The document summarizes several special-purpose diodes including Zener diodes, varactor diodes, optical diodes, Schottky diodes, PIN diodes, step recovery diodes, tunnel diodes, laser diodes, and current regulator diodes. It describes their basic operation and symbol, and provides examples of their applications such as using Zener diodes for voltage regulation, varactor diodes for tuning circuits, LEDs and photodiodes for optical applications, and laser diodes in fiber optic communications. The document aims to present an overview of different types of special-purpose diodes and their uses in electrical and electronic systems.

1. SPECIAL - PURPOSE DIODES
Presenting By: Submitted To:
Adnan Qayum Lecturer Engr. Iqbal Munir
13SWTBTECE156 Govt. College of Technology
B.Tech Electrical, Ist year Swat.
adnanqayum2005@gmail.com

2. It includes the followings:
Schottky diode
Pin diode
Tunnel diode
Step recovery diode/snap diode
Laser diode
Impatt diode
Gunn diode
Constant-current diodes
 Zener diodes
 Vericap / Varactor diodes/variable capacitance diode
 Optical diodes (i) light emitting diode (LED) (ii) photo
diode
 Other types of diodes

3. Zener Diodes
• A Zener diode is a type of diode that
permits current not only in the forward
direction like a normal diode, but also in the
reverse direction if the voltage is larger than
the breakdown voltage known as "Zener knee
voltage" or "Zener voltage". The device was
named after Clarence Zener, who discovered
this electrical property.

4. Symbol

5. Zener diode in a circuit

6. ZENER DIODE APPLICATIONS
• The zener diode is often used as a type of
voltage regulator for providing stable stable
reference voltages.

7. Varactor Diodes
Vericap, Varactor or variable capacitance diode
(operate in reverse bias) is a type
of diode whose capacitance varies as a
function of the voltage applied across its
terminals.
Vericap circuit symbol

8. Basic Operation

9. An Application
A major application of varactors is in tuning circuits. For example, electronic tuners in
TV and other commercial receivers utilize varactors. When use in a resonant circuit,
the varactor act as variable capacitor, thus allowing the resonant frequency to be
adjusted by a variable voltage level, where the varactor diode provides the total
variable capacitance in the parallel resonant band-pass filter

10. Optical Diodes
Two types
i) Light Emitting Diode (LED)
ii) Photo Diode
Light Emitting Diode (LED): operates in forward biased,
The light-emitting diode (LED) emits photons as visible light. It’s purpose is
for indication and other intelligible displays. Various impurities are
added during the doping process to vary the color output.

11. Symbol

12. Light Emission in LED

14. Applications
• Visual signals where light goes more or less directly from the
source to the human eye, to convey a message or meaning.
• Illumination where light is reflected from objects to give visual
response of these objects.
• Measuring and interacting with processes involving no human
vision.
• Narrow band light sensors where LEDs operate in a reverse-
bias mode and respond to incident light, instead of emitting
light

15. The Photodiode
A photodiode is a semiconductor device that
converts light into current. The current is generated
when photons are absorbed in the photodiode. A
small amount of current is also produced when no
light is present. Photodiodes may contain optical
filters, built-in lenses, and may have large or small
surface areas. Photodiodes usually have a slower
response time as its surface area increases. The
common, traditional solar cell used to generate
electric solar power is a large area photodiode. A
photodiode is designed to operate in reverse bias.

16. Symbol

17. Principle of operation
A photodiode is a p-n junction structure. When a photon of
sufficient energy strikes the diode, it creates an
electron, hole pair. This mechanism is also known as the
inner photoelectric effect. If the absorption occurs in the
junction's depletion region, or one diffusion length away
from it, these carriers are swept from the junction by the
built-in electric field of the depletion region. Thus holes
move toward the anode, and electrons toward the cathode,
and a photocurrent is produced. The total current through
the photodiode is the sum of the dark current (current that
is generated in the absence of light) and the photocurrent,
so the dark current must be minimized to maximize the
sensitivity of the device

18. Operation of a Photodiode

19. Other types of Diodes
Current Regulator Diode
It keeps constant current.
In Forward biased
With specific range of voltage i.e. from 1.5 V to 6 V.
Symbol

20. Schottky diode
A solid-state diode having a metal-semiconductor
junction, used in fast switching applications.

21. A PIN diode is a diode with a wide, lightly doped
'near' intrinsic semiconductor region between a p-type
semiconductor and an n-type semiconductor region

22. Step recoverydiode (SRD) is a semiconductor junction diode having the ability to
generate extremely short pulses.
• The step recovery diode or SRD is able to be
used as a microwave radio frequency
generator and pulse sharpener. Although used
in more specialist applications, the SRD is
nevertheless a very useful component that is
capable of some very high levels of
performance. As such it is a very useful tool in
the armoury of the RF design engineer to be
used when the occasion requires.

23. The tunnel diode exhibits a special characteristics known as negative resistance. This
features makes it useful in oscillator and microwave amplifier applications.
Negative resistance is a property of some electric circuits where an increase in
the current entering a port results in a decreased voltage across the same port. This is
in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the
same conditions

24. The Laser Diode
• A laser diode is electrically a P-i-n diode. The active region of the laser
diode is in the intrinsic (I) region, and the carriers, electrons and holes, are
pumped into it from the N and P regions respectively
• laser diode, or LD, is an electrically pumped semiconductor laser in which
the active medium is formed by a p-n junction of a semiconductor
diode similar to that found in a light-emitting diode.
• The laser diode is the most common type of laser produced. Laser diodes
have a very wide range of uses that include, but are not limited to, fiber
optic communications, barcode readers, laser pointers, CD/DVD/Blu-ray
reading and recording, laser printing, scanning and increasingly directional
lighting sources.

25. Laser diode (LD)
Symbol

26. x