What are Diodes?

A Diode is a semiconductor which can be used to change Electric Supply from Alternating Current to Direct Current. They allow current to pass through them in one direction.

Schematic Symbol

Rectifier diodes in a circuit

A Diode has two parts; the P-type semiconductor and the N-type semiconductor. The P-type semiconductor has holes (Positive charges) as the majority charge carriers and N-type has electrons (Negative charges) as the majority charge carriers.

Silicon or Germanium are the materials used to manufacture diodes. During manufacture, some impurity atoms are added to silicon. This is called doping. To produce the P-type semiconductor silicon is doped with Boron, for example, which introduces holes as majority charge carriers. Phosphorus can be added to silicon to produce N-type semiconductor. Phosphorus atoms offer their electrons to silicon in a covalent bond. There are also minority charge carriers in both semiconductors; the P-type has electrons as minority charge carriers and the N-type has holes as minority charge carriers.

P-type doping

N-type doping

Diode Bias

Diode bias is the condition of a diode.

No Bias

The No Bias condition, this is when there's no voltage applied. The layers of ions in the depletion region of the diode repel majority charge carriers, holes and electrons, of the P-type and N-type semiconductors and prevent them from crossing the junction.

Forward Bias (VD )
In the Forward Bias condition, also known as 'on' condition of the diode, a positive potential is applied to the P-type semiconductor and a negative potential is applied to N-type semiconductor. The ions in the depletion region get neutralized and eventually they will allow heavy flow of electrons. This is due to the pressurizing of electrons in the N-type semiconductor.


The forward bias voltage of a germanium diode is 0.3 volts and that of a silicon diode is 0.6 volts. It is important to note the maximum forward current which can be allowed to pass through the diode. If too large current is allowed to pass through a diode, it can  easily get damaged.

Reverse Bias
In the reverse bias, the depletion region enlarges. When a positive potential is applied to the N-type semiconductor and the negative potential to the P-type semiconductor, the uncovered positive and negative ions in the depletion region increases. This is because the negative terminal of the voltage supply repels electrons in the P-type semiconductor and the positive terminal attracts electrons in the N-type semiconductor. Therefore conduction is not possible.