This document discusses several types of special diodes: varactor diodes, which have a variable capacitance controlled by the applied reverse voltage; Schottky barrier diodes, which have a low forward voltage drop and fast switching speed due to a metal-semiconductor junction; photodiodes, including PIN and avalanche photodiodes, which convert light to electrical signals; and light-emitting diodes, which emit light when current passes through. Each diode type has distinct properties and applications in electronic circuits.

1. Special Diode Types
Dr. Varun Kumar
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2. Outlines
1 Varactors
2 Schottky-Barrier Diode
3 Photodiodes
PIN photodiode
Avalanche photodiode
4 Light-Emitting Diodes
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3. Varactor diode:
Key properties of Varactor diode
The varactor diode is also known as
Variable capacitance diode
Variable reactance diode
Tuning diode
Varactor diode always function in reverse biasing condition.
Mathematical, formulation of capacitance can be expressed as
C =
0r A
d
and V =
Q
C
Its doping level is high.
⇒ Junction width reduces by increasing the doping concentration.
⇒ Higher the reverse voltage, greater be the junction width.
⇒ By changing the applied voltage the capacitance can be varied.
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4. Symbol of Varactor diode
Symbol
Application
Tuning circuits. For instance VHF (Very high frequency), UHF (Ultra
High frequency), and satellite receivers.
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5. Schottky-Barrier Diode
Schottky Diode
Schottky diode is also known as schottky barrier diode, surface barrier
diode, majority carrier device, hot-electron diode, or hot carrier diode.
⇒ A metal-semiconductor junction formed between a metal and n-type
semiconductor creates a barrier or depletion layer known as a schottky
barrier.
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6. Continued–
⇒ A Schottky Diode is a metal-semiconductor diode with a low forward
voltage drop (0.2V-0.5V), pn-junction diode ∼ (0.6V-0.8V)
and a
very fast switching speed.
⇒ Barrier height depends on the combination of semiconductor and
metal.
⇒ The schottky barrier height of ohmic contact ∼ junction between two
metal or (non-rectifying barrier) is very low whereas the schottky
barrier height of non-ohmic contact (rectifying barrier) is high.
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7. Application of Schottky diode
⇒ Rectification
⇒ Signal conditioning
⇒ Switching, through to TTL and CMOS logic gates due mainly to their
low power and fast switching speeds.
.
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8. Photodiode
⇒ It is a form of light sensor that converts light energy into electrical
energy (voltage or current).
⇒ Between the p (positive) and n (negative) layers, an intrinsic layer is
present.
Types of photodiode:
1 PIN Photodiode
2 Avalanche Photodiode
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9. Advantages and Disadvantages of PIN/Avalanche photodiode
Advantage of PIN photodiode
High-frequency response.
It is inexpensive and the response time is in nanoseconds.
PIN diodes have a broad spectral response.
Used for detection of laser pulses in ultra-fast switching circuits.
Disadvantage of PIN photodiode
It requires large reverse bias for its operation which reduces the SNR.
Advantages and Disadvantages of Avalanche Photodiode
⇒ It can detect very weak signal due to high current-gain bandwidth
product.
⇒ The construction is quite complicated.
Application: It is widely used in fibre optic communication Systems.
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10. Light Emitting Diode (LED)
⇒ A light-emitting diode (LED) is a semiconductor light source that
emits light when current flows through it.
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11. References
A. S. Sedra, D. E. A. S. Sedra, K. C. Smith et al., Microelectronic circuits. New
York: Oxford University Press, 1998.
R. L. Boylestad, Electronic devices and circuit theory. Pearson Education India,
2009.
D. L. Schilling, C. Belove, T. Apelewicz, and R. J. Saccardi, Electronic circuits:
discrete and integrated. McGraw-Hill Kogakushaˆ eTokyo Tokyo, 1968.
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