Lec6

Lec6 - Subject_05:Outline...

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Carrier Drift in Semiconductors * Semiconductors * Carrier drift in an electric field * Carrier mobility * Resistivity Subject_05: Outline
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• Semiconductors are  POOR  electrical conductors in their  PURE  state having a conductivity that  INCREASES  with  increasing temperature in marked  CONTRAST  to metals  * Since the energy  GAP  that separates the highest filled band from the lowest empty     one is  SMALL  in semiconductors electrons can be  EXCITED  across it at higher     temperatures thereby increasing the conductivity  Semiconductors Phys. Rev. 75 , 865 (1949) • SHOWN RIGHT ARE MEASUREMENTS OF THE  RESISTIVITY OF SILICON-PHOSPHOROUS ALLOYS (NOTED THE INVERTED TEMPERATURE SCALE) • THE RESISTIVITY  INCREASES  WITH  DECREASING TEMPERATURE WHICH IS THE CHARACTERISTIC SIGNATURE OF A  SEMICONDUCTOR   NOTE LOG SCALE!
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Carrier Drift in an Electric Field • Having developed techniques for describing the statistics of carriers in semiconductors we would now like to  consider the nature of carrier  DYNAMICS  under a variety of conditions * We first consider the  DRIFT  of carriers that is induced by the application of an      ELECTRIC FIELD  across the semiconductor * The electric field superimposes a slow net  DRIFT  on top of the  RAPID  but  RANDOM      motion that the charge carriers execute due to their  THERMAL  energy • AT ROOM TEMPERATURE ELECTRONS AND HOLES IN A SEMICONDUCTOR UNDERGO A  RANDOM  THERMAL MOTION  THAT IS GENERATED AS THE CARRIERS  SCATTER  FROM  VARIOUS  IMPERFECTIONS  IN THE CRYSTAL STRUCTURE • IN SPITE OF THIS RAPID MOTION THE NET CURRENT FLOWING  THROUGH THE CRYSTAL IS  ZERO  IN THE ABSENCE OF AN  APPLIED ELECTRIC FIELD • WITH AN  ELECTRIC FIELD  APPLIED ACROSS THE CRYSTAL  HOWEVER A SLOW  DRIFTING  MOTION IS SUPERIMPOSED ON  THE RANDOM CARRIER WALKS GIVING RISE TO A  NET  CURRENT  FLOW t o t o +Δt v d   ×  Δt
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 A simple classical analysis suggests that the  DRIFT VELOCITY  v d  accumulated by the electron should vary  LINEARLY  with the magnitude of applied electric field E * If we assume that carriers in the crystal scatter after some average  RELAXATION     TIME  t their drift velocity may be easily estimated ) 1 . 5 ( * E v p d m q p τ = LINEAR
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Lec6 - Subject_05:Outline...

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