Transistors


 * See also: List of quasiparticles and Quasiparticle

A hole is a region with a net surplus of positive charges. An anti-hole is a region with a net surplus of negative charges. Electricity is the flow of holes and anti-holes.

A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention is needed for the direction of current that is independent of the type of charge carriers. The direction of conventional current is arbitrarily defined as the same direction as positive charges flow.



A p-type semiconductor only conducts holes. An n-type semiconductor only conducts anti-holes. Holes and anti-holes combine at the junction of a forward biased diode.
 * In the case of a light emitting diode the combining of electrons and holes results in the creation of light.

Electricity will not flow through a reverse biased diode because this would require holes and anti-holes to form at and move in opposite directions away from the junction.
 * However, in the case of a photodiode, current can be induced to flow by shining a light on the junction.



A transistor can be thought of as two diodes placed end-to-end (i.e. in series). When there is a voltage drop between the collector and the emitter then one diode is forward biased and the other diode is reverse biased. Because one of the diodes is reverse biased current will not flow. However current will flow from the collector to the emitter when a small amount of current is allowed to pass through the base.

To understand why this happens it helps to imagine that the forward biased diode is a light emitting diode and the reverse biased diode is a photodiode. (Such a transistor is called a photon coupled transistor.) When current is allowed to pass through the base light is created by the light emitting diode. This light is then absorbed by the photodiode and therefore current is able to pass from collector to emitter. This in turn creates still more light which allows still more current to pass. If the photodiode absorbed 100% of the light emitted than the current would flow forever. Since not all the light created by the light emitting diode is absorbed by the photodiode the current will decay rapidly.

The total amount of current that flows from collector to emitter will be some multiple of the current that originally flowed through the base.

From Exciton:

An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an electrically neutral quasiparticle that exists in insulators, semiconductors and in some liquids. The wavefunction of the bound state is said to be hydrogenic, an exotic atom state akin to that of a hydrogen atom. However, the binding energy is much smaller and the particle's size much larger than a hydrogen atom. This is because of both the screening of the Coulomb force by other electrons in the semiconductor (i.e., its dielectric constant), and the small effective masses of the excited electron and hole. Provided the interaction is attractive, an exciton can bind with other excitons to form a biexciton, analogous to a dihydrogen molecule.