1) Zener Diode:

A Zener Diode is a special kind of diode which permits current to flow in the forward
direction as normal, but will also allow it to flow in the reverse direction when the voltage is
above a certain value - the breakdown voltage known as the Zener voltage.
The Zener voltage of a standard diode is high, but if a reverse current above that value is
allowed to pass through it, the diode is permanently damaged. Zener diodes are designed
so that their Zener voltage is much lower - for example just 2.4 Volts. When a reverse
current above the Zener voltage passes through a Zener diode, there is a
controlled breakdown which does not damage the diode. The voltage drop across the Zener
diode is equal to the Zener voltage of that diode no matter how high the reverse bias
voltage is above the Zener voltage.

The illustration above shows this phenomenon in a Current vs. Voltage graph. With a zener
diode connected in the forward direction, it behaves exactly the same as a standard diode -
i.e. a small voltage drop of 0.3 to 0.7V with current flowing through pretty much unrestricted.
In the reverse direction however there is a very small leakage current between 0V and the
Zener voltage - i.e. just a tiny amount of current is able to flow. Then, when the voltage
reaches the breakdown voltage (Vz), suddenly current can flow freely through it.
Symbol the zener diode:

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 Application:
Zener diodes are widely used as voltage references and as shunt regulators to regulate the
voltage across small circuits. When connected in parallel with a variable voltage source so
that it is reverse biased, a zener diode conducts when the voltage reaches the diode's
reverse breakdown voltage. From that point on, the relatively low impedance of the diode
keeps the voltage across the diode at that value.

In this circuit, a typical voltage reference or regulator, an input voltage, UIN, is regulated
down to a stable output voltage UOUT. The breakdown voltage of diode D is stable over a
wide current range and holds UOUT relatively constant even though the input voltage may
fluctuate over a fairly wide range. Because of the low impedance of the diode when
operated like this, resistor R is used to limit current through the circuit.
In the case of this simple reference, the current flowing in the diode is determined using
Ohm's law and the known voltage drop across the resistor R.

2) Photodiodes:
These diodes respond to photons at a specific wavelength and are used to receive energy.
Most photodiodes operate in the infrared region although visible spectrum types do exist.
Symbol:

2
 Photo diode
Application:
Photodiodes are used in many different types of circuits and applications. Here are a few
examples of where photodiodes have been used.
Camera
• Light Meters
• Automatic Shutter Control
• Auto-focus
• Photographic Flash Control
Automotive
• Headlight Dimmer
• Twilight Detectors
• Medical
• CAT Scanners - X ray Detection
• Pulse Oximeters
• Blood Particle Analyzers
Safety Equipment
• Smoke Detectors
• Flame Monitors
• Security Inspection Equipment - Airport X ray
• Intruder Alert - Security Systemlimate Control - Sunlight Detector
Communications
• Fiber Optic Links
• Optical Communications
• Optical Remote Control
Industry
• Bar Code Scanners
• Light Pens
• Brightness Controls
• Encoders
• Position Sensors
• Surveying Instruments
• Copiers - Density of Toner
Typical Applications:

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Typical Application Circuits (cont.) s:

Fiber optic link:

Smoke detector circuit:

Light meter circuit:

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 3) Tunnel diode:

Symbol:

Applications:

1) One the most important application of the tunnel diode in the digital computer.
2) Another use of the tunnel diode in the radio communication. It is because of the high
speed frequency, excellent stability, and its ability to operate in critical environment
with the low power supply.
There are four demonstrations for the use of the tunnel diode in radio
communication.
FM receiver, FM transmitter, and Microsoft oscillator.

4)Light emitting diode:

Symbol:

5
 (LED)

Applications:
1) There are various materials that are used in the manufacturing of Light Emitting
Diodes. Most of the materials are gallium-based crystals and are used in high-
brightness applications.
2) Gallium is a minor metal noted by its low melting point of 29.8 ºC, the name being
derived from Gallia, the Latin for France, which was where it was discovered.
Among these include AlGaAs (Aluminum-Gallium-Arsenide), a semiconductor that
typically generates the red spectrum, often used in signs, displays and electronic
equipment.
3) InGaAlP (Indium-Gallium-Aluminum-Phosphide) produces the yellow-green
wavelength to red are often used in signs, auto interior as well as exterior, traffic
signals and cellphones.
4) InGaN (Indium-Gallium-Nitride) typically generates Blue, Green and white
spectrums and are used most often in full color signs, cell-phones, auto interior,
traffic signals. Furthermore, there is room for further improvement on the design of
traffic lights. The visible light from the LEDs in a traffic light can further be modulated
and encoded with information. Hence, it can be used for the broadcasting of audio
messages or any traffic or road information.
5) Essentially, all LED traffic lights can be used as Communications devices. InGaN
LEDs too has been made the light source of choice for many diagnostic and photo-
therapy applications from the Ultra-violet to the near Infrared.
6) Light-emitting diodes (LED) emit light in proportion to the forward current through
the diode. Light Emitting Diodes are the cutting edge technology of lighting today.

5) Varactor diode applications

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Although varactor diodes can be used within many types of circuit, they find applications
within two main areas:

Voltage controlled oscillators, VCOs:

Voltage controlled oscillators are used for a variety of applications. One major area
is for the oscillator within a phase locked loop - this is used in almost all radio, cellular and
wireless receivers. A varactor diode is a key component within a VCO.

RF filters:

Using varactor diodes it is possible to tune filters. Tracking filters may be needed in
receiver front end circuits where they enable the filters to track the incoming received signal
frequency. Again this can be controlled using a control voltage. Typically this might be
provided under microprocessor control via a digital to analogue converter.

6) Varicap diode
In electronic, a varicap diode, varactor diode, variable capacitance diode, variable reactance
diode or tuning diode is a type of diode which has a capacitance that varies as a function of
the voltage applied across its terminals.

Applications:
1) Varactors are used as voltage-controlled capacitors.
2) They are commonly used in parametric amplifiers, parametric
oscillators and voltage-controlled oscillators as part of phase-locked loops and
frequency. For example, varactors are used in the tuners of television sets to
electronically tune the receiver to different stations.
7) Schottky diode
This type of diode has a lower forward voltage drop than ordinary silicon PN junction
diodes. At low currents the drop may be somewhere between 0.15 and 0.4 volts as opposed
to 0.6 volts for a silicon diode. To achieve this performance they are constructed in a
different way to normal diodes having a metal to semiconductor contact. They are widely
used as clamping diodes, in RF applications, and also for rectifier applications

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Symbol:

Voltage clamping:
While standard silicon diodes have a forward voltage drop of about 0.7 volts and germanium
diodes 0.3 volts, Schottky diodes’ voltage drop at forward biases of around 1 mA is in the
range 0.15 V to 0.46 V, which makes them useful in voltage clamping applications and
prevention of transistor saturation. This is due to the higher current density in the Schottky
diode.
Reverse current and discharge protection
Because of a Schottky diodes’ low forward voltage drop, less energy is wasted as
heat making them the most efficient choice for applications sensitive to efficiency. For
instance, they are used in stand-alone ("off-grid") photovoltaic (PV) systems to
prevent batteries from discharging through the solar panels at night, and in grid-connected
systems with multiple strings connected in parallel, in order to prevent reverse current
flowing from adjacent strings through shaded strings if the bypass diodes have failed.
Power supply
They are also used as rectifiers in switched-mode power supplies; the low
forward voltage and fast recovery time leads to increased efficiency.
Schottky diodes can be used in power supply "OR"ing circuits in products that have both an
internal battery and a mains adapter input, or similar. However, the high reverse leakage
current presents a problem in this case, as any high-impedance voltage sensing circuit (e.g.
monitoring the battery voltage or detecting whether a mains adaptor is present) will see the
voltage from the other power source through the diode leakage
Laser diode

A laser diode is a laser whose active medium is a semiconductor similar to that found in a
light-emitting diode. The most common type of laser diode is formed from a p-n junction

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and powered by injected electric current. The former devices are sometimes referred to as
injection laser diodes to distinguish them from optically pumped laser diodes.
Application
Laser diodes can be arrayed to produce very high power outputs, continuous wave or
pulsed. Such arrays may be used to efficiently pump solid-state lasers for high average
power drilling, burning or for inertial confinement fusion.

Laser diodes are numerically the most common laser type, with 2004 sales of approximately
733 million units,as compared to 131,000 of other types of lasers.

Laser diodes find wide use in telecommunication as easily modulated and easily coupled
light sources for fiber optics communication. They are used in various measuring
instruments, such as rangefinders. Another common use is in barcode readers. Visible
lasers, typically red but later also green, are common as laser pointers. Both low and high-
power diodes are used extensively in the printing industry both as light sources for scanning
(input) of images and for very high-speed and high-resolution printing plate (output)
manufacturing. Infrared and red laser diodes are common in CD players, CD-ROMs and
DVD technology. Violet lasers are used in HD DVD and Blu-ray technology. Diode lasers
have also found many applications in laser absorption spectrometry (LAS) for high-
speed, low-cost assessment or monitoring of the concentration of various species in gas
phase. High-power laser diodes are used in industrial applications such as heat treating,
cladding, seam welding and for pumping other lasers, such as diode-pumped solid-state
lasers.

Uses of laser diodes can be categorized in various ways. Most applications could be served
by larger solid-state lasers or optical parametric oscillators, but the low cost of mass-
produced diode lasers makes them essential for mass-market applications. Diode lasers
can be used in a great many fields; since light has many different properties (power,
wavelength, spectral and beam quality, polarization, etc.) it is useful to classify applications
by these basic properties.

Many applications of diode lasers primarily make use of the "directed energy" property of an
optical beam. In this category one might include the laser printers, barcode readers, image
scanning, illuminators, designators, optical data recording, combustion ignition, laser
surgery, industrial sorting, industrial machining, and directed energy weaponry. Some of
these applications are well-established while others are emerging.

Laser medicine: medicine and especially dentistry have found many new uses for diode
lasers. The shrinking size of the units and their increasing user friendliness makes them
very attractive to clinicians for minor soft tissue procedures. The 800 nm – 980 nm units
have a high absorption rate for hemoglobin and thus make them ideal for soft tissue
applications, where good homeostasis is necessary.

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Uses which may make use of the coherence of diode-laser-generated light include
interferometric distance measurement, holography, coherent communications, and coherent
control of chemical reactions.

Uses which may make use of "narrow spectral" properties of diode lasers include range-
finding, telecommunications, infra-red countermeasures, spectroscopic sensing,
generation of radio-frequency or terahertz waves, atomic clock state preparation, quantum
key cryptography, frequency doubling and conversion, water purification (in the UV), and
photodynamic therapy (where a particular wavelength of light would cause a substance
such as porphyrin to become chemically active as an anti-cancer agent only where the
tissue is illuminated by light).

Uses where the desired quality of laser diodes is their ability to generate ultra-short pulses
of light by the technique known as "mode-locking" include clock distribution for high-
performance integrated circuits, high-peak-power sources for laser-induced breakdown
spectroscopy sensing, arbitrary waveform generation for radio-frequency waves, photonic
sampling for analog-to-digital conversion, and optical code-division-multiple-access systems
for secure communication.

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