Chapter 3 Lecture - Part 5

Drift of carriers in b and e fields drift

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Unformatted text preview: mobility is affected by the lattice and the impurities. – It has a temperature dependence because scattering It process have a temperature dependence. Drift of Carriers in B and E Fields Drift Conductivity • All charge carriers are in motion – for electrons and holes at equilibrium (no external fields) for the motion is random and lots of scattering occurs. the – Result: no net flow of charge Result: • With an applied external field, charge does flow With (drift): (drift): – It can be shown that for electrons, the current density Jx is given by: given Jx = -qn<vx> = q2ntEx/mx* = σEx and σ = qnµx and where: µn is the electron mobility and mx* is the where: 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 Drift • A similar analysis can be performed for holes, similar and the holes also have a mobility µ arrising from a current Jx for holes from p • Total current is the sum of electron and hole Total current: current: J = q(nµ + pµ )Ex = σEx n p • Also, there is an inverse relationship between Also, 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 Drift • Temperature Effects and Doping on Mobility – Recall that ther...
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