Please explain what is Photodiode, light emitting Diode and Zener Diode?

Photodiode

Photodiode is a p-n junction whose function is controlled by the light allowed to fall on it. Suppose, the wavelenght is such that te energy of a photon  , is sufficient to break a valence bond. When such light falls on the junction, new hole - electron pairs are created. The number of charge carriers increases and hence the conductivity of the junction increases. If the junction is connected in some circuit, the current in the circuit is controlled by the intensity of the incident light.

Light - emitting Diode

When a conduction electron makes a transition to the valence band to fill up a hole in a p-n junction, the extra energy may be emitted as a photon. If the wavelenght of this photon is in the visible range( 380 nm - 780 nm), one can see the emitted light. Such a p-n junction is known as light emitting diode abbreviated as LED. For silicon or germanium, the wavelength falls in the infrared region. LED's may be made from semiconducting compounds like gallium arsenide or indium phosphide. LED's are very commonly used in electronic gadgets as indicator lights.

Zener DIode

If the reverse - bias voltage across a p-n junction diode is increased, at a particular voltage the reverse current suddenly increases to a large value. This phenomenon is called breakdown of the diode and the voltage at which it occurs is called the breakdown voltage. At this voltage, the rate of creation of hole - electronpairs is increased lending to the increased current.

Tehre are two main processes by which breakdown may occur. The holes in the n - side and the conduction electrons in the p - side are accelerated due to the reverse - bias voltage. If these minority carriers acquire sufficient kinetic energy from the electric field and collide with a valence electron, the bond will be broken and the valence electron will be taken to the conduction band.