Diodes are electronic components functions as a one-way valve it means it allow current to flow in one direction. These diodes are manufactured by the semiconductor materials germanium, silicon and selenium.

Operation of diode can be classified in two ways, if it allows the current then it is forward biased otherwise it is reverse biased.

A basic bipolar transistor

Signal Diodes

This can be a small device with disproportional characteristics and whose applications are mainly involved at high frequency and very low currents devices such as radios and televisions etc.

To protect the diode from contamination it is enveloped with a glass so it is also named as Glass Passivated Diode which is extensively used as 1N4148.

Zenor Diodes

A zenor diode is a passive device that works under the principle of zener breakdown.

It is similar to normal diode in forward direction, it also allows current in reverse direction when the applied voltage reaches the breakdown voltage.

It is designed to prevent the other semiconductor devices from momentary voltage pulses. It acts as voltage regulator.

Ligth Emitting Diodes - LEDs

The LED is basically just a specialised type of diode as they have very similar electrical characteristics to a PN junction diode. This means that an LED will pass current in its forward direction but block the flow of current in the reverse direction.

Light emitting diodes are made from a very thin layer of fairly heavily doped semiconductor material and depending on the semiconductor material used and the amount of doping, when forward biased an LED will emit a coloured light at a particular spectral wavelength.

When the diode is forward biased, electrons from the semiconductors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic (single colour) of light.

Because of this thin layer a reasonable number of these photons can leave the junction and radiate away producing a coloured light output.

Schottky Diodes

Like other diodes, the Schottky diode controls the direction of current flow in a circuit.

These devices like other diodes let current pass only from anode to cathode. However, unlike standard diodes, the Schottky diode is known for its low forward voltage and fast switching ability. This makes them an ideal choice for radio frequency applications and any device with low voltage requirements.

One of the primary advantages of using a Schottky diode over a regular diode is their low forward voltage drop. This allows a Schottky diode to consume less voltage than a standard diode, using only 0.3-0.4V across its junctions. In the graph below you can see that a forward voltage drop of about 0.3V begins to increase current significantly in a Schottky diode. This current increase would not take effect until about 0.6V for a standard diode.