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Special Purpose

Special Purpose

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Zener Diode

Zener Diode

The zener diode – silicon pn-junction

device-designed for operate in the reverse-biased

region.

The breakdown voltage of a zener diode is set

by carefully controlling the level during

manufacture.

The basic function of zener diode is to

maintain a specific voltage across it’s

terminals within given limits of line or load

change.

Typically it is used for providing a stable

reference voltage for use in power supplies

and other equipment.

If a zener diode is forward biased it operates

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Zener Diode

Zener Diode

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Zener Diode

Zener Diode

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Zener Breakdown

Zener Breakdown

Zener diodes are designed to operate in reverse breakdown. Two

types of reverse breakdown in a zener diode are avalanche and zener. The avalanche break down occurs in both rectifier and zener diodes at a sufficiently high reverse voltage. Zener breakdown occurs in a

zener diode at low reverse voltages.

A zener diode is heavily doped to reduced the breakdown voltage. This causes a very thin depletion region. As a result, an intense electric field exists within the depletion region. Near the zener

breakdown voltage (Vz), the field is intense enough to pull electrons from their valence bands and create current. The zener diodes

breakdown characteristics are determined by the doping process. Low voltage zeners less than 5V operate in the zener breakdown range. Those designed to operate more than 5 V operate mostly in

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Breakdown Characteristics

Breakdown Characteristics

The reverse voltage (VR) is increased – the reverse current (IR) remains extremely small up to the “knee”of the curve

 Reverse current – the zener current, IZ

 At the bottom of the knee- the zener breakdown voltage (VZ) remains constant although it increase slightly as the zener current, IZ increase.

 IZK – min. current required to maintain voltage regulation

 IZM – max. amount of current the diode can handle without being damage/destroyed

 IZT – the current level at which the VZ rating of diode is measured

The zener diode maintains a constant voltage for value of reverse current rating from IZK to IZM

A zener diode operating in

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Zener Equivalent Circuit

Zener Equivalent Circuit

Z Z Z I V Z   

[image:7.720.17.687.69.506.2]
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Ex 3-1

Ex 3-1 A zener diode exhibits a certain change in V

Z

for a certain

change in I

Z

on a portion of

the linear characteristic curve between I

ZK

and I

ZM

as illustrated in Figure. What is the zener impedance?

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Zener Power Dissipation

Zener Power Dissipation

Zener diodes are specified to operate at a

maximum power called the maximum dc

power dissipation, P

D(max)

.

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Zener Diode Applications –

Zener Diode Applications –

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Zener Limiting

Zener Limiting

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Ex 3-8

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Varactor Diodes

Varactor Diodes

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Varactor Diodes

Varactor Diodes

Reverse bias voltage ↑ , depletion region ↑ , effectively the plate separation ↑ & the dielectric thickness ↑ , thus the

capacitance ↓ .

When the reverse bias voltage ↓ , the depletion region ↓ - the capacitance ↑

The capacitance is expressed by the following formula

; = plate area, = dielectric constant, = dielectric thickness

The capacitance parameters are controlled by the method of doping near the pn junction and the size and geometry of the diode’s construction.

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LEDs

LEDs

When the devices is FB – e- cross the pn

junction from the n-type material &

recombine with holes in p-type material,

Recombining release energy in the form of

heat and light.

A large exposed surface area on one layer

of the semi-conductive material permits

the photons to be emitted as visible light –

electroluminescence.

Various impurities are added during the

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LEDs

LEDs

LED Semiconductor Materials:

- GaAs, GaAsP, GaP

LED Biasing:

- The forward voltage across an LED is considerably greater than for a silicon diode

- The max. VF for LEDs is 1.2V – 3.2V

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LED Applications

LED Applications

Used for indicator lamps & readout displays on a wide variety

of instruments, ranging from consumer appliances to

scientific apparatus.

Common type of display device using LEDs – 7-segment

display.

IR light-emitting diodes – used in optical coupling applications

– often in conjunction with fiber optics.

Area of application include – industrial processing & control,

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Photodiode

Photodiode

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Photodiode

Photodiode

An increase in the amount of light intensity – irradiance (mW/cm2), produces an

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Current regulator diodes

Current regulator diodes

Current regulator diodes

keeps a constant

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Schottky diode

Schottky diode

The

Schottky diode’s

significant characteristic is it’s

fast switching speed.

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Pin Diode

Pin Diode

The Pin Diode is consists of heavily doped p & n regions separated

by an intrinsic (i) region

When reverse-biased – acts like constant capacitance.

When forward-biased – acts like a current controlled variable

resistance.

Low forward resistance of the intrinsic region decrease with

increasing current.

Used in mostly microwave frequency applications. Its variable

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Laser Diode

Laser Diode

Laser - Light amplification by stimulated emission of radiation Laser light is monochromatic – consists of a single color – called

coherent light

Normally emits coherent light – consists of a wide band of

wavelength, whereas the LED emits incoherent light

A pn junction is formed by two layers of doped GaAs

The length of the pn junction bears a precise relationship with the

wavelength of the light to emitted.

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All Diode Symbols

Figure

Figure (b) represents the practical model of a zener diode, where the

References

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