Diodes2, Special purpose diodes

• Current Regulator Diode
  A current regulator diode maintains a constant current in forward-biased. This is opposed to a zener diode which maintains constant voltage in reverse bias operation. The constant forward current is called the regulator current.
• Schottky Diode

A schottky diode is not a pn junction device. Rather it is formed by joining a n type material with a metal such as gold, silver or platinum. Its symbol is

So instead of a pn junction there is a metal-to-semiconductor junction. Schottky diodes are used mainly in fast-switching and high-frequency applications. They can be used in digital circuits to decrease switching times.

• PIN Diode

 A pin diode is made of heavily doped p and n regions with an intrinsic i region in between p and n regions. In reverse-bias position it acts like a constant capacitance while in forward bias position it acts like a current-controlled variable resistance. This low forward resistance decreases with increasing current.

The pin diode is used in dc-controlled microwave switch operated by rapid changes in bias. It is also used as a modulating device which uses variable forward-resistance characteristic. Some types of pin diodes are used as photo-detectors in fibre-optic systems.

• Step Recovery Diode

The step-recovery diode uses graded doping where the doping level of the semiconductor materials is reduced as the pn junction is approached. This produces an abrupt turn-off time by allowing a fast release of charge during switching from forward to reverse bias. A fast re-establishment of forward current is also achieved during switching from forward to reverse bias.

The step-recovery diode is used in VHF (very high frequency) and fast-switching applications.

• Tunnel Diode

The tunnel diode has a negative resistance characteristic. That is, an increase in voltage results in a decrease in current. This is opposite to Ohm's law, where an increase in voltage results in an increase in current. The negative resistance property of tunnel diode is useful in oscillator and microwave amplifier applications.  Its symbol is

• Laser Diodes

LASER (Light Amplification by Stimulated Emission of Radiation). Laser diodes made of doped gallium arsenide, emits a light that is monochromatic, which means that it consists of a single colour and not a mixture of colours. It is also a coherent light made up of only a single wavelength.

The length of a pn junction bears a precise relationship with the wavelength of the light. There is a highly reflective surface at one end of the junction and a partially reflective surface at the other end produced by polishing the ends. Laser diodes are used in forward biased operation.