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.
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.
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).