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Semiconductor p-n junction diode
If a crystal of p-type material is joined to a crystal of n-type material a p-n junction forms.
At the join, electrons from the n-type material fill the holes in the p-type material.
There are no charge carriers in this zone, called the depletion layer, and so it will not conduct. This is a semiconductor diode.
Reverse biased diode
If the diode is connected, as shown above, the electrons move towards the positive terminal and the holes move towards the negative terminal.
This causes the depletion layer to get bigger and so the diode does not conduct. This diode is said to be reverse biased.
Forward biased diode
If the diode is connected, as shown above, the electrons move towards the positive terminal and the holes move towards the negative terminal.
In this case electrons and holes both head for the depletion layer and fill it with charge carriers. This causes the depletion layer to disappear and the diode conducts. This diode is said to be forward biased.
Diodes only allow electricity to pass when they are connected in the forward bias manner. This means a diode will only allow current to flow in one direction.
When an electron fills a hole in the depletion layer it moves from the conduction band to the valance band, so energy is released. Depending on the impurity and semiconductor used, the difference in energy level between conduction and valence bands can be large enough to emit the energy as a photon of light. This is a light emitting diode, or LED.
Diodes can also be made so that the junction will absorb photons of light. A photon of light will cause an electron-hole pair to be created in the junction allowing the diode to conduct. This is a photodiode.
If a photodiode is connected in a circuit in reverse bias it will only conduct when light shines on the junction. This is known as the photoconductive mode.
A photodiode can be designed so energy from the incident photon promotes the electron from the electron-hole pair up to the conduction band. This generates an e.m.f. This is called the photovoltaic mode and is the basis of solar cells and PV (photovoltaic) panels used to generate electricity from sunlight.
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