Special-purpose diodes
Schottky diodes
Schottky diodes are constructed of
a metal-to-N junction rather than a P-N semiconductor junction.
Also known as hot-carrier diodes, Schottky diodes are
characterized by fast switching times (low reverse-recovery time), low
forward voltage drop (typically 0.25 to 0.4 volts for a metal-silicon
junction), and low junction capacitance.
The schematic symbol for a Schottky diode
is shown here:
In terms of forward voltage drop (VF),
reverse-recovery time (trr), and junction capacitance (CJ),
Schottky diodes are closer to ideal than the average "rectifying" diode.
This makes them well suited for high-frequency applications.
Unfortunately, though, Schottky diodes typically have lower forward
current (IF) and reverse voltage (VRRM and VDC)
ratings than rectifying diodes and are thus unsuitable for applications
involving substantial amounts of power.
Schottky diode technology finds broad
application in high-speed computer circuits, where the fast switching
time equates to high speed capability, and the low forward voltage drop
equates to less power dissipation when conducting.
Tunnel diodes
Tunnel diodes exploit a strange
quantum phenomenon called resonant tunneling to provide
interesting forward-bias characteristics. When a small forward-bias
voltage is applied across a tunnel diode, it begins to conduct current.
As the voltage is increased, the current increases and reaches a peak
value called the peak current (IP). If the voltage is
increased a little more, the current actually begins to decrease
until it reaches a low point called the valley current (IV).
If the voltage is increased further yet, the current begins to increase
again, this time without decreasing into another "valley." Both the
schematic symbol and a current/voltage plot for the tunnel diode are
shown in the following illustration:
The forward voltages necessary to drive a
tunnel diode to its peak and valley currents are known as peak voltage
(VP) and valley voltage (VV), respectively. The
region on the graph where current is decreasing while applied voltage is
increasing (between VP and VV on the horizontal
scale) is known as the region of negative resistance.
Tunnel diodes, also known as Esaki
diodes in honor of their Japanese inventor Leo Esaki, are able to
transition between peak and valley current levels very quickly,
"switching" between high and low states of conduction much faster than
even Schottky diodes. Tunnel diode characteristics are also relatively
unaffected by changes in temperature.
Unfortunately, tunnel diodes are not good
rectifiers, as they have relatively high "leakage" current when
reverse-biased. Consequently, they find application only in special
circuits where their unique tunnel effect has value. In order to exploit
the tunnel effect, these diodes are maintained at a bias voltage
somewhere between the peak and valley voltage levels, always in a
forward-biased polarity (anode positive, and cathode negative).
Perhaps the most common application of a
tunnel diode is in simple high-frequency oscillator circuits, where they
allow a DC voltage source to contribute power to an LC "tank" circuit,
the diode conducting when the voltage across it reaches the peak
(tunnel) level and effectively insulating at all other voltages.
Light-emitting diodes
Diodes, like all semiconductor devices,
are governed by the principles described in quantum physics. One of
these principles is the emission of specific-frequency radiant energy
whenever electrons fall from a higher energy level to a lower energy
level. This is the same principle at work in a neon lamp, the
characteristic pink-orange glow of ionized neon due to the specific
energy transitions of its electrons in the midst of an electric current.
The unique color of a neon lamp's glow is due to the fact that it's
neon gas inside the tube, and not due to the particular amount of
current through the tube or voltage between the two electrodes. Neon gas
glows pinkish-orange over a wide range of ionizing voltages and
currents. Each chemical element has its own "signature" emission of
radiant energy when its electrons "jump" between different, quantized
energy levels. Hydrogen gas, for example, glows red when ionized;
mercury vapor glows blue. This is what makes spectrographic
identification of elements possible.
Electrons flowing through a PN junction
experience similar transitions in energy level, and emit radiant energy
as they do so. The frequency of this radiant energy is determined by the
crystal structure of the semiconductor material, and the elements
comprising it. Some semiconductor junctions, composed of special
chemical combinations, emit radiant energy within the spectrum of
visible light as the electrons transition in energy levels. Simply put,
these junctions glow when forward biased. A diode intentionally
designed to glow like a lamp is called a light-emitting diode, or
LED.
Diodes made from a combination of the
elements gallium, arsenic, and phosphorus (called gallium-arsenide-phosphide)
glow bright red, and are some of the most common LEDs manufactured. By
altering the chemical constituency of the PN junction, different colors
may be obtained. Some of the currently available colors other than red
are green, blue, and infra-red (invisible light at a frequency lower
than red). Other colors may be obtained by combining two or more
primary-color (red, green, and blue) LEDs together in the same package,
sharing the same optical lens. For instance, a yellow LED may be made by
merging a red LED with a green LED.
The schematic symbol for an LED is a
regular diode shape inside of a circle, with two small arrows pointing
away (indicating emitted light):
This notation of having two small arrows
pointing away from the device is common to the schematic symbols of all
light-emitting semiconductor devices. Conversely, if a device is light-activated
(meaning that incoming light stimulates it), then the symbol will have
two small arrows pointing toward it. It is interesting to note,
though, that LEDs are capable of acting as light-sensing devices: they
will generate a small voltage when exposed to light, much like a solar
cell on a small scale. This property can be gainfully applied in a
variety of light-sensing circuits.
Because LEDs are made of different
chemical substances than normal rectifying diodes, their forward voltage
drops will be different. Typically, LEDs have much larger forward
voltage drops than rectifying diodes, anywhere from about 1.6 volts to
over 3 volts, depending on the color. Typical operating current for a
standard-sized LED is around 20 mA. When operating an LED from a DC
voltage source greater than the LED's forward voltage, a
series-connected "dropping" resistor must be included to prevent full
source voltage from damaging the LED. Consider this example circuit:
With the LED dropping 1.6 volts, there
will be 4.4 volts dropped across the resistor. Sizing the resistor for
an LED current of 20 mA is as simple as taking its voltage drop (4.4
volts) and dividing by circuit current (20 mA), in accordance with Ohm's
Law (R=E/I). This gives us a figure of 220 Ω. Calculating power
dissipation for this resistor, we take its voltage drop and multiply by
its current (P=IE), and end up with 88 mW, well within the rating of a
1/8 watt resistor. Higher battery voltages will require larger-value
dropping resistors, and possibly higher-power rating resistors as well.
Consider this example for a supply voltage of 24 volts:
Here, the dropping resistor must be
increased to a size of 1.12 kΩ in order to drop 22.4 volts at 20 mA so
that the LED still receives only 1.6 volts. This also makes for a higher
resistor power dissipation: 448 mW, nearly one-half a watt of power!
Obviously, a resistor rated for 1/8 watt power dissipation or even 1/4
watt dissipation will overheat if used here.
Dropping resistor values need not be
precise for LED circuits. Suppose we were to use a 1 kΩ resistor instead
of a 1.12 kΩ resistor in the circuit shown above. The result would be a
slightly greater circuit current and LED voltage drop, resulting in a
brighter light from the LED and slightly reduced service life. A
dropping resistor with too much resistance (say, 1.5 kΩ instead of 1.12
kΩ) will result in less circuit current, less LED voltage, and a dimmer
light. LEDs are quite tolerant of variation in applied power, so you
need not strive for perfection in sizing the dropping resistor.
Also because of their unique chemical
makeup, LEDs have much, much lower peak-inverse voltage (PIV) ratings
than ordinary rectifying diodes. A typical LED might only be rated at 5
volts in reverse-bias mode. Therefore, when using alternating current to
power an LED, you should connect a protective rectifying diode in series
with the LED to prevent reverse breakdown every other half-cycle:
As lamps, LEDs are superior to
incandescent bulbs in many ways. First and foremost is efficiency: LEDs
output far more light power per watt than an incandescent lamp. This is
a significant advantage if the circuit in question is battery-powered,
efficiency translating to longer battery life. Second is the fact that
LEDs are far more reliable, having a much greater service life than an
incandescent lamp. This advantage is primarily due to the fact that LEDs
are "cold" devices: they operate at much cooler temperatures than an
incandescent lamp with a white-hot metal filament, susceptible to
breakage from mechanical and thermal shock. Third is the high speed at
which LEDs may be turned on and off. This advantage is also due to the
"cold" operation of LEDs: they don't have to overcome thermal inertia in
transitioning from off to on or visa-versa. For this reason, LEDs are
used to transmit digital (on/off) information as pulses of light,
conducted in empty space or through fiber-optic cable, at very high
rates of speed (millions of pulses per second).
One major disadvantage of using LEDs as
sources of illumination is their monochromatic (single-color) emission.
No one wants to read a book under the light of a red, green, or blue
LED. However, if used in combination, LED colors may be mixed for a more
broad-spectrum glow.
Laser diodes
The laser diode is a further
development upon the regular light-emitting diode, or LED. The term
"laser" itself is actually an acronym, despite the fact it's often
written in lower-case letters. "Laser" stands for Light Amplification
by Stimulated Emission of Radiation, and refers to
another strange quantum process whereby characteristic light emitted by
electrons transitioning from high-level to low-level energy states in a
material stimulate other electrons in a substance to make similar
"jumps," the result being a synchronized output of light from the
material. This synchronization extends to the actual phase of the
emitted light, so that all light waves emitted from a "lasing" material
are not just the same frequency (color), but also the same phase as each
other, so that they reinforce one another and are able to travel in a
very tightly-confined, nondispersing beam. This is why laser light stays
so remarkably focused over long distances: each and every light wave
coming from the laser is in step with each other:
Incandescent lamps produce "white"
(mixed-frequency, or mixed-color) light. Regular LEDs produce
monochromatic light: same frequency (color), but different phases,
resulting in similar beam dispersion. Laser LEDs produce coherent
light: light that is both monochromatic (single-color) and
monophasic (single-phase), resulting in precise beam confinement.
Laser light finds wide application in the
modern world: everything from surveying, where a straight and
nondispersing light beam is very useful for precise sighting of
measurement markers, to the reading and writing of optical disks, where
only the narrowness of a focused laser beam is able to resolve the
microscopic "pits" in the disk's surface comprising the binary 1's and
0's of digital information.
Some laser diodes require special
high-power "pulsing" circuits to deliver large quantities of voltage and
current in short bursts. Other laser diodes may be operated continuously
at lower power. In the latter case, laser action occurs only within a
certain range of diode current, necessitating some form of
current-regulator circuit. As laser diodes age, their power requirements
may change (more current required for less output power), but it should
be remembered that low-power laser diodes, like LEDs, are fairly
long-lived devices, with typical service lives in the tens of thousands
of hours.
Photodiodes
Varactor diodes
Constant-current diodes
A constant-current diode, also
known as a current-limiting diode, or current-regulating diode,
does exactly what its name implies: it regulates current through it to
some maximum level. If you try to force more current through a
constant-current diode than its current-regulation point, it simply
"fights back" by dropping more voltage. If we were to build the
following circuit and plot diode current over diode current, we'd get a
graph that rises normally at first and then levels off at the current
regulation point:
One interesting application for a
constant-current diode is to automatically limit current through an LED
or laser diode over a wide range of power supply voltages, like this:
Of course, the constant-current diode's
regulation point should be chosen to match the LED or laser diode's
optimum forward current. This is especially important for the laser
diode, not so much for the LED, as regular LEDs tend to be more tolerant
of forward current variations.
Another application is in the charging of
small secondary-cell batteries, where a constant charging current leads
to very predictable charging times. Of course, large secondary-cell
battery banks might also benefit from constant-current charging, but
constant-current diodes tend to be very small devices, limited to
regulating currents in the milliamp range.
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