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20111ee2_1_2011_EE2_HW3_Solution

# 20111ee2_1_2011_EE2_HW3_Solution - Physics for Electrical...

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Physics for Electrical Engineers Prof. B. Jalali EE 2 Winter 2011 Homework #3 solution 1 a) Si can emit photon energy 1.12 g E E eV = Photon Energy is given as hc E hv λ = = So the emission wavelength is 34 8 19 6.626 10 3 10 1109.2 1.12 1.6 10 g hc m nm E λ × × × = = × × 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: E photon = Δ E electron =E c -E v =E g p photon = Δ p electron The energy conservation is easily satisfied, however, the momentum is not easily conserved,

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