2_1_2011_EE2_Mid-term Solution

2_1_2011_EE2_Mid-ter - Physics for Electrical Engineers Prof B Jalali EE 2 Mid-term Exam Solution 1 Consider the process of photon getting absorbed

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Physics for Electrical Engineers P r o f . B . J a l a l i E E 2 Mid-term Exam Solution February 08, 2011 1. Consider the process of photon getting absorbed in a direct-bandgap semiconductor with bandgap Eg. Draw a qualitative plot showing how the absorption strength (absorption coefficient) depends on: (a) Photon energy, E p ; (b) Photon wavelength, λ; (c) Relate the zero absorption energy and wavelength to the bandgap. An electron in semiconductor can be excited from valence band to conduction band through absorbing a photon. The photon energy p p hc hf E λ = = has to be at least equal to bandgap g E in order to be absorbed, which gives the zero absorption energy 0 pg EE = and zero absorption wavelength 0 p g hc E = . Usually we consider there are enough electrons in the valance band. Electrons are excited from valance band to empty states in conduction band. The density of states in conduction band D(E) is proportional to g in 3-dimentional material, therefore, the absorption strength is proportional to , or equivalently,
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This note was uploaded on 10/07/2011 for the course EE 2 taught by Professor Vis during the Spring '07 term at UCLA.

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2_1_2011_EE2_Mid-ter - Physics for Electrical Engineers Prof B Jalali EE 2 Mid-term Exam Solution 1 Consider the process of photon getting absorbed

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