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hw6_solutions - ECE 440 Homework VI Solutions Spring 2006...

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ECE 440 Homework VI Solutions Spring 2006 Due: Monday, February 20, 2006 1. (a) An n-type Si sample with N d = 1x10 17 /cm 3 is steadily illuminated such that g op = 3x10 21 EHP/cm 3 -s. If τ n = τ p =1 μs for the excitation, calculate the separation in the quasi-Fermi levels, (F n -F p ). Draw a band diagram such as Fig. 4-11. To find the excess carrier concentration we first need to check whether we can use the low level approximation. Since the inequality holds, we know . Using equations (3-25) we can calculate F n – E i and E i – F p .
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(b) At t=0, the illumination in part (a) is terminated. Assuming the carrier lifetimes remain the same, calculate the separation in the quasi-Fermi levels at t=4 μs. Draw a corresponding band diagram at 300 K. From homework #5, we know that the equations for the decay of excess carriers are . We can use these equations to calculate what the excess carrier concentrations are at t=4 μs. Now we just need to repeat part (a).
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