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ECE 3040 - Dr. Alan Doolittle Georgia Tech Lecture 9 Photogeneration, Absorption, and Nonequilibrium Reading: Pierret 3.3-3.4

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ECE 3040 - Dr. Alan Doolittle Georgia Tech Photogeneration Light with photon energy, h ν < Eg is not easily absorbed. A convenient expression for the energy of light is E=1.24/ λ where λ is the wavelength of the light in um. Light with energy, h ν > Eg is absorbed with the “unabsorbed” light intensity as a function of depth into the semiconductor is I(x) = I o e - α x where Io is the initial light intensity, x is distance and α is the absorption coefficient [1/cm]. h ν < Eg h ν > Eg
ECE 3040 - Dr. Alan Doolittle Georgia Tech Photogeneration

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ECE 3040 - Dr. Alan Doolittle Georgia Tech Photogeneration Each Photon with energy greater than Eg can result in one electron hole pair. Thus, we can say, ( ) Sec cm e G x G where x G t p t n x LO L L Light Light = = = 3 # ) , ( ) , ( α λ λ If α is small (near bandgap light), the generation profile can be approximately constant. If α is large (light with energy>> bandgap), the generation profile can be approximated as at the surface.
ECE 3040 - Dr. Alan Doolittle Georgia Tech Important Nomenclature n= n+n o and p= p+p o In Non-equilibrium, np does not equal n i 2 Low Level Injection p= n <<n o and n~n o in n-type material p= n <<p o and p~p o in p-type material

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ECE 3040 - Dr. Alan Doolittle Georgia Tech p can be >> p o If p>>p o , p~ p Carrier Concentrations after a “Perturbation” Steady State if perturbation has been applied for a long time Non-Steady State, Nonequilibrium Equilibrium p(t=0)>p o or p(t=0)<p o p(t=0)> p(t)> p o or p(t=0)< p(t)<p o
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