Semiconductor p-n junction diode
A p-n junction diode is formed by doping one half of the semiconductor crystal with p-type impurity and the other half with n-type impurity while the crystal is being formed.
Unbiased p-n junction
Unbiased conditions mean that there is no external energy source (no voltage).
In an unbiased diode, an electric field is set up across the Junction between the p-type and n-type materials of a diode in which an electric field is set up. This acts as a barrier that must be overcome before the diode can conduct. between the n-type and the p-type material. This is caused by the imbalance in free electrons due to the doping.
Reverse biased diode
In reverse bias the diode is connected with the p-type connected to the negative supply terminal and the n-type connected to the positive. The electric field across the depletion layer increases. This acts as a barrier that stops electron flow.
The valence band energy level in the p-type material is raised above the free electrons of the conduction band of the n-type. This is due to the combination of doping and electric field across the junction.
Forward biased diode
In forward bias, electrons in the conduction band of the n-type move towards the conduction band of the p-type.
Electrons drop from the conduction band to valence band of the p-type semiconductor. This drop in energy level causes energy to be released. In a normal p-n junction diode, this energy causes the diode to heat up when conducting.
LEDs - Light-emitting diodes
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.
Watch this video to find out more about how LEDs work.
How LEDs work
Photodiodes
Diodes can also be made so that the junction will absorb photons of light.
When a photon of light is absorbed, it provides energy that can cause electrons from the valence band of the p-type semiconductor to be promoted to the conduction band. The electrons flow towards the n-type semiconductor in the junction. This creates a potential difference across the diode and allows it to generate an EMF.
This is a photodiode or photovoltaic cell.
Watch this video to learn how a photodiode works.
How photodiodes work