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Unformatted text preview: ME 083 Spring 2001
Structure and Properties of Solids Practice Exam #4 April 25, 2001 E
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Problem 1: Semiconductors [((g‘ /
— . + + . _
(a) (b) Figure 1: pn junction diaode. Which of these two circuits will conduct electricity? Why? Of what engineering use is
such a diode? <4) FQPMBO 6/er W 76 (aw;— Hm
AfﬂoSS Yd/l/Cﬁbu / Egg/‘gaw'w (b) An intrinsic semiconductor has a conductivity of 111 (Q——m)‘l at 10 ”C and 172 (SI—m)"
at 17 "C. What is the band gape of this semicouductor? [0°C =£Q3L*/ ”2170;. £350 =a38667¢ __ £3. _; l /
C9=(90‘(3 9256?— Q/' = 6 4421—9877 =82€§6 far77: C "£34295 7:2. %(g2.= 03/5: ‘77—; / Eg=0886V (c) Suppose a piece of silicon is duped with phosphorous in a concentration of 1026 mi“. The
intrinsic carrier concentration of silicon at room temperature is 2 x 10"5 m‘3. Calculate the resulting hole concentration at room temperature. 2. 5) 1
Z 26 / ‘ x
_ A?— /x /0
P/5%sz>em~ae Liz—£6) 6 /
7:; A {/0 ) = Z} /0 —3 (c) GaAs (gallium arsenide) is a semiconducting material which can be made to convert elecn
tricity to light with a very high efﬁciency. Actually, it is a semiconducting laser Sketch
the band structure you would expect for GaAs assuming it is an intrinsic semiconductor.
Label all pertinent features. C‘EZJOW #3779763 014 Problem 2: Conductivity (a) A copper conductor is one mile long (1.62 x 105 em) and 0.3 cm in diameter. It carries a
current of 96 A when the potential difference between its ends is 440 V (DC). Calculate
the crosssectional area of aluminum wire which would conduct the same current under
the same conditions. The resistivity/6 of Aluminum is 6 x 10”“6 Q — cm _Ze ~I‘ . _ 1754?
4 x/ .Q /6.2>r/o ll (b) Pure germanium has a. room temperature conductivity of 2 (Sl—m)‘1, and an equal number
of negative and positive charge carriers with mobilities of 0.36 and 0.23 m2/ (V  sec),
respectively. What fraction of the conductivity is due to electrons? 61: m/e/Mci‘ P/(f/ﬂf, ME? WNQQ/
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C96 5 ”2/6/7697 (97: zW WP) 6%» 0,36
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C973?” (0.36 + 0:23) / 3%76 6 039/ 09,:75 7/70/55 Problem 3: Mechanical Properties Despite advances in the development of metallic alloys, over 95% of all metals used today are
iron and iron alloys. (a) A steel wire 10 inches in length with a diameter of 0.02 inches was pulled in tension with
the following results:
— yield point: 100 pounds
— elongation at yield: 0.11 inch — maximum load: 120 pounds Determine the wires (i) elastic modulus, (ii) yield strength, and (iii) ultimate tensile
strength. (L) (9:56 CC: 0'” 50,0” [email protected] ... M
7 /0 j Y 72: 0.03212
AME“ #Nwmgfo 75 M50,/ 6 '9‘ g: 6% g£?/[email protected]/WWW /o ,_ #2 my)
(#X‘) (9073* W=O.38/[email protected] (b) At temperatures greater than approximately half the melting point, metallic alloys exhibit
stress relaxation under load in a manner analogous to the low temperature viscoelastic
behavior of linear polymers. At 0.7 Tm a steel beam is stretched (elongated) by a tensile
stress of 50,000 psi, after which its length is held constant. If the time constant for stress
relaxation is 60 days, calculate the magnitude of the stress you would expect after 30 days
of exposure. IJ: (gage—(90.5? t /l 6:6004/ (90‘5‘30503/ 3004/
— goal
Qamﬁowe/ '5’ = agozxgqmm 4 =30/321Zg/4Fw300L (c) For small—scale deformations, engineering stress and strain are good approximations for
true stress and strain. Explain why. 3%: A}; (WW/Awavsﬂﬁa— 9/40 Wm
ﬁamw (Gasser/7w ma; 57.1% f f €520 g ...
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 Spring '04
 Zauscher

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