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University Of Hail
Community College
Electrical Engineering Department
Electronics Engineering and Instrumentation Program
PHYSICS OF APPLIED ELECTRONICS
(PHYS162)
Dr. Fawzy Hashem
Date:
SPECIAL-PURPOSE DIODES
THE ZENER DIODE
The zener diode is designed to operate in the reverse breakdown region. The
breakdown voltage (VZ) of the zener diode is set by carefully controlling the doping
level during manufacture. The symbol of the zener diode and its volt-ampere
characteristics are shown below:
Ideally, the reverse breakdown has a constant voltage (VZ), this makes it useful as
voltage reference, which is its primary application.
The ideal zener diode equivalent circuit model and the characteristic curve are shown
below:
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The practical model of the zener diode considers the zener impedance ZZ, which is
the ratio of change voltage in the breakdown region to the corresponding change in
current (ZZ= ΔVZ/ΔIZ). The practical zener diode equivalent circuit and its
characteristic curve are shown below:
Temperature Coefficient
The temperature coefficient of zener diode can specified in terms of change in voltage
per degree Celsius change in temperature (TC= ΔVZ/ΔT), where TC has units of
mV/oC . The temperature coefficient can be positive or negative.
For example, if a 1N756 is an (8.2 V at 25 oC) zener diode with positive temperature
coefficient TC= 5.4 mV/oC. The output voltage at 55 oC will be:
ΔVZ = TC x ΔT = 5.4 mV/oC x 30 oC = 162 mV
VZ = 8.2 V + 0.162 V = 8.362 V
ZENER DIODE APPLICATIONS
Zener Regulation with varying input voltage
In current applications, a zener diode can be used as basic regulator. To illustrate
this, lets use the ideal diode model of 1N4740A zener diode in the following circuit:
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As VIN changes IZ will change, the limitations on the input voltage variation
( VIN(min) and VIN(max) ) are set by the minimum and maximum current levels
( IZK and IZM ) with which the zener diode can operate.
The minimum current value IZK = 0.25 mA (from the 1N4740A zener diode
datasheet). The maximum current is not given on the data sheet, but can be calculated
from the power specification ratings of PD(max) = 1 W as follows:
IZM = PD(max) / VZ = 1W/10V=100 mA
For the minimum zener current, the voltage across the 220 Ω resistor is:
VR = IZK x R = 0.25 mA x 220 Ω = 55 mV
Since VIN = VR + VZ, then
VIN(min) = VR + VZ = 55 mV + 10 V = 10.055 V
For the maximum zener current, the voltage across the 220 Ω resistor is:
VR = IZM x R = 100 mA x 220 Ω = 22 V
Therefore
VIN(max) = VR + VZ = 22 V + 10 V = 32 V
This shows that this zener diode can ideally regulate voltage from 10.055 V to 32 V,
and maintain an approximate 10 V output.
Zener Regulation with a variable load
The following figure shows a zener voltage regulator with a variable load resistor
across its terminal:
The zener diode maintains a nearly constant voltage across RL as long as the zener
current IZ is greater than IZK and less than IZM.
When the output terminals of the zener regulator are open (RL = ∞), the load current
IL is zero and all of the current is through the zener, this is a no-load condition.
When a load resistor (RL) is connected, part of the total current IT is through the
zener IZ, and part through the load IL. As RL is decreased, the load current IL
increases and IZ decreases.
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The zener diode continues to regulate the voltage until IZ reaches its minimum-value
IZK. At this point the load current is maximum and a full-load condition exists.
Example
An 1N756 zener diode is used as an 12 V regulator in the circuit shown below:
What is the smallest load resistor that can be used before losing regulation? Assume
the ideal model for the zener diode.
Solution
The no load zener current
INL = (VIN – VZ) / R = (24 V-12 V) / 470 Ω = 25.5 mA
This is the maximum load current in regulation, therefore the minimum value of load
resistance RL(min)= VZ / INL = 12 V/ 25.5 mA = 470 Ω.
Keep in mind that if RL is less 470 Ω it will draw more of the total current away
from the zener diode and IZ will be reduced below IZK. This will cause the zener
diode to lose regulation.
### ZENER LIMITING
In addition to DC voltage regulation applications, zener diodes can be used in AC
application to limit voltage swings to desired levels. The following figure shows three
basic ways the limiting action of a zener diode can be used.
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Part (a) shows a zener used to limit the positive peak of a signal voltage to the
selected zener voltage VZ. During the negative alternative, the zener acts as a
forward-biased and limits the negative voltage to -0.7 V.
When the zener is turned around, as in part (b), the negative peak is limited to -VZ
and the positive voltage is limited to 0.7 V.
Two back-to-back zeners limit both peaks to (+VZ + 0.7 V) for the positive peak, and
to (-VZ - 0.7 V) for the negative peak, as shown in part (c). During the positive
alternation, D2 functioning as a zener limiter, and D1 as forward-biased diode. During
the negative alternation, the roles are reversed.
THE VARACTOR DIODE
A varactor is a special purpose diode that always operates in reverse-bias and is doped
to maximize the inherent capacitance of the depletion region. The depletion region
acts as a capacitor dielectric because of its nonconductive characteristic. The p and n
regions are conductive and act as the capacitor plates, as shown below:
Recall that the capacitance is determined by the parameters of plate area (A),
dielectric constant (ε) , and plate separation (d), as expressed by the formula:
C = (A x ε) / d
As the reverse-bias voltage increases the depletion region widens, effectively
increasing the plate separation, thus decreasing the capacitance.
Capacitance Ratio
The capacitance ratio is the ratio of the diode's capacitance at the minimum reverse
voltage (largest C) to the diode's capacitance at the maximum reverse voltage
(smallest C). Datasheets also include parameters such as maximum ratings for current,
power, and temperature.
Varactor Applications
A major application of varactors is in tuning circuits. For example VHF, UHF, and
satellite receivers utilize varactors. When used in a parallel resonant circuit, the
varactor acts as a variable capacitor. Thus, allowing the resonant frequency to be
adjusted by a variable voltage level.
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THE LIGHT-EMITTING DIODE (LED)
The basic operation of the light-emitting diode (LED) is as follows. When the device
is forward-biased, electrons cross the pn junction from the n-type material and
recombine with holes in the p-type material. When recombination takes place, the
recombining electrons release energy, in the form of photons. This process is called
electroluminescence.
The LED emits light in response to a sufficient forward current, as shown below:
The amount of power output translated into light is directly proportional to the
forward current as indicted in part (b).
Various impurities are added during the doping process to establish the wavelength of
the emitted light. The wavelength determines the color of the visible light. Some
LEDs emits photons that are not part of the visible spectrum but have longer
wavelengths and are in the infrared (IR) portion of the spectrum. The spectral output
curves are shown below:
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LED Applications
Standard LEDs are used for indicator lamps and readout displays on a wide variety of
instruments, ranging from consumer appliances to scientific apparatus. A common
type of display device using LEDs is the seven-segment display. Combinations of the
segments form the ten decimal digits as illustrated below:
Two types of LED circuit arrangements are the common anode and the common
cathode as shown in parts (b) and (c) above.
One common application of an infrared LED is in remote control units for TV,
DVD, gate opener , etc.
High Intensity LEDs
LEDs that produce much grater light outputs than standard LEDs are found in many
applications including traffic light, automotive lighting, indoor and outdoor
advertising and informational signs, and home lighting. For these applications LEDs
are usually used in the form of serial-parallel arrays. These arrays use optical lenses
and reflectors to help maximize the effect of the light output.
THE PHOTODIODE
The photodiode is a device that operates in reverse bias, it has a small transparent
window that allows light to strike the pn junction. A photodiode differs from a
rectifier diode in that when its pn junction is exposed to light, the reverse current
increase with the light intensity. When there is no incident light, the reverse current,
Iλ, is almost negligible and is called the dark current. An increase in the amount of
light intensity, expressed as irradiance (mW/cm2), produces an increase in the reverse
current as shown below:
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From the graph in part (b), you can see that for 0.5 mW/cm2 incident light the reverse
current is approximately 1.4 mA in case of reverse voltage VR=10 V. These values
indicates that the photodiode resistance:
RR = VR/ Iλ = 10V / 1.4 mA = 7.14 MΩ
At 20 mW/cm2 incident light the reverse current is approximately 55 mA in case of
reverse voltage VR=10 V. These values indicates that the photodiode resistance:
RR = VR/ Iλ = 10V / 55 mA = 182 KΩ
These calculations show that the photodiode can be used in various applications as a
variable resistance device controlled by light intensity.
OTHER TYPES OF DIODES
The Schottky Diode
Schottky diodes are high-current diodes used primarily in high-frequency and fastswitching applications. The LS family of the TTL logic (LS stands for Low-power
Schottky) is one type of digital integrated circuits that uses the Schottky diode. A
Schottky diode symbol is shown below:
A Schottky diode is formed by joining a doped semiconductor region (usually n-type)
with a metal such as gold, silver, or platinum. Rather than pn junction, there is a
metal-to-semiconductor junction as shown below:
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The PIN Diode
The pin diode consists of heavily doped p and n regions separated by an intrinsic ( i )
region, as shown below:
When reversed-biased, the pin diode acts like nearly a constant capacitance, and when
forward-biased, it acts like a current-controlled variable resistance. This illustrated in
part (a) and part (b) above respectively.
The low forward resistance of the intrinsic region decreases with increasing current.
The forward series resistance and the reverse capacitance characteristics are shown
below:
The pin diode is used as dc-controlled microwave switch operated by rapid changes in
bias. It can also be used in attenuator applications because its resistance can be
controlled by the amount of current.
The Tunnel Diode
The tunnel diode exhibit a special characteristic known as negative resistance. This
feature makes it useful in oscillator and microwave applications. The tunnel diode
symbols and characteristic curve are shown below:
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The tunnel diode's n and p regions are heavily doped, which results in an extremely
narrow depletion region. Thus, electrons can "tunnel" through the pn junction at very
low forward-bias voltages, and the diode acts as a conductor. This is shown by the
part of its characteristic curve, between points A and B.
Then the forward voltage begins to develop a barrier, and the current begins to
decrease as the forward voltage continues to increase. This is the negative-resistance
region (between points B and C). At point C, the diode begins to act as conventional
forward-biased diode.
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