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314-SampleProblems-F201000009

# 314-SampleProblems-F201000009 - cm The dopant is a group V...

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17. Thin film amorphous silicon-hydrogen alloy (a-Si:H) finds many practical applications including solar cells, sw itching transistors for flat panel displays and photoconductive coatings for xerography. It has a bandgap of 1.5 eV. Each incident photon w ith energy exceeding the bandgap w ill give rise to an electron-hole pair, w hich w hen flow ing in the external circuit constitutes photocurrent. (a) What is the threshold w avelength for absorption in a-Si:H? In w hich part of the electromagnetic spectrum does it lie? (b) If an a-Si:H solar cell is illuminated w ith monochromatic radiation from a 620 nm laser, can the cell generate a photocurrent? (c) What is the maximum photocurrent that can be generated by a cell of thickness 1 : m if the laser pow er is 100 mW. Given the absorption coefficient " = 2 x 10 cm 4 -1 at 8 = 620 nm. i 18. A sample of intrinsic silicon w ith n = 1.45 x 10 cm at T = 300 K is doped using ion 10 -3 implantation to a level of 10
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Unformatted text preview: cm . The dopant is a group V element that is an effective 15-3 dopant at room temperature. Before doping the electron and hole mobilities w ere found to be 1,800 cm /V-sec and 500 cm /V-sec respectively. After doping, these values reduce to 2 2 1,200 cm /V-sec and 200 cm /V-sec respectively. 2 2 G c For Si, E = 1.11 eV and N = 2.8 x 10 cm 19-3 v and N = 1.04 x 10 cm . 19 -3 (a) Does the Si become n-type or p-type after doping? (b) Why do the carrier mobilities decrease after doping? (c) Calculate the electron and hole concentrations at 300 K. doped (d) Calculate the conductivity F of the sample after doping. (e) Which carrier is the minority carrier and w hy is it called so? doped undoped (f) What is the ratio of F / F at 300 K. (g) Compute how far and tow ards w hich band the Fermi level has moved due to the doping. (h) What attributes of semiconductors does this problem point out?...
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