Special-Purpose Diodes
Objective of Lecture
•
•
•
•
What is Diode element ?
Ideal Vs Practical Diodes
Special Diodes
Application of Diodes
Diode Element
• A diode has two leads
connected to the external
circuit.
• Since a diode behaves
differently depending
upon forward or reverse
bias, it is critical to be
able to distinguish the
leads.
• The anode connects to
the p-type material, the
cathode to the n-type
material of the diode.
3
Ideal Diode
• In an ideal diode, current flow freely through the
device when forward biased, having no
resistance.
• In an ideal diode, there would be no voltage drop
across it when forward biased. All of the source
voltage would be dropped across circuit resistors.
• In an ideal diode, when reverse biased, it would
have infinite resistance, causing zero current flow.
Chapter 9 - Diodes and Diode Circuits
4
Practical Diodes
• A practical diode does offer some resistance to
current flow when forward biased.
• Since there is some resistance, there will be some
power dissipated when current flows through a
forward biased diode. Therefore, there is a practical
limit to the amount of current a diode can conduct
without damage.
• A reverse biased diode has very high resistance.
• Excessive reverse bias can cause the diode to
conduct.
Special Diodes
•
•
•
•
•
•
•
Zener Diode
LEDs
Photo Diode
Varactor Diode
Tunnel Diode
Schottky Diode
Schokley Diode
Zener Diode
• A Zener diode is a type of diode that permits current
not only in the forward direction like a normal diode,
but also in the reverse direction if the voltage is larger
than the breakdown voltage known as "Zener knee
voltage" or "Zener voltage". The device was named
after Clarence Zener, who discovered this electrical
property
• It is heavily doped in comparison with normal diode to
reduce the breakdown voltage.
• Breakdown voltage for commonly available zener
diodes can vary widely from 1.2 volts to 200 volts.
Zener Diodes – Operating Range
A zener diode is much like
a normal diode, the
exception being is that it
is placed in 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.
Operating range
Zener Diodes – Regulation Ranges
The zener diode’s
Zener zone Diode zone
breakdown characteristics
are determined by the
doping process. Low
voltage zeners (>5V),
operate in the zener
breakdown range. Those
designed to operate <5 V
operate mostly in
avalanche breakdown
Avalanche
range. Zeners are
zone
available with voltage
5V.
breakdowns of 1.8 V to This curve illustrates
the minimum and
200 V.
maximum ranges of current operation that the
zener can effectively maintain its voltage.
Zener Diodes – Equivalent Circuit
• Ideal Zener exhibits a
constant voltage,
regardless of current
draw.
• Ideal Zener exhibits no
resistance
characteristics.
Introduction
The basic function of zener diode is to maintain a specific
voltage across its 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.
RLoad
This particular zener circuit will work to maintain 10 V across the load.
Zener Diodes – Equivalent Circuit
• Zener exhibits a near
constant voltage, varied
by current draw
through the series
resistance ZZ.
• As Iz increases, Vz also
increases.
Zener Diode - Applications
Regulation
In this simple illustration of zener regulation circuit, the zener diode
will “adjust” its impedance based on varying input voltages. Zener
current will increase or decrease directly with voltage input changes.
The zener current, Iz, will vary to maintain a constant Vz.
Note: The zener has a finite range of current operation.
VZener
remains
constant
Zener Diode - Applications
Regulation
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.
VZener
remains
constant
See Ex. 3-5
Zener Diode - Applications
Vin
Vin
IZ
Vout
IZ
Vout
Zener Diode - Applications
**
1N4740
PDMAX = 1W. VZ = 10V.
IZK = 0.25mA to IZM = 100mA
VRmin = IZKR= .25mA x 220 = 55mV
VRmax = IZM = 100mA x 220 = 22V.
Calculate VZRegulate: (pg.118)
VinMIN = VR + VZ = 55mV + 10V = 10.055V.
VR = IZR = (100mA)(220) = 22V.
Vin(max) = 22V + 10V = 32V
See Ex. 3-5
VReg is ≈10V to 32V.
See Ex. 3-6
See Ex. 3-7
Zener Limiting
Zener diodes can used for limiting just as normal diodes. Recall
in previous chapter studies about limiters. The difference to
consider for a zener limiter is its zener breakdown
characteristics.
See Ex.3-8
Varactor Diode
• A junction diode which
act as a variable
capacitor under
changing reverse bias is
known as a varactor
diode.
Varactor Diodes
A varactor diode is best explained as a variable capacitor.
Think of the depletion region as a variable dielectric. The
diode is placed in reverse bias. The dielectric is “adjusted”
by reverse bias voltage changes.
Varactor Diodes
The varactor diode can be useful in filter circuits as the
adjustable component for resonance frequency selection.
Varactor
Bias adjust
Resonant Band-pass Filter w/ Varactor Diode
Parallel Resonant Tank
Series Resonant
Tank
31.6V
Varactor
VR = 2.85 to 28.7V
VBIAS = 2.9V to 29V
CVaractor = 17pF to 55pF
Resonant Frequency Range:
fr = 679kHz to 1.22MHz.
Varactor Bias
See Related Problem pg.130
What is LED?
LED are semiconductor p-n junctions that under forward bias conditions can emit
radiation by electroluminescence in the UV, visible or infrared regions of the
electromagnetic spectrum. The qaunta of light energy released is approximately
proportional to the band gap of the semiconductor.
How does it work?
P-n
junction
A typical LED needs a p-n junction
There are a lot of electrons and holes at
the junction due to excitations
Electrons from n need to be injected to p
to promote recombination
Junction is biased to produce even more
e-h and to inject electrons from n to p for
recombination to happen
Recombination
produces light!!
Electrical
Contacts
Optical Diodes
Electroluminescence, the process of
emitting photons from a parent material
(substrate), is the basis for LEDs.
Colors result from the choice of substrate
material and the resulting wavelength;
Todays LEDs (green,red, yellow) are based
on indium gallium aluminum phosphide
Blue uses silicon carbide or gallium nitride
IR (infrared) – GaAs (gallium arsenide)
LED Biasing: 1.2V to 3.2V is typical.
Note: Some newer LED’s run at higher
voltages and emit immense light energy.
Applications:
Traffic signals
Outdoor video screens
Runway markers
A strong +bias
encourages conductionband electrons in the Nmaterial to leap the
junction and recombine
with available holes
releasing light and heat.
Excitation
E
Electron (excited by the
biased forward voltage) is in
the conduction band
k
Hole is in valance
band
Normally the recombination takes place
between transition of electrons between the
bottom of the conduction band and the top
of the valance band (band exterma).
The emission of light is therefore;
hc/ = Ec-Ev = Eg(only direct band gap
Optical Diodes
The light-emitting diode (LED) emits photons as visible
light. Its purpose is for indication and other intelligible
displays. Various impurities are added during the doping
process to vary the color output.
Getting to know LED
Advantages of Light Emitting Diodes
(LEDs)
Longevity:
The light emitting element in a diode is a small
conductor chip rather than a filament which greatly
extends the diode’s life in comparison to an
incandescent bulb (10 000 hours life time compared
to ~1000 hours for incandescence light bulb)
Efficiency:
Diodes emit almost no heat and run at very low
amperes.
Greater Light Intensity:
Since each diode emits its own light
Cost:
Not too bad
Robustness:
Solid state component, not as fragile as incandescence light
bulb
Applications of LEDs
The seven segment display is an example of LEDs use for display
of decimal digits.
See “Light Emitting Diodes.pdf”
Applications of LEDs
Visible LED
Definition:
LED which could emit visible light, the band gap of the materials that we use
must be in the region of visible wavelength = 390- 770nm. This coincides with
the energy value of 3.18eV- 1.61eV which corresponds to colours as stated
below:
Colour of
an LED
should
emits
Violet
Blue
Green
Yellow
Orange
Red
~ 3.17eV
~ 2.73eV
~ 2.52eV
~ 2.15eV
~ 2.08eV
~ 1.62eV
The band gap,
Eg that the
semiconductor
must posses to
emit each light
PHOTODIODES
Photodiodes - a semiconductor p-n junction device whose region of
operation is limited to the reversed bias region. Converts nonelectrical energy such as light to electrical energy.
Photons - energy transmitted as discrete packages,has a level
directly related to the frequency of the travelling wave. This energy
associated with incident light waves is directly related to the
frequency of the travelling wave.
Dark current - current that will exist with no applied illumination.
Conductivity of semi-conductor is increased.
Current flow in the semi-conductor is induced.
BAE 5413
31 of 10
Photodiodes
Unlike LED’s, photodiodes receive light rather than produce light. The
photodiode varies it’s current in response to the amount of light that
strikes it. It is placed in the circuit in reverse bias. As with most
diodes, no current flows when in reverse bias, but when light strikes
the exposed junction through a tiny window, reverse current
increases proportional to light intensity (irradiance).
Note: Photodiodes all
exhibit a “reverse
leakage current”
which appears as an
inverse variable
resistance. Irradiance
causes the device to
exhibit a reduction in
the variable
resistance
characteristic.
Photodiodes
You have this
one in your kit.
Photodiode fundamentals
Based on PN or PIN junction diode
photon absorption in the depletion
region induces current flow
P
+
hole
h
I
electron
N
Spectral sensitivity
Material
Band gap
(eV)
Spectral sensitivity
silicon (Si)
1.12
250 to 1100 nm
indium arsenide (InGaAs)
~0.35
1000 to 2200 nm
Germanium (Ge)
.67
900 to 1600 nm
IL
RL
Schottky Diode
The Schottky diode’s (hot-carrier diodes) significant
characteristic is its fast switching speed. This is useful for high
frequencies and digital applications. It is not a typical diode in
that it does not have a p-n junction. Instead, it consists of a
lightly-doped n-material and heavily-doped (conduction-band
electrons) metal bounded together.
Response is very quick…high speed digital communications.
Lightly doped
Heavily doped (conductionband electrons)
Tunnel Diode
The tunnel diode exhibits 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 materials that create a
very thin depletion region which permits electrons to “tunnel” thru
the barrier region.
Tank circuits oscillate but “die out” due
to the internal resistance. A tunnel
diode will provide “negative resistance”
that overcomes the loses and maintains
the oscillations.
Germanium or Gallium
Tunnel Diodes
Tank circuits oscillate but “die out” due to the internal resistance. A
tunnel diode will provide “negative resistance” that overcomes the
loses and maintains the oscillations.
Other Diode Types
The laser diode (light amplification by stimulated emission of
radiation) produces a monochromatic (single color) “coherent”
light. Laser diodes in conjunction with photodiodes are used to
retrieve data from compact discs.
Forward bias the diode and electrons move thru the junction,
recombination occurs (as ordinary). Recombinations result in
photon release, causing a chain reaction of releases and
avalanching photons which form an intense laser beam.
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.
Troubleshooting
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.
Troubleshooting
With excessive zener impedance the voltage would be
higher than normal but less than the full unregulated
output.
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