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Lecture6_AbsorptionAndPhotoGeneration

# Lecture6_AbsorptionAndPhotoGeneration - Lecture Lecture 6...

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Lecture 6 Absorption and Photo-generation ECE 4833 - Dr. Alan Doolittle Georgia Tech

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Photogeneration Light with photon energy, h < Eg is not easily absorbed. A convenient expression for the energy of light is E=1.24/ where is the wavelength of the light in um. h < Eg h > Eg Light with energy, h > Eg is absorbed with the “unabsorbed” light intensity as a function of depth into the semiconductor is I(x) = I o e - x where Io is the initial light where Io is the initial light intensity, x is distance and is the absorption coefficient [1/cm]. ECE 4833 - Dr. Alan Doolittle Georgia Tech
Absorption The absorption depth is given by the inverse of the absorption coefficient, or α -1 . The absorption depth gives the distance into the material at which the light at that wavelength drops to about 36% of its original intensity (by a factor of 1/e) ECE 4833 - Dr. Alan Doolittle Georgia Tech about 36% of its original intensity (by a factor of 1/e).

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Absorption ECE 4833 - Dr. Alan Doolittle Georgia Tech
Real Energy band Diagrams: Direct verses Indirect Bandgaps The energy required to liberate an electron from the atom (the energy bandgap) is the same in all “escape directions” (di i h l l h ) (directions that an electron can leave the atom). Example: Electrons directed toward a neighboring atom would have a high escape energy, while electrons directed toward a channel in the crystal (a hole through the crystal) would have a lower escape energy. Thus, the energy band diagram is actually a function of ECE 4833 - Dr. Alan Doolittle Georgia Tech momentum. Additionally, both energy and momentum (directed mass motion) must be conserved during any transition.

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Real Energy band Diagrams: Direct verses Indirect Bandgaps Probability of a “direct transition” from valence band to conduction band i l b t if th l Probability of a Indirect Bandgap Direct Bandgap is low but if the valence electron is on an atom vibrating in a direction (I.e. has momentum) Probability of a “direct transition” from valence b d t that lowers the energy required, the probability increases! band to conduction band is high! ECE 4833 - Dr. Alan Doolittle Georgia Tech