The Electrons and holes performs a chaotic random thermal motion. The average kinetic energy of thermal motion per one electron is given by
3kBT/2
where T stands for temperature in degrees Kelvin and kB represents Boltzmann constant. The electron thermal velocity, Vthn, is found by equating the electron kinetic energy to 3kBT/2:
Thermal velocities Vth~ and Vthp tells us about the average magnitudes of electron and hole velocities induced by the thermal motion. Since the directions of this thermal motion are random, the same amount of carriers crosses any device cross-section in any direction so that the electric current is become equal to 0 level.
In an electric field, the electrons and holes gain an extra drift velocity caused by the electric field, which is superimposed on the chaotic thermally induced velocities. The drift velocity of the electrons and holes In a uniform semiconductor and the drift velocity of the electrons and holes in a weak electric field, F, vn and vp, are proportional to the electric field:
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