ECEG-2131 (AEI): Types of Diodes
Addis Ababa Institute of Technology (AAIT) School of Electrical and
Computer Engineering
Learning Outcomes
At the end of the lecture, students should
be able to know about:
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Zener Diodes.
Tunnel Diodes.
Varactor Diodes.
Schottky Diodes.
Shockley Diodes.
Light-Emitting Diodes
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Zener Diodes
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Are specifically designed to operate under reverse
breakdown conditions.
These diodes have a very accurate and specific reverse
breakdown voltage.
 The maximum reverse-bias potential that can be applied
before entering the breakdown region is called the peak
inverse voltage (PIV)
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Reverse Breakdown
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As the reverse voltage across the pn junction increases,
eventually “breakdown” occurs and a sudden, enormous
current is observed.
The breakdown resulting from a high voltage (and hence a
high electric field) can occur in any material. E.g. Lightning
A high electric field in the depletion region may impart enough
energy to the remaining covalent electrons to tear them from
their bonds.
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Reverse Breakdown: Zener Effect
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In order to create such high fields with reasonable
voltages, a narrow depletion region is required, which
implies high doping levels.
This type of breakdown is called the Zener Effect.
It appears for reverse bias voltages on the order of 3-8 V
It has a negative temperature coefficient for the breakdown
voltage. (i.e. as the temperature increases the breakdown
voltage decreases)
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Reverse Breakdown: Avalanche Effect
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It takes place when a reverse bias voltage increases across
a junction with moderate or low doping levels (<1015cm-3).
Because of small leakage current, each carrier entering the
depletion region experiences a very high electric field and
hence a large acceleration, gaining enough energy to break
the electrons from their covalent bonds.
This phenomenon called “impact ionization,” can lead to
avalanche.
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Reverse Breakdown
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Avalanche Effect has a positive temperature coefficient for
the breakdown voltage. (i.e. as the temperature increases
the breakdown voltage increases as well).
The Zener and avalanche breakdown effects do not damage
the diodes if the resulting current remains below a certain
limit given by the doping levels and the geometry of the
junction.
Both the breakdown voltage and the maximum allowable
reverse current are specified by diode manufacturers.
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Zener Diode
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A Zener is a diode operated in reverse bias at the Zener
voltage (VZ).
The voltage that causes a diode to enter the Zener region of
operation is called the Zener voltage.
Zener diodes are available having Zener potentials of 1.8 V
to 200 V with power ratings from 1⁄4 W to 50 W.
Zener Diodes are used in the design of voltage regulators.
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Tunnel Diode
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It is a high-conductivity two-terminal P-N junction diode
having doping density about 1000 times higher as
compared to an ordinary junction diode.
This doping level produces three unusual effects:
 reduces the width of the depletion layer to an
extremely small value (about 0.00001 mm).
 reduces the reverse breakdown voltage to a very small
value (approaching zero). i.e. the diode appears to be
broken down for any reverse voltage.
 produces a negative resistance section on the V/I
characteristic of the diode.
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Tunnel Diode
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Due to its extremely thin depletion layer, electrons are able
to tunnel through the potential barrier at relatively low
forward bias voltage (less than 0.05 V), hence its name.
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Tunnel Diode
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Tunnel diodes can be used as high frequency oscillators as
the transition between the high electrical conductivity is very
rapid. They can be used to create oscillation as high as 5Gz.
Normal diode transistors do not perform well in microwave
operation. So, for microwave generators and amplifiers
tunnel diode are preferred.
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and Computer Engineering
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Varactor Diode
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A Varactor diode is a P-N junction diode that changes its
capacitance as the bias applied to the diode is varied.
It is used in the reversed biased condition.
Its junction capacitance decreases with an increase in the
depletion region width caused due to an increase in the
reverse bias voltage (VR).
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Varactor Diode
 Varactor diodes are used in the design of Voltage Controlled
Oscillator.
 These diodes can be used as frequency modulators and RF
phase shifters.
 These diodes can be used as frequency multipliers in
microwave receiver.
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and Computer Engineering
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Schottky Diode
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It is also called Schottky barrier diode or hot-carrier diode.
It is a metal-semiconductor junction diode with no depletion
layer.
It is a unipolar device because it has electrons as majority
carriers on both sides of the junction.
Since no holes are available in metal, there is no depletion
layer to worry about. Hence, Schottky diode can switch OFF
faster than an ordinary PN junction diode.
It is very common in computers because of their ability to be
switched on and off so quickly.
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Shockley Diode
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It is a two-terminal four-layer diode, unlike other diodes
which are normally made with two layers.
It is generally used to control the average power delivered to
a load.
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Light Emitting Diode (LED)
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As the name indicates, it is a forward-biased P-N junction
which emits visible light when energized.
LEDs are designed with a very large bandgap so movement
of carriers across their depletion region emits photons of light
energy.
Lower bandgap LEDs emit infrared radiation, while LEDs with
higher bandgap energy emit visible light.
Have many applications in everyday life.
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Other Diodes
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Photodiode (Solar Cells)
PIN Diode
Gunn Diode
IMPATT Diode
Step Recovery Diode
…
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What to Do This Week?
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Reading Assignment
Practical Diode Circuits
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Rectifiers,
Clippers,
Clampers,
Zener Regulators.
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and Computer Engineering
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