Semiconductors

There are some elements that behave like a conductor and an insulator depending on its condition. These elements are grouped as semiconductors. Typical semiconductors like carbon, silicon and germanium have a unique electron structure in which each has four electrons in its outer orbital. These four electrons form a perfect covalent bond with four adjacent atoms and forms crystal lattice. Crystalline form of carbon is diamond.

    In pure state, semiconductors behave like insulators. However electron movement can be achieved by heating. As the temperature rises, the electrons leave their orbits and start to float. Then by applying voltage electrons can be made to flow in one direction. This is known as intrinsic conduction.

    Flow of current heats up the structure and increases conduction. If this is allowed to continue, it breaks down. This process is known as thermal runaway.

Doping: The process of intentionally introducing tiny amount of impurities (different atoms) to an extremely pure semiconductor is called doping. Doping improves conductivity of a semiconductor.

    There are two types of impurities that are explained below.

• N-type: Arsenic is introduced into silicon to improve its conductivity. Silicon has 4 valence electrons and arsenic has 5 valence electrons. When mixed, the fifth electron of the arsenic has nothing to bind to and so it will float as a free electron. Likewise, adding tiny amount of arsenic creates so many free electrons. These free electrons allow the current to flow. Since negatively charged extra electrons are floating, they are known as n-type semiconductors. Thus pentavalent (5 valence electrons) impurities produce n-type semiconductors.
• P-type: Boron or gallium is the dopant in p-type semiconductors. They have only 3 valence electrons. When mixed into the silicon they form a hole in the electron structure. This missing electron forms a positive charge effect and is called p-type semiconductors. Thus trivalent (3 valence electrons) impurities produce p-type semiconductors.