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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 20130215 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 : pe h , where E a ne( 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) 20130215 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 20130215 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 20130215 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 20130215 10:47:37 70/73 Chapter 18.pdf (70/73) 20130215 10:47:37 71/73 Chapter 18.pdf (71/73) 20130215 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 pe h 2 10 20 atoms / m 3  e  0.14m 2 /(V s) 4.48 (  m)1 or b) 20130215 10:47:38 73/73 Chapter 18.pdf (73/73)...
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This note was uploaded on 03/24/2013 for the course ENGINEER 101 taught by Professor Preston during the Winter '13 term at McMaster University.
 Winter '13
 Preston

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