• Draw the forward and reverse characteristics of a diode and explain
which type of current flows during conduction.

• What are applications of a Diode? Explain the working of diode as a

rectifier with a diagram.

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