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98_exam2fall - YOUR NAME Department of Electrical...

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YOUR NAME________________________________ Department of Electrical Engineering and Computer Science Massachusetts Institute of Technology 6.012 Electronic Devices and Circuits Exam No. 2 Notes: 1. Unless otherwise indicated, assume room temperature and that kT/q is 0.025 V. You may also approximate [(kT/q) ln 10] as 0.06 V. 2. Open book; 6.012 text and any other notes permitted. 3. All of your answers and any relevant work must appear on these pages. Any additional paper you hand in will not be graded. 4. Make reasonable approximations and assumptions. State and justify any such assumptions and approximations you do make. 5. Be careful to include the correct units with your answers when appropriate. 6. Be certain that you have all eight (8) pages of this exam booklet and make certain that you write your name at the top of this page in the space provided. 6.012 Staff Use Only PROBLEM 1 (of 28 possible) PROBLEM 2 (of 36 possible) PROBLEM 3 (of 36 possible) TOTAL
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Page 2 of 8 Problem 1 (28points; 4 points each for Parts a, b, c, e, f; 8 points for d) The first several parts of this problem concern the photodiode illustrated to the left below. It is illuminated with light which generates M hole-electron pairs/cm 2 -s across the plane at x = 1 µm, as indicated in the figure. There contacts are ohmic, and the n-type region between x = 0 and 2 µm is doped to a level of 10 16 cm -3 . The p-region between x = - 1µm and 0 is doped to a level of 10 16 cm -3 . Throughout the device the minority carrier diffusion length is much greater than 2 µm, µ e is 1600 cm 2 /V-s, and µ h is 600 cm 2 /V-s; the cross-sectional area is A. The terminals are shorted, as shown. M p'(1µm) p'(x) I n p x (µm) x (µm) -1 0 1 2 -1 0 1 2 a) Plot the excess minority carrier profile in the diode on the axes provided above on the right. b) What is the current, I, measured in the external short circuit? I = c) In Exam #1 we looked at adding a 0.2 µm thick n+ region (N D = 10 18 cm -3 ) before the ohmic contact on one end of an n-type bar, as shown below to the left. Doing this greatly reduces the minority carrier flux to the right-hand ohmic contact.
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