ABSTRACT
Let usnowconsiderthecaseofphoton(3),whichenergyishigherthanEG. ™is photon can be absorbedinthesemiconductor,butbecausewithinthebandstheelectronicstatesformacontinuum, theproducedcarrierswilleasilymigratetolowerenergystates,transferringtheenergyexcesstothe latticeinformofheat(phononemission).Inanyconventionalsemiconductor,carrierswillrelaxbythis thermalizationprocesstothebandgapedgesinsub-picosecondstimeandtheremainingcollectable energyfromphoton(3)willbethesameasthatinthecaseofphoton(1).Wefacethenatrade-off:if wechooseasemiconductorofhighEG, such as GaAs, few photons will be absorbed, whereas if a lowbandgapmaterialasGeischosen,wewillcollectmorecarriers,butagreaterpartoftheirenergywillbe lost by thermalization.
