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 its variable capacitance
Special-Purpose Diodes 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 laser diode.
Introduction Zener Diodes
The basic function of zener diode is to maintain a specific A zener diode is much like a
voltage across its terminals within given limits of line or load normal diode, the exception
change. Typically it is used for providing a stable reference being is that it is placed in
voltage for use in power supplies and other equipment. the circuit in reverse bias and
operates in reverse
breakdown. This typical
characteristic curve
illustrates the operating
range for a zener. Note that
its forward characteristics
are just like a normal diode.
This particular zener circuit will work to maintain 10 V across the load.
Zener Diodes Zener Diodes
The zener diode’s
breakdown characteristics
are determined by the
doping process. Low voltage
zeners less than 5V operate As with most devices, zener diodes have given characteristics
in the zener breakdown such as temperature coefficients and power ratings that have
range. Those designed to to be considered. The data sheet provides this information.
operate more than 5 V
operate mostly in avalanche
breakdown range. Zeners
are available with voltage
breakdowns of 1.8 V to 200
V. This curve illustrates the minimum and maximum
ranges of current operation that the zener can
effectively maintain its voltage.
Zener Diode Applications
Regulation
Zener Limiting
In this simple illustration of zener regulation circuit, the zener diode Zener diodes can used for limiting just as normal diodes. Recall
will “adjust” its impedance based on varying input voltages and loads in previous chapter studies about limiters. The difference to
(RL) to be able to maintain its designated zener voltage. Zener current consider for a zener limiter is its zener breakdown
will increase or decrease directly with voltage input changes. The characteristics.
zener current will increase or decrease inversely with varying loads.
Again, the zener has a finite range of operation.
Varactor Diodes Varactor Diodes
The varactor diode can be useful in filter circuits as
A varactor diode is best explained as a variable capacitor. Think the adjustable component.
of the depletion region a variable dielectric. The diode is placed
in reverse bias. The dielectric is “adjusted” by bias changes.
Optical Diodes Optical Diodes
The light-emitting diode (LED) emits photons as visible The seven segment display is an example of
light. Its purpose is for indication and other intelligible LEDs use for display of decimal digits.
displays. Various impurities are added during the doping
process to vary the color output.
Optical Diodes Other Diode Types
The photodiode is used to vary current by the amount of light that
strikes it. It is placed in the circuit in reverse bias. As with most Current regulator diodes keeps a constant current
diodes when in reverse bias, no current flows when in reverse bias, value over a specified range of forward voltages
but when light strikes the exposed junction through a tiny window, ranging from about 1.5 V to 6 V.
reverse current increases proportional to light intensity.
Other Diode Types Other Diode Types
The Schottky diode’s significant characteristic is its fast The pin diode is also used in mostly microwave
switching speed. This is useful for high frequencies and digital frequency applications. Its variable forward series
applications. It is not a typical diode in that it does not have a resistance characteristic is used for attenuation,
p-n junction. Instead, it consists of a heavily-doped n-material modulation, and switching. In reverse bias it exhibits a
and metal bound together. nearly constant capacitance.
Other Diode Types Other Diode Types
The tunnel diode has negative resistance. 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
The step-recovery diode is also used for fast switching materials that creates a very thin depletion region.
applications. This is achieved by reduced doping at the
junction.
Other Diode Types Troubleshooting
Although precise power supplies typically use IC type
regulators, zener diodes can be used alone as a voltage
The laser diode (light amplification by stimulated regulator. As with all troubleshooting techniques we must
emission of radiation) produces a monochromatic know what is normal.
(single color) light. Laser diodes in conjunction with
photodiodes are used to retrieve data from compact
discs.
A properly functioning zener will work to maintain the output voltage
within certain limits despite changes in load.
Troubleshooting Troubleshooting
With an open zener diode, the full unregulated voltage
With excessive zener impedance the voltage would be higher
will be present at the output without a load. In some
than normal but less than the full unregulated output.
cases with full or partial loading an open zener could
remain undetected.
Summary Summary
The zener diode operates in reverse breakdown. Light emitting diodes (LED) emit either infrared or visible
A zener diode maintains a nearly constant voltage across light when forward-biased.
its terminals over a specified range of currents. Photodiodes exhibit an increase in reverse current with
Line regulation is the maintenance of a specific voltage light intensity.
with changing input voltages. The laser diode emits a monochromatic light
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).
