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# T curve of such a semiconductor will have three

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Unformatted text preview: Chapter 18.pdf (65/73) 1. Consider a boron doped (p type) semiconductor. It is well known that the conductivity vs. T curve of such a semiconductor will have three regions; freeze out region (low T), the saturation region (intermediate T) and intrinsic region (high T). Estimate the temperature at which the saturation zone ends and the intrinsic zone begins given that the concentration of boron is 1020 /m3, Eacceptor is 0.01 eV/atom, Egap is 1.6 eV/atom, the number of electrons in the valance band at 0 K is 1030/m3, the mobility of electrons is 0.14m2 /V s and that of holes is 0.05m2 /V s . a) 273 K b) 328 K c) 380 K d) 401 K e) 280 K 2013-02-15 10:47:36 66/73 Chapter 18.pdf (66/73) To solve this question, equate the conductivity in the saturation region to the conductivity in the intrinsic region : p|e| h , where E a n|e|( e h) 10 20 exp Ea |e| kT h 1030 exp Eg 2kT |e|( h e ) 0.01 eV/atom and E g 1.6 eV/atom Solve for T to get 380 K or c) 2013-02-15 10:47:37 67/73 Chapter 18.pdf (67/73) 2. A one meter long stainless steel wire has electrical conductivity of 0.200x107 (ohm m) 1, and end to end resistance of 10.0 ohms. What is the diameter of the wire? a) 0.126 mm b) 0.252 mm c) 0.396 mm d) 0.505 mm e) 0.936 mm 2013-02-15 10:47:37 68/73 Chapter 18.pdf (68/73) First, solve for the radius using the given data. 1 Assume and A r 2 . Then : r RA l l R (0.2 107 ( 1 m) 1 ) 10 0.126mm Therefore, the diameter is twice the radius : d d 2013-02-15 10:47:37 2 r 2 0.126mm 0.252mm or b) 69/73 Chapter 18.pdf (69/73) 3. According to Matthiessen’s rule the conductivity of a metal decreases with: (i) Increasing temperature (ii) increasing impurity content (iii) increasing number of dislocations a) i only b) ii only c) i and ii only d) i, ii and iii 2013-02-15 10:47:37 70/73 Chapter 18.pdf (70/73) 2013-02-15 10:47:37 71/73 Chapter 18.pdf (71/73) 2013-02-15 10:47:38 72/73 Chapter 18.pdf (72/73) Since Selenium is two columns to the right of Silicon we know two important indications from this : 1) It is a N - Type Extrinsic Semiconductor 2) Each atom of Selenium donates two electrons. Therefore, the number of atoms per m 3 must be multiplied by two in order to determine the number of donated electrons. 2 p|e| h 2 10 20 atoms / m 3 | e | 0.14m 2 /(V s) 4.48 ( - m)-1 or b) 2013-02-15 10:47:38 73/73 Chapter 18.pdf (73/73)...
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