MidtemPrelim_2008_Keyl - MIDTERM EXAM ENGRI 1110...

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MIDTERM EXAM Name__________________________ ENGRI 1110 NANOTECHNOLOGY Oct. 16, 2008 Your exam should have 7 pages. You will have 1.5 hours to complete the exam. If you need extra space, please write on the other side of the page. PUT YOUR NAME ON EACH PAGE! You will not be penalized if you do not evaluate equations to get a numerical answer. However make sure your work is written neatly. You may use a 4-function or scientific calculator, but no graphing calculators are allowed. Suggestions for how to structure your answers: Write simple and concise answers Some answers only require a single word or number Some answers only require a well labeled diagram Semiconductors 1. a) Rank the following from smallest in energy to largest in energy (3 pts): i) Energy difference between the highest energy level of the valence band and the lowest energy level of the conduction band of Si ii) Energy difference between two adjacent electron energy levels within the valence band of Si iii) Energy difference between the energy level of a donor impurity and the lowest energy level of the conduction band of Si Smallest to largest: ii < iii < i b) Assign a value in eV to the three energies above (3 pts) i = ~1.1eV ii = ~10 -23 eV iii = ~0.03ev (answers within a factor of two correct) 2.P-n Junction a) (6 pts) Draw a schematic of a p-n junction. Using the symbol “h” for holes, “e” for electrons. “-“ and “+” for the charges that are not mobile, show the location of holes, electrons and all other charges for the p-n junction in equilibrium Page 1 of 8
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MIDTERM EXAM Name__________________________ ENGRI 1110 NANOTECHNOLOGY Oct. 16, 2008 b) In a sketch you draw below, draw both: i) the electron energy levels for the p-n junction in equilibrium and ii) the electron energy levels for the p-n junction in the condition of reverse bias (8 pts total) Left is i) Right is ii) c) (5 pts) The color of light emitted by a light emitting diode will depend on the size of the band gap of the diode material. Explain why. LEDs function by radiative recombination of an electron and hole. The electron comes from the bottom of the conduction band, while the hole comes from the top of the valence band. Thus the light emitted has the energy of the band gap. The color of the light (if it’s in the visible spectrum) is a direct result of what the energy value is in eV or wavelength in nm. d) (3 pts) Give one example of a compound semiconductor used in light emitting diodes: _________ GaAs _____________ (many possibilities here) 3a). Germanium (Ge) is a semiconductor and being a Group IV element, behaves much like Si. It has a bandgap of 0.67 eV. I have an infrared-camera that is only sensitive to light less than 0.1 eV in energy. The camera is damaged if light greater than 0.8 eV is transmitted into it. I can put either a coating of Ge or a coating of Si on the outer entrance lense of this infrared camera so that the light passes through the coating before it reaches the camera. Which is a better choice, Ge or
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This note was uploaded on 03/07/2010 for the course MS&E 111 at Cornell University (Engineering School).

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MidtemPrelim_2008_Keyl - MIDTERM EXAM ENGRI 1110...

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