23-08-2013, 03:25 PM
CONDUCTION IN SEMICONDUCTORS
CONDUCTION IN SEMICONDUCTORS.ppt (Size: 605.5 KB / Downloads: 121)
Drift and Diffusion
We now have some idea of the number density of charge carriers (electrons and holes) present in a semiconductor material from the work we covered in the last chapter. Since current is the rate of flow of charge , we shall be able calculate currents flowing in real devices since we know the number of charge carriers. There are two current mechanisms which cause charges to move in semiconductors. The two mechanisms we shall study in this chapter are drift and diffusion.
Carrier Mobility
A perfect crystal has a perfect periodicity and therefore the potential seen by a carrier in a perfect crystal is completely periodic.
So the crystal has no resistance to current flow and behaves as a superconductor. The perfect periodic potential does not impede the movement of the charge carriers. However, in a real device or specimen, the presence of impurities, interstitials, subtitionals, temperature , etc. creates a resistance to current flow.
The presence of all these upsets the periodicity of the potential seen by a charge carrier.
Random motion result no current.
Since there is no applied field, the movement of the charge carriers will be completely random. This randomness result no net current flow. As a result of thermal energy there are almost an equal number of carriers moving right as left, in as out or up as down.
Carrier Diffusion
Diffusion current is due to the movement of the carriers from high concentration region towards to low concentration region. As the carriers diffuse, a diffusion current flows. The force behind the diffusion current is the random thermal motion of carriers.
Carrier recombination and diffusion length
By means of introducing excess carriers into an intrinsic s/c, the number of majority carriers hardly changes, but the number of minority carriers increases from a low- to high-value.
When we illuminate our sample (n-type silicon with 1015 cm-3 ) with light that produces 1014 cm-3 electron-hole pairs.
The electron concentration (majority carriers) hardly changes, however hole concentration (minority carriers) goes from 1.96 x 105 to 1014 cm-3.