Diode – a device that allows current flow in one direction only.

Diodes are categorized as either vacuum

Vacuum Tube Diode – invented by Sir John Ambrose Fleming in 1904 diode. It is also called Fleming
valve or thermionic tube. It consists of two electrodes, a cathode, and an anode or plate. The cathode
emits the free electrons. Hence, it is called as emitter. The anode collects the free electrons. Hence, it is
called as collector. The cathode and anode are enclosed in an empty glass envelope. The anode is a
hollow cylinder made of molybdenum or nickel and cathode is a nickel cylinder coated with strontium
and barium oxide. The anode surrounds the cathode. In between the cathode and anode an empty
space is present, through which the free electrons or electric current flow. Although considered as
almost obsolete due to the introduction of semiconductor type, it find its use in some audio equipment,
such as audio amplifiers.

Symbol of a vacuum tube

Operation

Forward Bias

If voltage is applied to the vacuum diode, in such a way, that anode is connected to a positive terminal
and cathode is connected to a negative terminal (anode is more positive with respect to the cathode),
the free electrons in the vacuum gains enough kinetic energy to reach the anode.

Vacuum tube diode at forward bias

Reverse Bias
If voltage is applied to the vacuum diode, in such a way, that anode is connected to the negative
terminal and cathode is connected to the positive terminal (anode is more negative with respect to
cathode), the free electrons in the vacuum gains enough kinetic energy to reach the anode. However,
anode repels the free electrons that try to move towards it.

Vacuum tube diode at reverse bias

Semiconductor Diode – also known as junction diode or crystal diode, it is constructed of doped
semiconductor materials, commonly, Silicon, and occasionally, Germanium. It also sometimes called as
solid state diode.

Diode schematics and symbol

Forward Bias

If voltage is applied to the diode, in such a way, that anode is connected to a positive terminal and
cathode is connected to a negative terminal (anode is more positive with respect to the cathode), the
current will flow from anode to cathode.

Diode at forward bias

If voltage is applied to the diode, in such a way, that anode is connected to the negative terminal and
cathode is connected to the positive terminal (anode is more negative with respect to cathode), current
will stop flowing.

Diode at reverse bias

Types of Diode

Signal or Switching Diode

It is a type of P-N junction diode which operates on low voltage signals. Its junction area is very small.
Due to which, the junction has less capacitance & low charge storing capacity. It enables the small signal
diode to have high switching speed with very fast recovery time. However, its limitations are low voltage
and current parameters.

Signal Diode
Rectifier Diode

A rectifier diode is a type of P-N junction diode, whose P-N junction area is very large. This results in high
capacitance in reverse direction. It has low switching speed. These types of diodes can handle heavy
current and are used in converting AC into DC (Rectification).

Rectifier Diode
Schottky Diode

Named after German physicist Walter H. Schottky, is a type of diode which consists of a small junction
between an N-type semiconductor and a metal. It has no P-N junction. Schottky has very low forward
voltage drop. As there is no capacitive junction (P-N junction), the Schottky diode switching speed is very
fast. The limitation of Schottky diode is that it has low reverse breakdown voltage and high reverse
leakage current. The packaging is similar to a rectifier diode.

Symbol

Light Emitting Diode (LED)

The light emitting diode is also a type of P-N junction diode that emits light in the forward bias. The
semiconductor material used in LED is Gallium Arsenide (GaAs), Gallium Phosphide (GaP) or Gallium
Arsenide Phosphide (GaAsP). At forward bias, the charge carriers (electrons) cross the barrier and
recombine with electron holes on the other side, they emit photon particles (light) whose color depends
on the energy gap of the semiconductor. LED converts electrical energy into light energy.

Symbol

Light Emitting Diode

Photodiode

The photodiode is a type of P-N junction diode that converts the light energy into electrical current. Its
operation is opposite to that of an LED. In the photodiode, there is a special opening that allows the light
to enter its sensitive part. When the light (Photon particles) strikes the PN junction, it creates an
electron-hole pair. These electron and hole flow out as electrical current. It called a photo-voltaic effect
which the foundation of solar electricity.

Symbol
 Photodiode

Tunnel or Esaki Diode

Tunnel diode was invented by Leo Esaki in 1958. A tunnel diode is a heavily doped P-N junction diode. It
works on the principle of the tunneling effect. Due to heavy doping concentration, the junction barrier
becomes very thin. This allows the electron to easily escape through the barrier. This phenomenon is
known as tunneling effect. The Tunnel diode has a region in its VI curve where the current decreases as
the voltage increases. This region is known as the negative resistance region. The tunnel diode operates
in this region in different applications such as an oscillator and a microwave amplifier.

Symbol

Tunnel or Esaki Diode

Zener Diode

Named after Clarence Malvin Zener who discovered the Zener effect. It is a type of diode, which not only
allows the flow of current in the forward direction but also in reverse direction. When the reverse
voltage reaches the breakdown voltage known as Zener voltage it allows the current flow. The Zener
diode has heavier doping concentration than a normal P-N junction diode. Hence, it has a very thin
depletion region. In forward bias, it operates as a simple P-N junction diode (Rectifier). In reverse bias, it
blocks until the reverse voltage reaches breakdown. After that, it allows the current flow with a constant
voltage drop. A Zener diode is mainly used in reverse bias configuration. It provides a stabilized voltage
for protection of circuits from overvoltage.

Symbol
Varactor Diode or Variable Capacitance Diode (Varicap)

Varactor diode also known as Varicap diode are voltage controlled capacitors. They have a P-N junction
with variable junction capacitance. The varactor diode operates under reverse bias conditions. The
depletion layer between the P and N-type material is varied by changing the reverse voltage. All diode’s
junction capacitance varies with reverse voltage but Varactor diode is able to use this effect with a high
range of capacitance. The applications of Varactor diodes are as a voltage controlled oscillator in the
phase-lock loop, in RF tuning filters and frequency multipliers.

Symbol

Varactor Diode

P Intrinsic N (PIN) Diode

PIN diode is a three-layer diode. The intrinsic semiconductor layer is placed between heavily doped P
and an N-type semiconductor. The electron and holes from N and P-type region respectively flow to the
intrinsic region. Once the “I” region fills completely with electron-holes, the diode starts conduction. In
reverse bias, the wide intrinsic layer in the diode can block and tolerate high reverse voltages. At higher
frequency, the PIN diode will act as a linear resistor. It is because of the fact that the PIN diode has poor
reverse recovery time. The reason is that the heavily charged “I” region does not get enough time to
discharge during fast cycles. While at low frequency, it acts as a rectifier diode. Because it gets enough
time to discharge & turn off during the cycle.

Symbol

PIN Diode
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