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Unformatted text preview: mobility is affected by the lattice and the impurities.
– It has a temperature dependence because scattering
process have a temperature dependence. Drift of Carriers in B and E Fields
Conductivity • All charge carriers are in motion
– for electrons and holes at equilibrium (no external fields)
the motion is random and lots of scattering occurs.
– Result: no net flow of charge
Result: • With an applied external field, charge does flow
– It can be shown that for electrons, the current density Jx is
Jx = -qn<vx> = q2ntEx/mx* = σEx
and σ = qnµx
where: µn is the electron mobility and mx* is the
conductivity effective mass (not DOS EM!)
conductivity Randomized Electron Motion Thermal motion of an electron in a solid. Drift of Carriers in B and E Fields
• A similar analysis can be performed for holes,
and the holes also have a mobility µ arrising
from a current Jx for holes
p • Total current is the sum of electron and hole
J = q(nµ + pµ )Ex = σEx
n p • Also, there is an inverse relationship between
effective mass and mobility --> less massive
particles are more mobile than massive ones.
particles Current Flow Conventions Drift of electrons and holes in a semiconductor bar. Drift of Carriers in B and E Fields
• Temperature Effects and Doping on Mobility
– Recall that ther...
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- Spring '10