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Chapter 4 Lecture - Part 1

Donor levels are designated by a plus sign and

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Unformatted text preview: 4-9 Includes dopants and impurity-based traps Some elements introduce two trapping levels Energy levels of impurities in Si The energies are measured from the nearest band edge (Ev or Ec). Donor levels are designated by a plus sign and acceptors by a minus sign. Steady State Carrier Generation Steady • Quasi Fermi Levels – Terms: • Equilibrium – no external excitation except T and no net Equilibrium charge motion charge • Steady State – nonequilibrium conditions where all Steady processes are constant and balanced by opposing processes processes – Recombination occurs under these cases too • Example: Thermal and optical excitation – Fermi Levels • In general the Fermi level is only meaningful when no In excess charge carriers are present. – Recall that excess charge carriers refer to those in excess of Recall Steady State Carrier Generation Steady • Quasi Fermi Levels (cont.) – Take the same form as equilibrium Fermi level – Used for steady state conditions – Denoted as Fn and Fp – Steady state carrier concentrations: n = nie(Fn – Ei)/kT p = pie(Ei – Fn)/kT – Quasi Fermi levels are useful in visualizing the Quasi deviation from equilibrium caused by an external stimulus. stimulus. Steady State Carrier Generation Quasi-Fermi levels Fn and Fp for a Si sample with n0 = 1014 cm23, tp = 2 ms, and gop = 1019 EHP/cm3-s and optically excited. Fn is only slightly above EF while FP is significantly below EF. This indicates that the incident photons cause a large change in holes and small change in electrons. Photoconductive Devices Photoconductive • Mode of Operation – Resistance change upon exposure to light • Choice of SC – SC are most sensitive to photons with energies equal to SC or slightly higher than the bandgap or • E < Eg no absorption • E >> Eg absorbed only at the surface – Common photodetector materials • Visible: CdS (2.42 eV) Visible: • IR: Ge (0.67 eV), InSb (0.18 eV) – Sensitivity: higher for higher mobilities and higher Sensitivity: lifetimes lifetimes – Time response: related to recomb times and trapping...
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