Semiconductor devices have revolutionized the field of electronics. Before semiconductors, vacuum tubes were the only devices available for signal amplification, switching, and other applications. But vacuum tubes are bulky, require a high operating voltage, and are inefficient. Semiconductors, on the other hand, are materials that are in between conductors and insulators when it comes to their ability to conduct electrical current.
The most commonly used semiconductor material in the electronics industry is silicon, followed by a compound known as gallium arsenide. Though germanium was used extensively in the early years of semiconductor technology, it’s unstable at high temperatures, which is why silicon became more widely used.
Semiconductor materials have two current carriers, free electrons and holes. In an intrinsic semiconductor material, free electrons are produced when the material receives sufficient thermal energy that provides valence electrons from the valence band enough energy to jump to the conduction band and turn into free electrons. When valence electrons jump to the conduction band, they leave vacancies in the valence band. These vacancies are called holes. The number of holes in the valence band is just equal to the number of free electrons in the conduction band in this undoped, intrinsic material.
However, semiconductor materials, in their intrinsic state, do not conduct current well due to the limited number of free electrons and holes in them. But through a process known as doping, the conductivity of a semiconductor can be increased by adding impurities with either more free electrons or holes to the intrinsic semiconductor material.
The number of free electrons in an intrinsic semiconductor material with four valence electrons, such as silicon, is increased in the doping process by adding pentavalent impurity atoms, or atoms with five valence electrons such as arsenic, phosphorus, bismuth, or antimony. By adding pentavalent impurity atoms to an intrinsic semiconductor material, the number of free electrons can be increased as well as the conductivity of the semiconductor material. Semiconductors doped with pentavalent atoms are n-type semiconductors since the majority of their current carriers are electrons.
In order for an intrinsic semiconductor material with four valence electrons, such as silicon, to have more holes, they are doped with trivalent impurity atoms such as boron, indium, and gallium. By adding more trivalent impurity atoms to an intrinsic semiconductor material, it increases the number of holes and improves the conductivity of the semiconductor material. Semiconductors doped with trivalent atoms are p-type semiconductors since the majority of their current carriers are holes.
The doping process converts an intrinsic semiconductor material into extrinsic and produces either an n-type or a p-type semiconductor material. When you dope an intrinsic semiconductor p-type and then, directly adjacent to that, n-type, the boundary where the p-type and n-type doped material touches is known as a p-n junction. This p-n junction is the basis for different semiconductor devices widely used today like diodes, transistors, and thyristors.
In summary, semiconductors are materials that have revolutionized the field of electronics. We discussed the basics of semiconductors, the intrinsic semiconductor and its poor conductivity, how doping increases the number of current carriers in a semiconductor material and improves its conductivity. We also briefly mentioned how different semiconductor devices were created based on the p-n junction.