A single P-N junction creates a diode. Most diodes exhibit the
properties described above for P-N junctions in general. Basically, a
diode acts like a check valve, it allows current to flow in one
direction but not in the other.
Small-signal diodes and switching diodes
Most diodes are referred to as "small signal" or "switching" diodes. A
small signal diode is made to be used with low currents. The term
"switching diode" comes from the fact that a forward-biased diode
appears to be a closed switch and a reverse-biased diode appears to be
an open switch. A switching diode is designed to switch between the
"on" and "off" states efficiently.
Varactor Diodes
As mentioned while discussing the P-N junction, when a diode is reverse biased the depletion region gets larger; the greater the
reverse-biased voltage, the bigger the depletion region. This means
that you have two conducting regions separated by an insulating region.
We have just described a capacitor. You can change the width of this
insulating region by changing the voltage. This makes a reverse-biased
diode a voltage controlled variable capacitor. A varactor diode (also
called a varicap) is optimized to be used as a variable capacitor.
Increasing the reverse-bias voltage with a varactor diode will decrease
the capacitance and vice versa.
Light Emitting Diodes (LEDs)
Light Emitting Diodes produce light when forward biased. A typical LED
takes about 20 mA to operate and produces about 40 millicandles of
light output. However, "super bright" and high wattage LEDs use much
more current and produce much more light.
The color of an LED is determined by the semiconductor materials it
is
made from. For example, gallium arsenide (a mixture of gallium and
arsenic) produces infrared light. Aluminum gallium arsenide produces
infrared or red light. Indium gallium nitride can produce green or blue
light. Aluminum nitride or carbon (diamond) can produce ultraviolet
light. Originally, the only colors available were red, green yellow and
orange. In the 1990s the first commercial blue LEDs were produced and
today, high-brightness blue LEDs are inexpensive. Theoretically, white
LEDs can be produced by combining red, blue and green LEDs. However,
white LEDs are usually made with blue or ultraviolet LEDs that shine
through a phosphor-coated window. This window converts the blue or UV
light to
broad-spectrum white light. This is why white LEDs appear yellow when
off. When turned on much of the blue light leaks through the phosphor
making the output bluish white. Many LED lights made for area lighting
are color balanced to give a more natural white.
LED televisions and computer monitors actually use liquid crystal
displays (LCDs). LEDs are used only as backlights for the displays.
However, OLED displays use LEDs made from organic films. These displays
use the actual LEDs to form the image.
Photodiodes
Diodes are typically light sensitive. A photodiode is optimized to take
advantage of this light sensitivity. Photodiodes can work in two
different modes. In the photoconductor mode the diode is reverse biased
and acts as an open circuit in the dark. When light falls on the diode
it will conduct. The more light that falls on the diode the more it
conducts. A photodiode acts somewhat different from a photoresistor.
The amount of current that flows through a photodiode is much more
dependent on how much light falls on the diode than how much voltage is
applied to it. In photovoltaic mode a photodiode produces a voltage
when light strikes it. Photocells or solar cells are basically
large-area photodiodes.
Opto-isolator / opto-coupler
An opto-isolator (also called an opto-coupler) is a combination of an
LED and a photo diode in an opaque package (to exclude outside light).
A signal is connected to the LED. The LED shines onto the photodiode
which develops a current proportional to the light intensity.
Opto-isolators are use to get information from one circuit to another
keeping them completely isolated electrically.
Silicon Controlled Rectifiers (SCRs)
A Silicon Controlled Rectifier, also known as a thyristor, has three
leads, the anode, the cathode, and the gate. It acts like a normal
diode when reverse biased, but when forward biased, current will not flow
unless a trigger voltage is applied to the gate. Once current is
flowing it will not stop until the power is removed. Think of it as a
voltage controlled switch that cannot be turned off once it is turned
on.
Triac
An SCR will only conduct when it is forward biased. A triac works like
an SCR except it works in either direction. It works like two SCRs
connected in parallel pointing in opposite directions.
Shockley diode
A Shockley diode is like an SCR with no gate lead. It will turn on when
a certain forward-biased voltage is reached. These are no-longer
manufactured but a high power equivalent called a dynistor is.
Schottky diode
A Schottky diode (not to be confused with a Shockley diode) is made
with a junction of semiconductor with metal. They have a low
forward-biased voltage and a fast switching time. The low
forward-biased voltage results in less wasted energy where this is
significant. However, they have more leakage current when reverse
biased than silicon diodes.
Diac
A DIAC is much like a triac without a gate lead. Like the Shockley
diode, it will turn on when a certain voltages is reached (around 30V)
and therefore requires no gate lead. It is something like a Shockley
diode that works in either direction. DIACs are often placed on the
gate lead of an SCR to control the trigger point of the SCR.
Tunnel diode
A tunnel diode has characteristics different from other diodes. When
reverse biased it has a linear voltage-to-current curve. In other
words, it looks much like a resistor when reverse biased; double the
voltage and you double the current. When forward biased a tunnel diode
also looks like
a resistor at low voltages. Once a certain voltage is reached, further
increasing the voltage causes a decrease in current. This is the
opposite of practically any other device, where an increase in voltage
causes an increase in current. The range of voltage where this occurs
is called the "negative resistance" region. When the voltage approaches
0.7 volts it begins to act like a normal forward-biased diode.
Characteristic curve of a tunnel diode.
Tunnel diodes are often used as the active component of very high
frequency oscillators. They can be biased in such a way that the
voltage will oscillate rapidly in the negative resistance region.
