Download CHAPTER 3 Special-Purpose Diodes

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 current
produces a small change in zener voltage V
Z
 By ohm’s law:

VZ
ZZ 
I Z
 Normaly -Zz is specified at IZT

(3-1)
Zener impedance
I Z 
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,
I ZM 
PD (max)
VZ
I ZK  0.25mA, PD (max)  1W ,VZ  10V
 100mA
For min. zener current, the voltage across the 220 ohm resistor:
Therefore:
VR  I ZK R  (0.25)(220)  5mV
VIN (min)  VR  VZ  55mV  10V  10.055mV
For max. zener current,
The voltage across resistor is:
VR  I ZM R  (100mA)(220)  22V
Therefore:
VIN (max)  22V  10V  32V
i/p
10.055V – 32V
maintain
o/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 decrease
directly 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 long
as 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 level
a) 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 -0.7V
b) Zener diode is turn around
-The –ve peak is by zener action & +ve voltage is limited to +0.7V
c) 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.
The reverse-biased
varactor diode acts
as a variable capacitor
Varactor diode symbol
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 capacitance is expressed by the following formula
A
C
d
•
;
, the depletion region
A = plate area, 
-
dielectric constant,
the capacitance
d
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.
3-3 Varactor Diodes (cont.)
(An Application)
 Major applications – tuning circuits – e.g.: electronic tuning in TV, other commercial
receiver.
 When used in a resonant circuit –acts as a variable capacitor (see Figure) – allowed
the resonant frequency to be adjusted by a variable voltage level.
 Varactor diode provides the total variable capacitance in the parallel resonant bandpass filter.
 C1,C2,C3,C4 – coupling capacitor – prevent the dc bias circuit
1
 The parallel resonant frequency is f r 
2 LC
prevent a dc path
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 varactor
through the inductor prevent a dc
path from the
wiper to gnd
from the wiper to a
load on the o/p
through the inductor
as a variable
dc voltage divider
for biasing the
varactor
Figure 3-23 A resonant
band-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 material
permits 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
Electroluminescence
in 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)
LEDs emits light
in response to a
sufficient forward
current
The amount of power
o/p translated into
light is directly proportional
to the forward current
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 conjunction
with 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 pairs
in 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 amount
of 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,
Ip
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 diode
symbol
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
VF begin to developed
a barrier- the current
A to B –
acts as
conductor
act as a conventional
forward biased diode
Two alternate symbols of tunnel diode
3-5 Other Types of Diodes (cont.)
(The Tunnel Diode)
Application: Parallel resonant circuit
Parallel equivalent of the series
winding resistance of the coil
Tunnel diode is
placed series with
the tank circuit
When the tank circuit is ”shocked”
into oscillation by an application of
Voltages, a damped sinusoidal o/p
results. 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
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 between
the filtered o/p of the power
supply & 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 diode
During –ve i/p voltage, D1 is zener limiter, D2 is FB diode
7.0V
- 8.4V