Semiconductor definitions/descriptions

Intrinsic and Extrinsic Semiconductors


Intrinsic semiconductors are pure and undoped. They have an equal number of conduction electrons and holes. Thermal excitation is required for conduction.


Extrinsic semiconductors are doped with impurity atoms, which generate majority carriers. The type of majority carrier is dependent on the dopant used. These are as follows:

N-type- Majority carriers are electrons, minority carriers and holes.

P-type- Majority carriers are holes, minority carriers and electrons.


Drift velocity of carriers

Drift velocity is the average velocity that a charge carrier, such as an electron, attains in a material due to an electric field.


Band-gap energy

The band-gap energy is the energy required to move an electron from the valence band to the conduction band.


Electron-hole recombination and the frequency of light emitted by an LED.

Electron-hole recombination

When an electron which has previously been excited from the valence, to the conduction band, falls back into an empty state in the valence band.

Frequency of light emitted by an LED

When an electron moves back from the conduction band to the valence band, energy equal to the band-gap energy is emitted.

This energy produces a frequency which is equal to the band-gap energy divided by Planck’s constant. In LEDs, the frequency produced is in the form of light (Hence the name Light Emitting Diode).




As the name suggests semiconductors are partially conductive materials, and lay somewhere between the conductivity of conductive metals and insulators.

N-type semiconductors

N-type semiconductors are generally composed of silicon, or germanium, doped in antimony. The doping provides a  free electron which increases the conductivity of the material.

The name derives from negative charge. This is because of electric current, where electrons break free from their atoms, and create a direction flow of electrons, with the aid of an electric field. Electrons flow towards vacancies in positively biased materials.

P-type semiconductors

P-type semiconductors are also generally composed of silicon, or germanium. However, Instead of being doped in antimony, they are doped in an element such as indium. This has the opposite effect to that of doping in antimony, as indium has a vacancy in its outer shell of electrons, to which a free electron can easily occupy.

The letter ‘P’ represents the fact that this material has a positive charge.


Under normal circumstances, In N-type semiconductors free electrons flow away from the materiel. Whereas In P-type semiconductors, free electrons flow towards the material to occupy the vacancies.

In terms of conventional current, current flows from P-type (positive) semiconductors, and current flows towards N-type (negative) semiconductors.