16 Solid State Lighting II

16 Solid State Lighting II - Solid State Lighting II P. D....

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Solid State Lighting II P. D. Dapkus
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White LED II
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Radiative Recombination Light emitting devices rely on recombination of electrons and holes to produce photons. Radiative recombination events that produce photons conserve energy and momentum. Thus, direct materials are preferred for high efficiency. At equilibrium in direct semiconductors, recombination and generation processes are in balance. Thus the rates of generation and recombination are equal and the concentration of electrons and holes are the equilibrium concentrations, n o and p o . For undoped materials, n o = p o = n i but n o x p o = n i 2 . Under non equilibrium conditions the electron and hole concentrations are perturbed by an external sources and the rate of recombination, R, is proportional to the product of the electron and hole concentrations and . Generally . Then . Under steady state conditions the generation rate by the external source is constant, , and ( )( ) Thus, the exc oo ext th ext o o th th o o n n n p p p n p R Bnp g R g g Bnp B n n p p g R Bn p   2 ess recobination rate owing to the external source of generation is ( ) . exc o o R B n n p B n
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Under low level conditions , , and ( ). When the external excitation is time depenedent, ( ), then ( ) ( ( )) ( ( )) ( ) ( ) ( ) ( ) oo exc o o ext tot th ext th exc ext exc ext o o n p n p R B n n p gt dn g R t g g t R R t dt g t R t g t B n p n    / () Assume the external excitation is a step function tha t ends at t = 0, then for t > 0, ( ) ( ) or ( ) (0) where is the lifetime of the excess carriers 1 1 fo t
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16 Solid State Lighting II - Solid State Lighting II P. D....

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