CHAPTER 3

Special-Purpose Diodes

Chapter Objectives:

Describe the characteristics of a zener diode and analyze its operation Explain how a zener is used in voltage regulation and limiting Describe the varactor diode and it’s variable capacitance characteristics Discuss the operation and characteristics of LEDs and photodiodes Discuss the basic characteristics of the current regulator diode, the pin diode, the step-recovery diode, the tunnel diode, and the laserdiode.

3-1 Zener Diodes (Introduction)

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 The zener diode – silicon pn-junction device-designed for operate in the reverse-biased region

Schematic diagram shown that this particular zener circuit will work to maintain 10 V across the load

Zener diode symbol

3-1 Zener Diodes (cont.) (Introduction)

Breakdown voltage – set by controlling the doping level during manufacture When diode reached reverse breakdown – voltage remains constant- current change drastically If zener diode is FB – operates the same as a rectifier diode A zener diode is much like a normal diode – but if it is placed in the circuit in reverse bias and operates in reverse breakdown. Note that it’s forward characteristics are just like a normal diode.

1.8V – 200V

3-1 Zener Diodes (cont.) (Zener Breakdown)

Two type of reverse breakdown: avalanche breakdown & zener breakdown i) avalanche breakdown - occurs in both rectifier & zener diodes at high reverse voltage - breakdown voltages greater than approximately 5V ii) zener breakdown - occurs at relatively low reverse voltage - n-type & p-type of materials of a zener diode are heavily doped - very thin depletion region – can break down at lower reverse voltage (VR) - as a result – intense electric field exits within the depletion region - the field have enough energy to pull electrons from their valence bands and create current.

Note: both type called zener diode (breakdown voltages of 1.8V – 200 V)

3-1 Zener Diodes (cont.) (Breakdown Characteristics)

The reverse voltage (VR) is increased – the reverse current (IR) remains extremely small up to the “knee”of the curve Reverse current – called 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 (specified on a data sheet) The zener diode maintains a constant voltage for value of reverse current rating from IZK to IZM

3-1 Zener Diodes (cont.) (Zener Equivalent Circuit)

Since the actual voltage is not ideally vertical, the change in zener currentproduces a small change in zener voltage By ohm’s law:

Normaly -Zz is specified at IZT

ZI ZV

Z

ZZ I

VZ

Zener impedance

(3-1)

3-1 Zener Diodes (cont.) (Temperature Coefficient & Zener Power Dissipation and Derating)

As with most devices, zener diodes have given characteristics such as temperature coefficients and power ratings that have to be considered. The data sheet provides this information (refer Figure 3-7).

3-2 Zener Diodes Applications (Introduction)

Zener diode can be used as

1. Voltage regulator for providing stable reference voltages

2. Simple limiters or clippers

3-2 Zener Diodes Applications (cont.) (Zener Regulation with a Varying Input Voltage)

- as i/p voltage varies (within limits) – zener diode maintains a constant o/p voltage -but as VIN changes, IZ will change, so i/p voltage variations are set by the min. & max. current value (IZK & IZM) with which the zener can operate-Resistor, R –current limiting resistor

3-2 Zener Diodes Applications (cont.) (Zener Regulation with a Varying Input Voltage)

- To illustrate regulation:From the data sheet

So,

For min. zener current, the voltage across the 220 ohm resistor:

Therefore:

For max. zener current,The voltage across resistor is:

Therefore:

VVWPmAI ZDZK 10,1,25.0 (max)

mAV

PI

Z

DZM 100(max)

mVRIV ZKR 5)220)(25.0(

mVVmVVVV ZRIN 055.101055(min)

VmARIV ZMR 22)220)(100(

VVVVIN 321022(max)

i/p10.055V – 32V

maintaino/p voltage

3-2 Zener Diodes Applications (cont.) (Zener Regulation with a Variable Load)

In this simple illustration of zener regulation circuit, the zener diode will “adjust”its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decreasedirectly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation.

Note: The zener diode maintains a nearly constant voltage across RL as longas the zener current is greater than IZK and less than IZM

3-2 Zener Diodes Applications (cont.) (From No Load to Full Load)

• When the o/p terminal of the zener diode is open (RL=∞)-load current is

zero and all of the current is through the zener

• When a load resistor (R) is connected, current flow through zener & load

• RL , IL , IZ

• The zener diode continues to regulate the voltage until IZ reaches its

min value , IZK

• At this point, the load current is max. , the total current through R remains

essentially constant.

3-2 Zener Diodes Applications (cont.) (Zener Limiting)

Zener diode also can be used in ac applications to limit voltage swings to desired levela) To limit the +ve peak of a signal voltage to the selected zener voltage - During –ve alternation, zener arts as FB diode & limits the –ve voltage to -0.7Vb) Zener diode is turn around -The –ve peak is by zener action & +ve voltage is limited to +0.7Vc) Two back-to-back zeners limit both peaks to the zener voltage ±7V -During the +ve alternation, D2 is functioning as the zener limiter – D1 is functioning as a FB diode. -During the –ve alternation-the roles are reversed

3-2 Zener Diodes Applications (cont.)

Example 1:

Determine the o/p voltage for zener limiting circuit.

7.7V

6.3V

3-3 Varactor Diodes (Introduction)

• A varactor diode is best explained as a variable capacitor – the junction

capacitance varies with the amount of reverse-bias voltage.

• Think of the depletion region a variable dielectric.

• The diode is placed in reverse bias- the dielectric is “adjusted” by bias changes.

• Operates in reverse-bias and is doped to maximize the inherent capacitance of

depletion region.

• The depletion region-widened by the reverse bias – acts as a capacitor dielectric

because of its nonconductive characteristic.

• The p & n regions are conductive – acts as the capacitor plates.

Varactor diode symbol

The reverse-biasedvaractor diode acts as a variable capacitor

3-3 Varactor Diodes (cont.) (Basic Operation)

• 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.

d

AC

A d

3-3 Varactor Diodes (cont.) (An Application)

Major applications – tuning circuits – e.g.: electronic tuning in TV, other commercialreceiver. When used in a resonant circuit –acts as a variable capacitor (see Figure) – allowedthe resonant frequency to be adjusted by a variable voltage level. Varactor diode provides the total variable capacitance in the parallel resonant band-pass filter. C1,C2,C3,C4 – coupling capacitor – prevent the dc bias circuit The parallel resonant frequency is

as a variable dc voltage divider for biasing the varactor

prevent a dc path from potentiometer wiper back to the ac source through the inductor & R1

prevent a dc path from cathode to the anode of the varactorthrough the inductor

prevent a dc path from the wiper to a load on the o/p through the inductor

prevent a dc path from the wiper to gnd

LCfr 2

1

Figure 3-23 A resonantband-pass filter

3-4 Optical Diodes (Introduction)

Two types of optoelectronic devices:

1. The light-emitting diode (LED) – a light emitter

2. Photodiode – a light detector

3-4 Optical Diodes (cont.) (LED)

Basic operation:- 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 semiconductive materialpermits the photons to be emitted as visible light - electroluminescence - Various impurities are added during the doping process to establish the wavelength of the emitted light – determine the color of the light and if it is visible or infrared (IR)

Symbol of LED Electroluminescencein a FB LED

3-4 Optical Diodes (cont.) (LED)

LED Semiconductor Materials:- Early LEDs - GaAs - 1st visible red LEDs – gallium arsenide phosphide (GaAsP) on a GaAs subtrate- GaP substrate – brighter red LEDs & orange LEDs- GaAs LEDs – emit infrared (IR) readiation – invisible- Others LEDs – refer textbook pg. 131.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- Reverse breakdown << for a silicon rectifier diode (typically 3V – 10V)

The amount of powero/p translated intolight is directly proportionalto the forward current

LEDs emits lightin response to asufficient forwardcurrent

3-4 Optical Diodes (cont.) (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.by forward-biasing selected recombinations of segments-decimal digit/point can be form IR light-emitting diodes – used in optical coupling applications – often in conjunctionwith fiber optics. Area of application include – industrial processing & control, position encoder, bar Graph readers and optical switching.

3-4 Optical Diodes (cont.) (The Photodiode)

- The photodiode is a divice that operate in reverse bias- Has a small transparent window that allows light to strike the pn junction- When reverse-biased – the photodiode has a very small reverse leakage current- The reverse-biased current is produced by thermally generated electron-hole pairsin the depletion region – swept across the pn junction by the electric field create by the reverse biased.- The reverse current increase with the light intensity- When no incident light , reverse current is almost negligible – called dark current

reverse current

3-4 Optical Diodes (cont.) (The Photodiode)

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

No reverse current when no incidence light,When light beam strike the photodiode - conduct an amountof reverse current

3-5 Other Types of Diodes (Introduction)

Current Regulator Diode

Schottky Diode

PIN Diode

Step-recovery Diode

Tunnel Diode

Laser Diode

3-5 Other Types of Diodes (cont.) (Current Regulator Diode) Referred to as a constant-current diode – maintains

a constant current

Operates in forward bias & the forward current becomes

a specified constant value at forward voltages ranging

about 1.5 V – 6 V

Constant forward current – called regulator current, pI

3-5 Other Types of Diodes (cont.) (The Schottky Diode) The Schottky diode’s significant characteristic is its fast switching speed –

also known as hot-carrier diodes This is useful for high frequencies and digital applications. It is not a typical diode in that it does not have a pn junction. Form by joining a doped n-material with a metal such as gold,silver or platinium Forward voltage drop is around 0.3 V Operate only with majority carrier – no minority carrier/ no reverse leakage current When forward biased – the higher energy e- in the n region are injected into the

metal region where they give up their excess energy very rapidly

Schottky diodesymbol

Basic internal construction

3-5 Other Types of Diodes (cont.) (The PIN Diode) 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.The pin diode is also used in mostly microwave frequency applications. Its variable forward series resistance characteristic is used for attenuation,

modulation, and switching.

3-5 Other Types of Diodes (cont.) (The Step-Recovery Diode)

The step-recovery diode is used in very high frequency (VHF)

and also used for fast switching applications. This is achieved by reduced doping at the junction.

3-5 Other Types of Diodes (cont.) (The Tunnel Diode)

The tunnel diode exhibits a special characteristic known as negative resistance. Useful in oscillator & microwave amplifier applications. It will actually conduct well with low forward bias. With further increases in bias it

reaches the negative resistance range where current will actually go down. This is achieved by heavily-doped p and n materials (with Ge/ GaAs) that creates a

very thin depletion region. The heavy doping allows conduction for all reverse voltage – no breakdown effects

Two alternate symbols of tunnel diode

A to B – acts as conductor

VF begin to developeda barrier- the current

act as a conventional forward biased diode

3-5 Other Types of Diodes (cont.) (The Tunnel Diode)

Application: Parallel resonant circuit

Parallel equivalent of the serieswinding resistance of the coil

When the tank circuit is ”shocked”into oscillation by an application of Voltages, a damped sinusoidal o/presults. The damping due to resistance of the tank-prevent a sustained oscillation

Constant sinusoidal voltage will result on the o/p due to the –ve resistance charac. of the tunnel diode counteracts the +ve resistance charac. of the tank resistance

Tunnel diode is placed series withthe tank circuit

3-5 Other Types of Diodes (cont.) (The 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.

3-5 Other Types of Diodes (cont.) (The Laser Diode)

Basic operation: The laser diode is FB by an external voltage source Electron move through the junction –recombine with holes – photons are released Released photon can strike an atoms – cause another photons to be released Some of the photons that are randomly drifting within the depletion region strike the

reflected surfaces perpendicularly This back-and-forth movement of photons increase as the generation of photons

“snowballs” until a very intense beam of laser light is formed by the photons that pass

through the partially reflective of the pn junction Therefore, a single wavelength of intense light emerges from the laser diod

Application: Used in the pick-up system of compact disk (CD) players

3-6 Troubleshooting

Although precise power supplies typically use IC type regulators, zener diodes can be

used alone as a voltage regulator. As with all troubleshooting techniques we must know

what is normal.

A properly functioning zener will work to maintain the output voltage within certain

limits despite changes in load.

3-6 Troubleshooting (cont.)

With an open zener diode, the full unregulated voltage will be present at the output

without a load. In some cases with full or partial loading an open zener could remain

undetected.

no voltage dropped betweenthe filtered o/p of the powersupply & the o/p terminal

undetected

3-6 Troubleshooting (cont.)

With excessive zener impedance the voltage would be higher than normal but less

than the full unregulated output.

The zener has failed such that its internal impedance is more than it should be.

Summary The zener diode operates in reverse breakdown. A zener diode maintains a nearly constant voltage across its terminals over a specified range of currents. Line regulation is the maintenance of a specific voltage with changing input voltages. Load regulation is the maintenance of a specific voltage for different loads. There are other diode types used for specific RF purposes such as varactor diodes (variable capacitance), Schottky diodes (high speed switching), and PIN diodes (microwave attenuation and switching).

Summary Light emitting diodes (LED) emit either infrared or visible light when forward-biased. Photodiodes exhibit an increase in reverse current with light intensity. The laser diode emits a monochromatic light

Solution 1:During +ve i/p voltage, D2 is zener limiter, D1 is FB diodeDuring –ve i/p voltage, D1 is zener limiter, D2 is FB diode

7.0V

- 8.4V