Physics for Electrical Engineers Prof. B. Jalali EE 2 Winter 2011 Homework #3 solution 1 a) Si can emit photon energy 1.12gEEeV≥=Photon Energy is given as hcEhvλ==So the emission wavelength is 348196.626103 10118.104.22.1680ghcmnmEλ−−×××≤==××Figure 1. Band diagram of Si and GaAs. b) Figure 1 (or figure 3-10 in the textbook) shows the band structure of Si and GaAs. As you can see, Si is an indirect bandgap material, therefore, it is not a good choice for LED. When emitting a photon from Si, an electron in the conduction band has to jump down to the valence band. Also, this emitting process has to obey both energy conservation and momentum conservation: Ephoton=ΔEelectron=Ec-Ev=Eg pphoton=ΔpelectronThe energy conservation is easily satisfied, however, the momentum is not easily conserved,
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