Resistors, capacitors and inductors are known as passive components . Electronic components which produces changes in circuit conditions by reacting to applied signals are known as active components . The majority of active components used in electronic circuits use semiconductors .
A semiconductor can sometimes act like a conductor, and at other times like an insulator in the same circuit.
The term semiconductors arises from the ability of these materials to conduct “part
time.” Their versatility lies in the fact that the conductivity can be controlled to produce effects such as amplification, rectification, oscillation, signal mixing, and switching.
Very simply, a semiconductor material is one which can be given a predominance of mobile negative charges or electrons, or positive charges or holes. Current can flow through the material from the movement of both electrons and holes.
Semiconductor properties can be given to a strictly limited number of materials by doping with minute traces of impurities.
Doping can produce a material with either a predominance of .Positive charges (holes) resulting in a P-type material (p channel); or with a predominance of a negative charges (electrons), known as an N-type material (n channel).
This does not become particularly significant until a single crystal (of germanium or silicon) is treated with both a P-type dope and an N-type dope.
Since these regions have opposite charges there is a tendency for electrons to migrate from the N-zone to the P-zone, and holes to migrate from the P-zone into the N-zone. The effect is a cancellation of charges in the region of the junction of the P- and N-zones, forming what is called a depletion layer .
Depletion layer contains no free electrons or holes, then acts as a barrier between the P-zone and the N-zone, preventing any further migration of either electrons or holes. In effect, depletion layer sets up a potential difference between the two regions and the device remains in a stable state until an external voltage is applied to it.
If a positive voltage is applied to P channel and a negative voltage to the N-channel, depletion layer disappear and N-type material constantly feeds electrons to the P-type in an attempt to create an electron balance, and the battery or power supply
keeps robbing electrons from the P-type material, this is known as forward bias.
When the polarity is switched so the N-type material is positive with and P channel negative, electrons in the N-type material are pulled towards the positive charge, away from the junction;and holes are pulled toward the negative charge, also away from the junction type,this is called reverse bias.
In this case the depletion region acts like an insulator.