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CHAPTER 12
ELECTRICAL PROPERTIES
PROBLEM SOLUTIONS
Ohm's Law Electrical Conductivity
12.1 This problem calls for us to compute the electrical conductivity and resistance of a silicon specimen. (a) We use Equations 12.3 and 12.4 for the conductivity, as
= 1 Il = = VA Il d 2 V 2
=
(0.1 A)( 38 x 10-3 m) 5.1 x 10-3 m (12.5 V)() 2
2
= 14.9 ( - m) -1
(b) The resistance, R, may be computed using Equations 12.2 and 12.4, as
l A
R=
=
51 x 10-3 m 2 -3 m 14.9 ( - m ) -1 () 5.1 x 10 2
= 167.6
12.2 For this problem, given that an aluminum wire 10 m long must experience a voltage drop of less than 1.0 V when a current of 5 A passes through it, we are to compute the minimum diameter of the wire. Combining Equations 12.3 and 12.4 and solving for the cross-sectional area A leads to
Il V
2
A=
d From Table 12.1, for aluminum = 3.8 x 10 (-m) . Furthermore, inasmuch as A = 2
7 -1
for a cylindrical
wire, then
d 2
or
2
=
Il V
d =
4I l V
=
(4)(5 A)(10 m ) ()(1 V) 3.8 x 107 ( - m) -1
= 1.3 x 10-3 m = 1.3 mm
12.3 This problem asks that we compute, for a plain carbon steel wire 3 mm in diameter, the maximum length such that the resistance will not exceed 20 . From Table 12.1, for a plain carbon steel, = 0.6 x 107 (-m)1
. If d is the diameter then, combining Equations 18.2 and 18.4 leads to d l = RA = R 2
2
-3 2 0.6 x 107 ( - m ) -1 () 3 x 10 m = 848 m = (20 ) 2
12.4 Let us demonstrate, by appropriate substitution and algebraic manipulation, that Equation 12.5 may be made to take the form of Equation 12.1. Now, Equation 12.5 is just
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J = E
I . Also, the A
But, by definition, J is just the current density, the current per unit cross-sectional area, or J = electric field is defined by E =
V . And, substituting these expressions into Equation 12.5 leads to l I V = A l
But, from Equations 12.2 and 12.4 l RA
= and
l V I = A RA l
Solving for V from this expression gives V = IR, which is just Equation 12.1.
12.5 (a) In order to compute the resistance of this copper wire it is necessary to employ Equations 12.2 and 12.4. Solving for the resistance in terms of the conductivity, l l = = A A l d 2 2
-1
R=
From Table 12.1, the conductivity of copper is 6.0 x 10 (-m) , and l d 2 2 2 m -3 2 6.0 x 107 ( - m )-1 () 3 x 10 m 2 = 4.7 x 10-3
7
R=
=
(b) If V = 0.05 V then, from Equation 12.1
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I =
0.05 V V = = 10.6 A R 4.7 x 10-3
(c) The current density is just I = A I d 2
2
J =
=
10.6 A -3 m 3 x 10 2
2
= 1.5 x 106 A/m 2
(d) The electric field is just
E=
0.05 V V -2 = = 2.5 x 10 V/m 2 m l
Electronic and Ionic Conduction
12.6 When a current arises from a flow of electrons, the conduction is termed electronic; for ionic conduction, the current results from the net motion of charged ions.
Energy Band Structures in Solids
12.7 For an isolated atom, there exist discrete electron energy states (arranged into shells and subshells); each state may be occupied by, at most, two electrons, which must have opposite spins. On the other hand, an electron band structure is found for solid materials; within each band exist closely spaced yet discrete electron states, each of which may be occupied by, at most, two electrons, having opposite spins. The number of electron states in each band will equal the total number of corresponding states contributed by all of the atoms in the solid.
Conduction in Terms of Band and Atomic Bonding Models
12.8 This question asks that we explain the difference in electrical conductivity of metals, semiconductors, and insulators in terms of their electron energy band structures.
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For metallic materials, there are vacant electron energy states adjacent to the highest filled state; thus, very little energy is required to excite large numbers of electrons into conducting states. These electrons are those that participate in the conduction process, and, because there are so many of them, metals are good electrical conductors. There are no empty electron states adjacent to and above filled states for semiconductors and insulators, but rather, an energy band gap across which electrons must be excited in order to participate in the conduction process. Thermal excitation of electrons will occur, and the number of electrons excited will be less than for metals, and will depend on the band gap energy. For semiconductors, the band gap is narrower than for insulators; consequently, at a specific temperature more electrons will be excited for semiconductors, giving rise to higher conductivities.
Electron Mobility
12.9 The drift velocity of a free electron is the average electron velocity in the direction of the force imposed by an electric field. The mobility is the proportionality constant between the drift velocity and the electric field. It is also a measure of the frequency of scattering events (and is inversely proportional to the frequency of scattering).
12.10 (a) The drift velocity of electrons in Ge may be determined using Equation 12.7. Since the room temperature mobility of electrons is 0.38 m2/V-s (Table 12.2), and the electric field is 1000 V/m (as stipulated in the problem),
vd = e E = (0.38 m 2 /V - s)(1000 V/m) = 380 m/s
(b) The time, t, required to traverse a given length, l (= 25 mm), is just
l 25 x 10-3 m = = 6.6 x 10-5 s 380 m / s v
d
t =
12.11 (a) The number of free electrons per cubic meter for copper at room temperature may be computed using Equation 12.8 as
n =
|e |
e
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=
6.0 x 107 ( - m )-1
(1.602
x 10-19 C )( 0.0030 m 2 / V - s)
= 1.25 x 1029 m-3
(b) In order to calculate the number of free electrons per copper atom, we must first determine the number of copper atoms per cubic meter, NCu. From Equation 5.2 (and using the atomic weight of Cu found inside the front cover--viz 63.55 g/mol) NA A
Cu
N Cu =
=
( 6.023
x 1023 atoms / mol )( 8.9 g / cm 3 )(106 cm 3 / m 3 ) 63.55 g / mol = 8.43 x 1028 m-3
The number of free electrons per copper atom is just n/NCu n N
Cu
=
1.25 x 1029 m -3 8.43 x 1028 m-3
= 1.48
12.12 (a) This portion of the problem asks that we calculate, for silver, the number of free electrons per cubic meter (n) given that there are 1.3 free electrons per silver atom, that the electrical conductivity is 6.8 x 107 (' m)-1, and that the density (Ag ) is 10.5 g/cm3. (Note: in this discussion, the density of silver is represented by ' Ag in order to avoid confusion with resistivity which is designated by .) Since n = 1.3NAg, and NAg is defined
in Equation 5.2 (and using the atomic weight of Ag found inside the front cover--viz 107.87 g/mol), then
' Ag N A n = 1.3N Ag = 1.3 AAg
(10.5 g / cm 3 )( 6.023 x 1023 atoms / mol ) = 1.3 107.87 g / mol
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= 7.62 x 1022 cm-3 = 7.62 x 1028 m-3 (b) Now we are asked to compute the electron mobility, e. Using Equation 12.8
e =
6.8 x 107 ( - m )-1
n | e|
=
( 7.62
x 1028 m -3 )(1.602 x 10-19 C )
= 5.57 x 10-3 m 2 /V - s
Electrical Resistivity of Metals
12.13 We want to solve for the parameter A in Equation 12.11 using the data in Figure 12.36. From Equation 12.11
A=
c (1 - c )
i i
i
However, the data plotted in Figure 12.36 is the total resistivity, total, and includes both impurity (i) and thermal (t) contributions (Equation 12.9). The value of t is taken as the resistivity at ci = 0 in Figure 12.36, which has a value of 1.7 x 10-8 (-m); this must be subtracted out. Below are tabulated values of A determined at c = 0.10,
i
0.20, and 0.30, including other data that were used in the computations. ci 0.10 0.20 0.30 1 ci 0.90 0.80 0.70
total (-m)
3.9 x 10 5.3 x 10
-8 -8 -8
i (-m)
2.2 x 10 3.6 x 10
-8 -8 -8
A (-m)
2.44 x 10 7 2.25 x 10 7 2.12 x 10 7
6.15 x 10
4.45 x 10
So, there is a slight decrease of A with increasing c .
i
12.14 (a) Perhaps the easiest way to determine the values of 0 and a in Equation 12.10 for pure copper in Figure 12.8, is to set up two simultaneous equations using two resistivity values (labeled t1 and t2) taken at two corresponding temperatures (T1 and T2). Thus,
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t 1 = 0 + aT1 t 2 = 0 + aT2
And solving these equations simultaneously lead to the following expressions for a and 0:
a =
t1 - t 2 T - T
1 2
- t2 0 = t 1 - T1 t 1 T1 - T 2 - t2 = t 2 - T2 t1 T1 - T 2
From Figure 12.8, let us take T1 = 150C, T2 = 50C, which gives t1 = 0.6 x 10-8 (-m), and t2 = 1.25 x 10-8 (-m). Therefore
a =
t1 - t 2 T - T
1 2
=
[(0.6 x 10-8 ) - (1.25 x 10-8 )]( - m)
-150C - (- 50C)
6.5 x 10-11 (-m)/C
and
- t2 0 = t 1 - T1 t 1 T1 - T 2
= ( 0.6 x 10-8 ) - (-150)
[(0.6 x 10-8 ) - ( 1.25 x 10-8 )]( - m)
- 150C - ( -50C)
= 1.58 x 10-8 (-m)
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(b) For this part of the problem, we want to calculate A from Equation 12.11
i = Aci (1 - c i )
In Figure 12.8, curves are plotted for three ci values (0.0112, 0.0216, and 0.0332). Let us find A for each of these ci's by taking a total from each curve at some temperature (say 0C) and then subtracting out i for pure copper at this same temperature (which is 1.7 x 10-8 -m). Below is tabulated values of A determined from these three ci values, and other data that were used in the computations.
ci 0.0112 0.0216 0.0332
1 ci 0.989 0.978 0.967
total (-m)
3.0 x 10 4.2 x 10 5.5 x 10
-8 -8 -8
i (-m)
1.3 x 10 2.5 x 10 3.8 x 10
-8 -8 -8
A (-m)
1.17 x 10 6 1.18 x 10 6 1.18 x 10 6
The average of these three A values is 1.18 x 10-6 (-m). (c) We use the results of parts (a) and (b) to estimate the electrical resistivity of copper containing 1.75 at% Ni (ci = 0.0175) at100C. The total resistivity is just
total = t + i
Or incorporating the expressions for t and i from Equations 12.10 and 12.11, and the values of 0, a, and A determined above, leads to
total = ( 0 + aT ) + Aci (1 - ci )
=
{1.58 x 10 -8 ( - m) + [6.5 x 10 -11 ( - m) / C] (100C)} + {[1.18 x 10 -6 ( - m)] (0.0175)(1 - 0.0175)}
= 4.26 x 10-8 (-m)
12.15 We are asked to determine the electrical conductivity of a Cu-Ni alloy that has a tensile strength of 275 MPa. From Figure 8.16(a), the composition of an alloy having this tensile strength is about 8 wt% Ni. For this
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composition, the resistivity is about 14 x 10-8 -m (Figure 12.9). And since the conductivity is the reciprocal of the resistivity, Equation 12.4, we have 1 1 = = 7.1 x 106 ( - m) -1 14 x 10-8 - m
=
12.16 This problem asks for us to compute the room-temperature conductivity of a two-phase Cu-Sn alloy which composition is 89 wt% Cu-11 wt% Sn. It is first necessary for us to determine the volume fractions of the and phases, after which the resistivity (and subsequently, the conductivity) may be calculated using Equation 12.12. Weight fractions of the two phases are first calculated using the phase diagram information provided in the problem. We may represent a portion of the phase diagram near room temperature as follows:
Applying the lever rule to this situation C - C0 37 - 11 = = 0.703 37 - 0 C - C
W =
C - C 11 - 0 = = 0.297 W = 0 C - C 37 - 0
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We must now convert these mass fractions into volume fractions using the phase densities given in the problem statement. (Note: in the following expressions, density is represented by ' in order to avoid confusion with resistivity which is designated by .) Utilization of Equations 10.6a and 10.6b leads to W V = ' W W
+ ' '
0.703 = 8.94 g / cm 3 0.297 0.703 + 8.25 g / cm 3 8.94 g / cm 3
= 0.686
V = W
W ' W
+ ' '
0.297 = 8.25 g / cm 3 0.703 0.297 + 8.94 g / cm 3 8.25 g / cm 3
= 0.314
Now, using Equation 12.12
= V + V = (1.88 x 10 -8 - m)(0.686) + (5.32 x 10 -7 - m) (0.314) = 1.80 x 10-7 -m
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Finally, for the conductivity (Equation 12.4) 1 1 = = 5.56 x106 ( - m) -1 1.80 x 10-7 - m
=
12.17 We are asked to select which of several metals may be used for a 2 mm diameter wire to carry 10 A, and have a voltage drop less than 0.03 V per foot (300 mm). Using Equations 12.3 and 12.4, let us determine the minimum conductivity required, and then select from Table 12.1, those metals that have conductivities greater than this value. The minimum conductivity is just Il = VA Il d 2 V 2
=
=
(10 A)( 300 x 10-3 m ) 2 x 10-3 m (0.03 V) () 2
2
= 3.2 x 107 ( - m) -1
Thus, from Table 12.1, only aluminum, gold, copper, and silver are candidates.
Intrinsic Semiconduction
12.18 (a) For this part of the problem, we first read, from Figure 12.15, the number of free electrons (i.e., the intrinsic carrier concentration) at room temperature (298 K). These values are ni(Ge) = 5 x 1019 m-3 and ni(Si) = 3 x 1016 m-3. Now, the number of atoms per cubic meter for Ge and Si (N and N , respectively) may be determined
Si
Ge
' using Equation 5.2 which involves the densities ( Ge and ' ) and atomic weights (A and A ). (Note: here we Si Ge Si
use ' to represent density in order to avoid confusion with resistivity, which is designated by . Also, the atomic weights for Ge and Si, 72.59 and 28.09 g/mol, respectively, are found inside the front cover.) Therefore,
N Ge =
N A ' Ge A
Ge
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=
( 6.023
x 1023 atoms / mol )( 5.32 g / cm 3 )(106 cm 3 / m 3 ) 72.59 g / mol
= 4.4 x 1028 atoms/m3
Similarly, for Si N A 'Si A
Si
N Si =
=
( 6.023
x 1023 atoms / mol )( 2.33 g / cm 3 )(106 cm 3 / m 3 ) 28.09 g / mol
= 5.00 x 1028 atoms/m3 Finally, the ratio of the number of free electrons per atom is calculated by dividing ni by N. For Ge ni (Ge) N Ge 5 x 1019 electrons/ m 3 4.4 x 1028 atoms / m 3
=
1.1 x 10-9 electron/atom
And, for Si ni (Si) N Si 3 x 1016 electrons/ m 3 5.00 x 1028 atoms / m 3
=
= 6 x 10-13 electron/atom
(b) The difference is due to the magnitudes of the band gap energies (Table 12.2). The band gap energy at room temperature for Si (1.11 eV) is larger than for Ge (0.67 eV), and, consequently, the probability of excitation across the band gap for a valence electron is much smaller for Si.
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12.19 This problem asks that we make plots of ln ni versus reciprocal temperature for both Si and Ge, using the data presented in Figure 12.15, and then determine the band gap energy for each material realizing that the slope of the resulting line is equal to Eg/2k. Below is shown such a plot for Si.
The slope of the line drawn is equal to 6400 which leads to an Eg value of Eg = 2k (slope) = - 2( 8.62 x 10-5 eV / K)( - 6400 ) = 1.10 eV The value cited in Table 12.2 is 1.11 eV. Now for Ge, an analogous plot is shown below.
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The slope of this line segment is 4085, which leads to be band gap energy value of
Eg = - 2 k (slope) = - 2( 8.62 x 10-5 eV / K)( - 4085 ) = 0.70 eV
This value is in good agreement with the 0.67 eV cited in Table 12.2.
12.20 The factor 2 in Equation 12.35a takes into account the creation of two charge carriers (an electron and a hole) for each valence-band-to-conduction-band intrinsic excitation; both charge carriers may participate in the conduction process.
12.21 In this problem we are asked to compute the intrinsic carrier concentration for PbS at room temperature. Since the conductivity and both electron and hole mobilities are provided in the problem statement, all we need do is solve for n and p (i.e., ni) using Equation 12.15. Thus,
ni = | e| ( e +
h
)
=
25 ( - m) -1
(1.602 x 10-19 C )(0.06 + 0.02) m 2 / V - s
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= 1.95 x 1021 m-3
12.22 Yes, compound semiconductors can exhibit intrinsic behavior. They will be intrinsic even though they are composed of two different elements as long as the electrical behavior is not influenced by the presence of other elements.
12.23 This problem calls for us to decide for each of several pairs of semiconductors, which will have the smaller band gap energy and then cite a reason for the choice. (a) Germanium will have a smaller band gap energy than C (diamond) since Ge is lower in row IVA of the periodic table (Figure 2.6) than is C. In moving from top to bottom of the periodic table, Eg decreases. (b) Indium antimonide will have a smaller band gap energy than aluminum phosphide. Both of these semiconductors are III-V compounds, and the positions of both In and Sb are lower vertically in the periodic table (Figure 2.6) than Al and P. (c) Gallium arsenide will have a smaller band gap energy than zinc selenide. All four of these elements are in the same row of the periodic table, but Zn and Se are more widely separated horizontally than Ga and As; as the distance of separation increases, so does the band gap. (d) Cadmium telluride will have a smaller band gap energy than zinc selenide. Both are II-VI compounds, and Cd and Te are both lower vertically in the periodic table than Zn and Se. (e) Cadmium sulfide will have a smaller band gap energy than sodium chloride since Na and Cl are much more widely separated horizontally in the periodic table than are Cd and S.
Extrinsic Semiconduction
12.24 These semiconductor terms are defined in the Glossary. Examples are as follows: intrinsic--high purity (undoped) Si, GaAs, CdS, etc.; extrinsic--P-doped Ge, B-doped Si, S-doped GaP, etc.; compound--GaAs, InP, CdS, etc.; elemental--Ge and Si.
12.25 For this problem we are to determine the electrical conductivity of and n-type semiconductor, given that n = 3 x 1018 m-3 and the electron drift velocity is 100 m/s in an electric field of 500 V/m. The conductivity of this material may be computed using Equation 12.16. But before this is possible, it is necessary to calculate the value of e from Equation 12.7. Thus, the electron mobility is equal to
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v e = d E = 100 m / s 2 = 0.20 m / V - s 500 V / m
Thus, from Equation 12.16, the conductivity is = n| e | e = ( 3 x 1018 m -3 )(1.602 x 10-19 C )( 0.2 0 m 2 / V - s) = 0.0961 (-m)-1
12.26 The explanations called for are found in Section 12.11.
12.27 (a) No hole is generated by an electron excitation involving a donor impurity atom because the excitation comes from a level within the band gap, and thus, no missing electron is created within the normally filled valence band. (b) No free electron is generated by an electron excitation involving an acceptor impurity atom because the electron is excited from the valence band into the impurity level within the band gap; no free electron is introduced into the conduction band.
12.28 Nitrogen will act as a donor in Si. Since it (N) is from group VA of the periodic table (Figure 2.6), and an N atom has one more valence electron than an Si atom. Boron will act as an acceptor in Ge. Since it (B) is from group IIIA of the periodic table, a B atom has one less valence electron than a Ge atom. Sulfur will act as a donor in InSb. Since S is from group VIA of the periodic table, it will substitute for Sb; also, an S atom has one more valence electron than an Sb atom. Indium will act as a donor in CdS. Since In is from group IIIA of the periodic table, it will substitute for Cd; and, an In atom has one more valence electron than a Cd atom. Arsenic will act as an acceptor in ZnTe. Since As is from group VA of the periodic table, it will substitute for Te; furthermore, an As atom has one less valence electron than a Te atom.
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12.29 (a) In this problem, for a Si specimen, we are given values for p (1.0 x 1023 m-3) and [103 (m)-1], while values for h and e (0.05 and 0.14 m2/V-s, respectively) are found in Table 12.2. In order to solve for n we must use Equation 12.13, which, after rearrangement, leads to - p| e | h | e|
e
n =
=
103 ( - m) -1 - (1.0 x 1023 m -3 )(1.602 x 10-19 C )( 0.05 m 2 / V - s)
(1.602
x 10-19 C)( 0.14 m 2 / V - s)
= 8.9 x 1021 m-3 (b) This material is p-type extrinsic since p (1.0 x 1023 m-3) is greater than n (8.9 x 1021 m-3).
12.30 (a) This germanium material to which has been added 1024 m-3 As atoms is n-type since As is a donor in Ge. (Arsenic is from group VA of the periodic table--Ge is from group IVA.) (b) Since this material is n-type extrinsic, Equation 12.16 is valid. Furthermore, each As atom will donate a single electron, or the electron concentration is equal to the As concentration since all of the As atoms are ionized at room temperature; that is n = 1024 m-3, and, as given in the problem statement, = 0.1 m2/V-s. Thus
e
= n | e | e = (10 24 m -3 )(1.602 x 10 -19 C)( 0.1 m 2 /V - s) = 1.6 x 104 (-m)-1
12.31 In order to solve for the electron and hole mobilities for InP, we must write conductivity expressions for the two materials, of the form of Equation 12.13--i.e., = n | e | e + p | e | h
For the intrinsic material 2.5 x 10-6 ( - m) -1 = (3.0 x 1013 m -3 )(1.602 x 10-19 C) e
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+ ( 3 x 1013 m -3 )(1.602 x 10-19 C) h
which reduces to 0.52 = e + h
Whereas, for the extrinsic InP 3.6 x 10-5 ( - m) -1 = (4.5 x 1014 m -3 )(1.602 x 10-19 C) e + (2.0 x 1012 m -3 )(1.602 x 10-19 C) h
which may be simplified to 112.4 = 225 e + h
Thus, we have two independent expressions with two unknown mobilities. Upon solving these equations simultaneously, we get = 0.50 m2/V-s and = 0.02 m2/V-s.
e h
The Temperature Dependence of Carrier Concentration
12.32 In order to estimate the electrical conductivity of intrinsic silicon at 80C, we must employ Equation 12.15. However, before this is possible, it is necessary to determine values for ni, e, and h. According to Figure 12.15, at 80C (353 K), ni = 1.5 x 1018 m-3, whereas from the "<1020 m-3" curves of Figures 12.18a and 12.18b, at 80C (353 K), e = 0.11 m2/V-s and h = 0.035 m2/V-s (realizing that the mobility axes of these two plot are scaled logarithmically). Thus, the conductivity at 80C is
= n i | e |( e + h ) = ( 1.5 x 1018 m-3 )(1.602 x 10-19 C)( 0.11 m 2 /V - s + 0.035 m 2 / V - s) = 0.035 ( - m) -1
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12.33
This problem asks for us to assume that electron and hole mobilities for intrinsic Ge are
temperature-dependent, and proportional to T -3/2 for temperature in K. It first becomes necessary to solve for C in Equation 12.36 using the room-temperature (298 K) conductivity [2.2 (-m)-1] (Table 12.2). This is accomplished by taking natural logarithms of both sides of Equation 12.36 as Eg 3 ln T - 2 2 kT
ln = ln C -
and after rearranging and substitution of values for Eg (0.67 eV, Table 12.2), and the room-temperature conductivity, we get Eg 3 ln T + 2 kT 2
ln C = ln +
= ln (2.2) +
3 0.67 eV ln (298) + 2 (2)(8.62 x 10-5 eV / K )(298 K)
= 22.38
Now, again using Equation 12.36, we are able to compute the conductivity at 423 K (150C) Eg 3 ln T - 2 kT 2
ln = ln C -
= 22.38 -
0.67 eV 3 ln (423 K) - 2 (2)( 8.62 x 10-5 eV / K)(423 K)
= 4.12
which leads to = e4.12 = 61.4 (-m)-1.
12.34 This problem asks that we determine the temperature at which the electrical conductivity of intrinsic Ge is 40 (-m)-1, using Equation 12.36 and the results of Problem 12.33. First of all, taking logarithms of Equation 12.36
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ln = ln C -
Eg 3 ln T - 2 2 kT
And, from Problem 12.33 the value of ln C was determined to be 22.38. Using this and = 40 (-m)-1, the above equation takes the form ln 40 = 22.38 - 0.67 eV 3 ln T - 2 (2) (8.62 x 10-5 eV / K)(T )
In order to solve for T from the above expression it is necessary to use an equation solver. For some solvers, the following set of instructions may be used:
ln(40) = 22.38 1.5*ln(T) 0.67/(2*8.62*10^-5*T)
The resulting solution is T = 400, which value is the temperature in K; this corresponds to T(C) = 400 273 = 127C.
12.35 This problem asks that we estimate the temperature at which GaAs has an electrical conductivity of 3.7 x 10
-3
(-m)-1 assuming that the conductivity has a temperature dependence as shown in Equation 12.36. From
the room temperature (298 K) conductivity [10-6 (-m)-1] and band gap energy (1.42 eV) of Table 12.2 we determine the value of C (Equation 12.36) by taking natural logarithms of both sides of the equation, and after rearrangement as follows: Eg 3 ln T + 2 kT 2
ln C = ln +
= ln 10-6 ( - m )-1 +
[
]
3 1.42 eV ln (298 K) + 2 (2)(8.62 x 10-5 eV / K )(298 K)
= 22.37 Now we substitute this value into Equation 12.36 in order to determine the value of T for which = 3.7 x 10-3 (m)-1, thus Eg 3 ln T - 2 kT 2
ln = ln C -
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ln 3.7 x 10 -3 ( - m)-1 = 22.37 -
[
]
3 1.42 eV ln T - 2 (2)( 8.62 x 10-5 eV / K) (T)
This equation may be solved for T using an equation solver. For some solvers, the following set of instructions may be used:
ln(3.7*10^3) = 22.37 1.5*ln(T) 1.42/(2*8.62*10^5*T)
The resulting solution is T = 437; this value is the temperature in K which corresponds to T(C) = 437 K 273 = 164C.
12.36 This question asks that we compare and then explain the difference in temperature dependence of the electrical conductivity for metals and intrinsic semiconductors. For metals, the temperature dependence is described by Equation 12.10 (and converting from resistivity to conductivity using Equation 12.4), as 1 + aT
0
=
That is, the electrical conductivity decreases with increasing temperature. Alternatively, from Equation 12.8, the conductivity of metals is equal to = n| e | e As the temperature rises, n will remain virtually constant, whereas the mobility (e) will decrease, because the thermal scattering of free electrons will become more efficient. Since |e| is independent of temperature, the net result will be diminishment in the magnitude of .
For intrinsic semiconductors, the temperature-dependence of conductivity is just the opposite of that for metals--i.e, conductivity increases with rising temperature. describes the conductivity; i.e., = n | e | ( e + h ) = p | e |( e + h ) = n i | e |( e + h ) One explanation is as follows: Equation 12.15
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Both n and p increase dramatically with rising temperature (Figure 12.15), since more thermal energy becomes available for valence band-conduction band electron excitations. The magnitudes of e and h will diminish somewhat with increasing temperature (per the upper curves of Figures 12.18a and 12.18b), as a consequence of the thermal scattering of electrons and holes. However, this reduction of e and h will be overwhelmed by the increase in n and p, with the net result is that increases with temperature. An alternative explanation is as follows: for an intrinsic semiconductor the temperature dependence is represented by an equation of the form of Equation 12.36. This expression contains two terms that involve temperature--a preexponential one (in this case T -3/2) and the other in the exponential. With rising temperature the preexponential term decreases, while the exp (Eg/2kT) parameter increases. With regard to relative magnitudes, the exponential term increases much more rapidly than the preexponential one, such that the electrical conductivity of an intrinsic semiconductor increases with rising temperature.
Factors That Affect Carrier Mobility
12.37 This problems asks that we determine the room-temperature electrical conductivity of silicon that has been doped with 1023 m-3 of arsenic atoms. Inasmuch as As is a group VA element in the periodic table (Figure 2.6) it acts as a donor in silicon. Thus, this material is n-type extrinsic, and it is necessary to use Equation 12.16), with n = 1023 m-3 since at room temperature all of the As donor impurities are ionized. electron The mobility, from Figure 12.17 at an impurity concentration of 1023 m-3, is 0.065 m2/V-s. Therefore, the conductivity is equal to = n | e | e = (1023 m -3 )(1.6 x 10-19 C )( 0.065 m 2 / V - s) = 1040 ( - m) -1
12.38 Here we are asked to calculate the room-temperature electrical conductivity of silicon that has been doped with 2 x 1024 m-3 of boron atoms. Inasmuch as B is a group IIIA element in the periodic table (Figure 2.6) it acts as an acceptor in silicon. Thus, this material is p-type extrinsic, and it is necessary to use Equation 12.17, with p = 2 x 1024 m-3 since at room temperature all of the B acceptor impurities are ionized. The hole mobility, from Figure 12.17 at an impurity concentration of 2 x 1024 m-3, is 0.0065 m2/V-s. Therefore, the conductivity is equal to = p | e | e = ( 2 1024 m -3 )(1.6 10-19 C)( 0.0065 m 2 / V - s) = 2080 ( - m) -1
12.39 In this problem we are to estimate the electrical conductivity, at 75C, of silicon that has been doped with 1022 m-3 of phosphorous atoms. Inasmuch as P is a group VA element in the periodic table (Figure 2.6) it acts as a donor in silicon. Thus, this material is n-type extrinsic, and it is necessary to use Equation 12.16; n in this
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expression is 1022 m-3 since at 75C all of the P donor impurities are ionized. The electron mobility is determined using Figure 12.18a. From the 1022 m-3 impurity concentration curve and at 75C (348 K), e = 0.08 m2/V-s. Therefore, the conductivity is equal to = n | e | e = (1022 m-3 )(1.6 x 10-19 C)( 0.08 m 2 / V - s) = 128 ( - m) -1
12.40 In this problem we are to estimate the electrical conductivity, at 135C, of silicon that has been doped with 1024 m-3 of aluminum atoms. Inasmuch as Al is a group IIIA element in the periodic table (Figure 2.6) it acts as an acceptor in silicon. Thus, this material is p-type extrinsic, and it is necessary to use Equation 12.17; p in this expression is 1024 m-3 since at 135C all of the Al acceptor impurities are ionized. The hole mobility is determined using Figure 12.18b. From the 1024 m-3 impurity concentration curve and at 135C (408 K,) h = 0.007 m2/V-s. Therefore, the conductivity is equal to = p | e | h = (1024 m -3 )(1.6 10-19 C )( 0.007 m 2 / V - s) = 1120 ( - m) -1
The Hall Effect
12.41 (a) This portion of the problem calls for us to determine the electron mobility for some hypothetical metal using the Hall effect. This metal has an electrical resistivity of 3.3 x 10-8 (-m), while the specimen thickness is 15 mm, I = 25 A and B = 0.95 tesla; under these circumstances a Hall voltage of 2.4 x 10-7 V is
x z
measured. It is first necessary to convert resistivity to conductivity (Equation 12.4). Thus 1 1 = = 3.0 x 107 ( - m) -1 -8 ( - m ) 3.3 x 10
=
The electron mobility may be determined using Equation 12.20b; and upon incorporation of Equation 12.18, we have e = RH VH d I B
x z
=
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- 2.4 x 10-7 V (15 x 10-3 m) 3.0 x 107 ( - m )-1 = (25 A)(0.95 tesla) = 0.0045 m 2 /V - s
[
]
(b) Now we are to calculate the number of free electrons per cubic meter. From Equation 12.8 we have | e|
n =
e
=
3.0 x 107 ( - m) -1
(1.602
x 10-19 C)( 0.0045 m 2 / V - s) = 4.17 x 1028 m -3
12.42 In this problem we are asked to determine the magnetic field required to produce a Hall voltage of 3.5 x 10 V, given that = 1.2 x 107 (-m)-1, e = 0.0050 m2/V-s, Ix = 40 A, and d = 35 mm. Combining Equations 12.18 and 12.20b, and after solving for Bz, we get
VH d I
x e
-7
Bz =
=
-3.5 x 10-7 V 1.2 x 107 ( - m) -1 ( 35 x 10-3 m) (40 A) (0.0050 m 2 / V - s)
[
]
= 0.74 tesla
Semiconducting Devices
12.43 The explanations called for are found in Section 12.15.
12.44 The energy generated by the electron-hole annihilation reaction, Equation 12.21, is dissipated as heat.
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12.45 In an electronic circuit, a transistor may be used to (1) amplify an electrical signal, and (2) act as a switching device in computers.
12.46 The differences in operation and application for junction transistors and MOSFETs are described in Section 12.15.
Conduction in Ionic Materials
12.47 We are asked in this problem to determine the electrical conductivity for the nonstoichiometric Fe(1 - x)O, given x = 0.060 and that the hole mobility is 1.0 x 10-5 m2/V-s. It is first necessary to compute the number of vacancies per cubic meter for this material. For this determination let us use as our basis 10 unit cells. For the sodium chloride crystal structure there are four cations and four anions per unit cell. Thus, in ten unit cells of FeO there will normally be forty O2- and forty Fe2+ ions. However, when x = 0.06, (0.06)(40) = 2.4 of the Fe2+ sites will be vacant. (Furthermore, there will be 4.8 Fe3+ ions in these ten unit cells inasmuch as two Fe3+ ions are created for every vacancy). Therefore, each unit cell will, on the average contain 0.24 vacancies. Now, the number of vacancies per cubic meter is just the number of vacancies per unit cell divided by the unit cell volume; this volume is just the unit cell edge length (0.437 nm) cubed. Thus # vacancies m3 0.24 vacancies / unit cell
=
(0.437 10-9 m)3
= 2.88 x 1027 vacancies/m 3 Inasmuch as it is assumed that the vacancies are saturated, the number of holes (p) is also 2.88 x 1027 m-3. It is now possible, using Equation 12.17, to compute the electrical conductivity of this material as = p| e | h = (2.88 x 10 27 m -3 )( 1.602 x 10-19 C)( 1.0 x 10-5 m 2 /V - s) = 4600 ( - m)-1
12.48 For this problem, we are given, for NaCl, the activation energy (173,000 J/mol) and preexponential (4.0 x 10-4 m2/s) for the diffusion coefficient of Na+ and are asked to compute the mobility for a Na+ ion at 873 K.
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The mobility, Na+, may be computed using Equation 12.23; however, this expression also includes the diffusion coefficient DNa+, which is determined using Equation 6.8 as Q = D0 exp - d RT
D
Na +
173, 000 J / mol = ( 4.0 x 10-4 m 2 /s ) exp - (8.31 J / mol - K)(873 K) = 1.76 x 10-14 m 2 /s Now solving for Na+ yields
n
Na +
eD
= Na +
Na +
kT
=
(1 )(1.602 x 10-19 C / atom )(1.76 x 10-14 m 2 / s)
(1.38
x 10-23 J / atom - K ) (873 K)
= 2.34 x 10-13 m 2 /V - s (Note: the value of nNa+ is unity, since the valence for sodium is one.)
Capacitance
12.49 We want to compute the plate spacing of a parallel-plate capacitor as the dielectric constant is increased form 2.5 to 4.0, while maintaining the capacitance constant. Combining Equations 12.26 and 12.27 yields r 0 A
l
C=
Now, let us use the subscripts 1 and 2 to denote the initial and final states, respectively. Since C = C , then
1 2
r1 0 A
l
1
=
r 2 0 A
l
2
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And, solving for l2 r 2 l1
r1
l2 =
=
(4.0)(1 mm ) = 1.6 mm 2.5
12.50 This problem asks for us to ascertain which of the materials listed in Table 12.4 are candidates for a parallel-plate capacitor that has dimensions of 38 mm by 65 mm, a plate separation of 1.3 mm so as to have a minimum capacitance of 7 x 10-11 F, when an ac potential of 1000 V is applied at 1 MHz. Upon combining Equations 12.26 and 12.27 and solving for the dielectric constant r we get
lC A
0
r =
=
(1.3 x 10-3 m)(7 x 10-11 F ) ( 8.85 x 10-12 F / m)(38 x 10-3 m )(65 x 10-3 m)
= 4.16
Thus, the minimum value of r to achieve the desired capacitance is 4.16 at 1 MHz. Of those materials listed in the table, titanate ceramics, mica, steatite, soda-lime glass, porcelain, and phenol-formaldehyde are candidates.
12.51 In this problem we are given, for a parallel-plate capacitor, its area (3225 mm2), the plate separation (1 mm), and that a material having an r of 3.5 is positioned between the plates. (a) We are first asked to compute the capacitance. Combining Equations 12.26 and 12.27, and solving for
C yields
C =
r 0 A
l
=
(3.5) (8.85 x 10-12 F / m )( 3225 mm 2 )(1 m 2 / 106 mm 2 ) 10-3 m = 10-10 F = 100 pF
(b) Now we are asked to compute the electric field that must be applied in order that 2 x 10-8 C be stored on each plate. First we need to solve for V in Equation 12.24 as
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V =
Q 2 x 10-8 C = = 200 V C 10-10 F
The electric field E may now be determined using Equation 12.6; thus 200 V V = = 2.0 x 105 V/m -3 m l 10
E =
12.52 This explanation is found in Section 12.19.
Field Vectors and Polarization Types of Polarization
12.53 Shown below are the relative positions of Na+ and Cl- ions, without and with an electric field present.
Now,
d = r
Na +
+ r - = 0.102 nm + 0.181 nm = 0.283 nm Cl
and d = 0.05 d = (0.05)(0.283 nm) = 0.0142 nm = 1.42 x 10 -11 m
From Equation 12.28, the dipole moment, p, is just
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p = q d = (1.602 x 10-19 C)( 1.42 x 10-11 m) = 2.26 x 10-30 C-m
12.54 (a) In order to solve for the dielectric constant in this problem, we must employ Equation 12.32, in which the polarization and the electric field are given. Solving for r from this expression gives P + 1 E
0
r =
=
1.0 x 10-6 C / m 2
( 8.85
x 10-12 F / m)( 5 x 104 V / m)
+ 1
= 3.26
(b) The dielectric displacement may be determined using Equation 12.31, as D = 0E + P
=
( 8.85
x 10-12 F/m )( 5 x 104 V/m ) + 1.0 x 10 -6 C/m 2 = 1.44 x 10-6 C/m2
12.55 (a) We want to solve for the voltage when Q = 3.5 x 10 5.0. Combining Equations 12.24, 12.26, and 12.27 yields A A Q = = r 0 l l V
-11
C, A = 160 mm , l = 3.5 mm, and r =
2
C =
Or A Q = r 0 l V
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And, solving for V, and incorporating values provided in the problem statement, leads to Ql A
r 0
V =
=
( 3.5
x 10-11 C)( 3.5 x 10-3 m)
(5.0) (8.85 x 10-12 F / m )(160 mm 2 )(1 m 2 / 106 mm 2 )
= 17.3 V
(b) For this same capacitor, if a vacuum is used Ql A
0
V =
=
( 3.5 ( 8.85
x 10-11 C)( 3.5 x 10-3 m)
x 10-12 F / m)(160 x 10-6 m 2 )
= 86.5 V
(c) The capacitance for part (a) is just 3.5 x 10-11 C Q = = 2.0 x 10 -12 F 17.3 V V
C =
While for part (b) 3.5 x 10-11 C Q = = 4.0 x 10 -13 F 86.5 V V
C =
(d) The dielectric displacement may be computed by combining Equations 12.31, 12.32 and 12.6, as V
0 r
D = 0 E + P = 0 E + 0 ( r - 1)E = 0 r E =
l
And incorporating values for r and l provided in the problem statement, as well as the value of V computed in part (a)
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D=
( 8.85 x 10-12 F / m) (5.0)(17.3 V)
3.5 x 10-3 m
= 2.2 x 10-7 C/m2
(e) The polarization is determined using Equations 12.32 and 12.6 as V P = 0 ( r - 1)E = 0 ( r - 1) l
=
( 8.85 x 10-12 F / m) (5.0 - 1)(17.3 V)
3.5 x 10-3 m
= 1.75 x 10-7 C/m2
12.56 (a) For electronic polarization, the electric field causes a net displacement of the center of the negatively charged electron cloud relative to the positive nucleus. With ionic polarization, the cations and anions are displaced in opposite directions as a result of the application of an electric field. Orientation polarization is found in substances that possess permanent dipole moments; these dipole moments become aligned in the direction of the electric field. (b) Only electronic polarization is to be found in gaseous argon; being an inert gas, its atoms will not be ionized nor possess permanent dipole moments. Both electronic and ionic polarizations will be found in solid LiF, since it is strongly ionic. In all probability, no permanent dipole moments will be found in this material. Both electronic and orientation polarizations are found in liquid H2O. The H2O molecules have permanent dipole moments that are easily oriented in the liquid state. Only electronic polarization is to be found in solid Si; this material does not have molecules with permanent dipole moments, nor is it an ionic material.
12.57 (a) This portion of the problem asks that we compute the magnitude of the dipole moment associated with each unit cell of BaTiO3, which is illustrated in Figure 12.34. The dipole moment p is defined by Equation 12.28 as p = qd in which q is the magnitude of each dipole charge, and d is the distance of separation between the charges. Each Ti4+ ion has four units of charge associated with it, and thus q = (4)(1.602 x 10-19 C) = 6.41 x 10-19 C. Furthermore, d is the distance the Ti4+ ion has been displaced from the center of the unit cell, which is just 0.006 nm + 0.006 nm = 0.012 nm [Figure 12.34(b)]. Hence
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p = qd = (6.41 x 10-19 C)(0.012 x 10 -9 m) = 7.69 x 10-30 C-m
(b) Now it becomes necessary to compute the maximum polarization that is possible for this material. The maximum polarization will exist when the dipole moments of all unit cells are aligned in the same direction. Furthermore, it is computed by dividing the value of p by the volume of each unit cell, which is equal to the product of three unit cell edge lengths, as shown in Figure 12.34. Thus p V
P =
C
=
7.69 x 10-30 C - m
( 0.403
x 10-9 m )( 0.398 x 10-9 m )( 0.398 x 10-9 m)
= 0.121 C/m2
Frequency Dependence of the Dielectric Constant
12.58 For this soda-lime glass, in order to compute the fraction of the dielectric constant at low frequencies that is attributed to ionic polarization, we must determine the r within this low-frequency regime; such is tabulated in Table 12.4, and at 1 MHz its value is 6.9. Thus, this fraction is just r (low ) - r (high ) (low )
r
fraction =
=
6.9 - 2.3 = 0.67 6.9
Ferroelectricity
12.59 The ferroelectric behavior of BaTiO3 ceases above its ferroelectric Curie temperature because the unit cell transforms from tetragonal geometry to cubic; thus, the Ti4+ is situated at the center of the cubic unit cell, there is no charge separation, and no net dipole moment.
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DESIGN PROBLEMS
Electrical Resistivity of Metals
12.D1 This problem asks that we calculate the composition of a copper-nickel alloy that has a room temperature resistivity of 2.5 x 10-7 -m. The first thing to do is, using the 90 Cu-10 Ni resistivity data, determine the impurity contribution, and, from this result, calculate the constant A in Equation 12.11. Thus, total = 1.90 x 10-7 ( - m) = i + t
From Table 12.1, for pure copper, and using Equation 12.4 1 1 = = 1.67 x 10-8 ( - m) 7 ( - m) -1 6.0 x 10
t =
Thus, for the 90 Cu-10 Ni alloy i = total - t = 1.90 x 10-7 - 1.67 x 10-8 = 1.73 x 10-7 (-m)
In the problem statement, the impurity (i.e., nickel) concentration is expressed in weight percent. However, Equation 12.11 calls for concentration in atom fraction (i.e., atom percent divided by 100). Consequently,
conversion from weight percent to atom fraction is necessary. (Note: we now choose to denote the atom fraction of ' nickel as cNi , and the weight percents of Ni and Cu by CNi and CCu, respectively.) Using these notations, this conversion may be accomplished by using a modified form of Equation 5.6a as
c'
C' =
Ni
Ni
100
=
C Ni ACu C
Ni Cu
A
+C
Cu Ni
A
Here ANi and ACu denote the atomic weights of nickel and copper (which values are 58.69 and 63.55 g/mol, respectively). Thus
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c'
Ni
=
(10 wt %)( 63.55 g / mol ) (10 wt %)( 63.55 g / mol ) + (90 wt %)(58.69 g / mol)
= 0.107
Now, solving for A in Equation 12.11
A =
i c ' 1 - c ' Ni Ni
=
1.73 x 10-7 ( - m ) = 1.81 x 10-6 ( - m) (0.107 )(1 - 0.107 )
Now it is possible to compute the c ' determine i as
Ni
to give a room temperature resistivity of 2.5 x 10-7 -m. Again, we must
i = total - t = 2.5 x 10-7 - 1.67 x 10-8 = 2.33 x 10-7 ( - m)
If Equation 12.11 is expanded, then i = A c' - A c'
2
Ni
Ni
and, solving for c '
Ni
(using the quadratic equation solution)
c'
A =
A - 4 A i 2A
2
Ni
Again, from the above A = 1.81 x 10-6 (-m) i = 2.33 x 10-7 (-m) which leads to
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' cN i =
1.81 x 10-6
(1.81 x 10-6 ) 2 - (4)(1.81 x 10-6 )( 2.33 x 10-7 ) (2) (1.81 x 10-6 )
And, taking the negative root, c'
Ni
= 0.152
Or, in terms of atom percent, C' = 100c '
Ni
Ni
= (100)(0.152) = 15.2 at %
While the concentration of copper is
' CCu = 100 - C '
Ni
= 100 - 15.2 = 84.8 at%
Now, converting this composition to weight percent Ni, requires that we use Equation 5.7a as
CNi =
C ' ANi
Ni
x 100
' C ' ANi + CCu ACu Ni
=
(15.2 at%)( 58.69 g / mol ) x 100 (15.2 at %)(58.69 g / mol ) + (84.8 at%)(63.55 g / mol)
= 14.2 wt%
12.D2 This problem asks that we determine the electrical conductivity of an 85 wt% Cu-15 wt% Zn alloy at 100C using information contained in Figures 12.8 and 12.36. In order to solve this problem it is necessary to employ Equation 12.9 which is of the form total = t + i since it is assumed that the alloy is undeformed. Let us first determine the value of i at room temperature (25C) which value will be independent of temperature. From Figure 12.8, at 25C and for pure Cu, t(25) = 1.75 x 10-8
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-m. Now, since it is assumed that the curve in Figure 12.36 was generated also at room temperature, we may take as total(25) at 85 wt% Cu-15 wt% Zn which has a value of 4.7 x 10-8 -m. Thus i = total (25) - t (25) = 4.7 x 10-8 - m - 1.75 x 10-8 - m = 2.95 x 10-8 - m Finally, we may determine the resistivity at 100C, total(100), by taking the resistivity of pure Cu at 100C from Figure 12.8, which gives us t(100) = 0.90 x 10-8 -m. Therefore total (-100) = i + t (-100)
= 2.95 x 10-8 - m + 0.90 x 10-8 - m = 3.85 x 10-8 - m
And, using Equation 12.4 the conductivity is calculated as 1 1 = = 2.60 x 10 7 ( - m) -1 -8 - m 3.85 x 10
=
12.D3 To solve this problem, we want to consult Figures 8.16(b) and 12.9 in order to determine the Ni concentration ranges over which the yield strength is greater than 130 MPa (19,000 psi) and the conductivity exceeds 4.0 x 10 (-m) . From Figure 8.16(b), a Ni concentration greater than about 23 wt% is necessary for a yield strength in excess of 130 MPa. In Figure 12.9 is plotted the resistivity versus the Ni content. Since conductivity is the 1 -8 . According to reciprocal of resistivity, the resistivity must be less than 25 x 10 -m--i.e., 6 ( - m ) -1 4.0 x 10 the figure, this will be the case for Ni concentrations less than 17 wt%. Hence, it is not possible to prepare an alloy meeting the criteria; for the stipulated yield strength the required Ni content must be greater than 23 wt%, whereas for the required conductivity, less than 17 wt% Ni is necessary.
6 -1
Extrinsic Semiconduction Factors That Affect Carrier Mobility
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12.D4 First of all, those elements which, when added to silicon render it n-type, lie one group to the right of silicon in the periodic table; these include the group VA elements (Figure 2.6)--i.e., nitrogen, phosphorus, arsenic, and antimony. Since this material is extrinsic and n-type, n >> p, and the electrical conductivity is a function of the hole concentration according to Equation 12.16. Also, the number of free electrons is about equal to the number of donor impurities, Nd. That is n ~ Nd
From Equation 12.16, the conductivity is a function of both the electron concentration (n) and the electron mobility (e). Furthermore, the room-temperature electron mobility is dependent on impurity concentration (Figure 12.17). One way to solve this problem is to use an iterative approach--i.e., assume some donor impurity concentration (which will also equal the value of n), then determine a "calculated" electron mobility from Equation 12.16--i.e., n | e|
e =
and, finally, compare this mobility with the "measured" value from Figure 12.17, taken at the assumed n (i.e., Nd) value. Let us begin by assuming that Nd = 1022 m-3. Thus, the "calculated" mobility value is 200 ( - m) -1 = = 0.125 m 2 / V - s n | e | (1022 m -3 )(1.602 x 10-19 C )
e =
From Figure 12.17, at an impurity concentration of 1022 m-3 the "measured" electron mobility is 0.10 m2/V-s, which is slightly lower than the "calculated" value. For our next choice, let us assume a higher impurity concentration, say 1023 m-3. At this higher
concentration there will be a reduction of both "calculated" and "measured" electron mobilities. The "calculated" value is just 200 ( - m) -1 = = 0.0125 m 2 / V - s n | e | (1023 m -3 )(1.602 x 10-19 C)
e =
Whereas, Figure 12.17 yields a "measured" e of 0.05 m2/V-s, which is higher than the "calculated" value. Therefore, the correct impurity concentration will lie somewhere between 1022 and 1023 m-3 probably closer to the lower of these two values. At 1.3 x 1022 m-3, "measured" and "calculated" e values are about equal (0.095 m2/Vs).
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It next becomes necessary to calculate the concentration of donor impurities in atom percent. This computation first requires the determination of the number of silicon atoms per cubic meter, NSi, using Equation 5.2, which is as follows
N Si =
N A ' Si A
Si
=
( 6.023
x 1023 atoms / mol )( 2.33 g / cm 3 )(106 cm 3 / m 3 ) 28.09 g / mol
= 5 x 1028 m-3
(Note: in the above discussion, the density of silicon is represented by ' in order to avoid confusion with Si resistivity, which is designated by .) The concentration of donor impurities in atom percent (C' ) is just the ratio of Nd and (Nd + NSi) d multiplied by 100 as Nd N
d
' Cd =
+ N
x 100
Si
=
1.3 x 1022 m-3
(1.3 x 1022 m -3 ) + (5 x 1028 m -3 )
x 100 = 2.6 x 10-5 at%
Now, conversion to weight percent (Cd) is possible using Equation 5.7a as C ' Ad d C ' Ad + C ' ASi d Si
Cd =
x 100
where Ad and ASi are the atomic weights of the donor and silicon, respectively. Thus, the concentration in weight percent will depend on the particular donor type. For example, for nitrogen C' AN N C' AN + C ' ASi N Si
CN =
x 100
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=
(2.6 x 10-5 at% ) (14.01 g / mol) ( 2.6 x 10-5 at%) (14.01 g / mol ) + (99.999974 at%)( 28.09 g / mol )
= 1.3 x 10-5 wt%
x 100
Similar calculations may be carried out for the other possible donor impurities which yield CP = 2.87 x 10-5 wt% C As = 6.93 x 10-5 wt% CSb = 1.127 x 10-4 wt%
12.D5 This problem asks for us to determine the temperature at which boron is to be diffused into highpurity silicon in order to achieve a room-temperature electrical conductivity of 1000 (-m)-1 at a distance 0.2 m from the surface if the B concentration at the surface is maintained at 1.0 x 1025 m-3. It is first necessary for us to compute the hole concentration (since B is an acceptor in Si) at this 0.2 m location. From Equation 12.17, the conductivity is a function of both the hole concentration (p) and the hole mobility (h). Furthermore, the room-temperature hole mobility is dependent on impurity concentration (Figure 12.17). One way to solve this problem is to use an iterative approach--i.e., assume some boron concentration, NB (which will also equal the value of p), then determine a "calculated" hole mobility from Equation 12.17--i.e., p| e |
h =
and then compare this mobility with the "measured" value from Figure 12.17, taken at the assumed p (i.e., NB). Let us begin by assuming that NB = 1024 m-3. Thus, the "calculated" mobility value is 1000 ( - m) -1 = = 0.0062 m 2 / V - s p | e | (1024 m -3 )(1.602 x 10-19 C)
h =
From Figure 12.17, at an impurity concentration of 1024 m-3 the "measured" hole mobility is 0.01 m2/V-s, which is higher than the "calculated" value. For our next choice, let us assume a lower boron concentration, say 1023 m-3. At this lower concentration there will be an increase of both "calculated" and "measured" hole mobilities. The "calculated" value is just
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h =
1000 ( - m) -1 = = 0.062 m 2 / V - s p | e | (1023 m -3 )(1.602 x 10-19 C )
Whereas, Figure 12.17 yields a "measured" h of 0.034 m2/V-s, which is lower than the "calculated" value. Therefore, the correct impurity concentration will lie somewhere between 1023 and 1024 m-3. At 4.0 x 1023 m-3, "measured" and "calculated" values are about equal (0.016 m2/V-s).
With regard to diffusion, the problem is one involving the nonsteady-state diffusion of B into the Si, wherein we have to solve for temperature. Temperature is incorporated into the diffusion coefficient expression given in the problem. But we must first employ the solution to Fick's second law for constant surface composition boundary conditions, Equation 6.5; in this expression C0 is taken to be zero inasmuch as the problem stipulates that the initial boron concentration may be neglected. Thus, x Cx - C0 = 1 - erf 2 Dt Cs - C0 4.0 x 1023 m-3 - 0
x = 1 - erf 2 Dt 1.0 x 1025 m -3 - 0
which reduces to x 0.96 = erf 2 Dt x 2 Dt
In order to solve this expression for a value
of it is necessary to interpolate using data in Table 6.1. Thus
z 1.4 z 1.5
erf(z) 0.9523 0.9600 0.9661
0.9600 - 0.9523 z - 1.4 = 1.5 - 1.4 0.9661 - 0.9523
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From which, z = 1.4558; which is to say x 2 Dt
1.4558 =
Inasmuch as there are 3600 s/h (= t) and x = 0.2 m (= 2 x 10-7 m) the above equation becomes 2 x 10-7 m 2 (D)(3600 s)
1.4558 =
which, when solving for the value of D leads to 2 10-7 m s (2)(1.4558)
2
1 D= 3600
= 1.31 10-18 m 2 / s
Now, equating this value to the expression for D given in the problem gives 347, 000 J / mol D = 1.31 x 10-18 m 2 /s = (2.4 x 10-4 ) exp - (8.31 J / mol - K)(T )
To solve for T, let us take the natural logarithms of both sides of the above equation; this leads to ln (1.31 x 10-18 ) = ln ( 2.4 x 10-4 ) - 347, 000 8.31T
-41.176 = - 8.335 -
4.176 x 104 T
which yields a value for T of 1271 K (998C).
12.D6 This problem asks, for the nonstoichiometric Fe(1 - x)O, given the electrical conductivity [2000 (m)-1] and hole mobility (1.0 x 10-5 m2/V-s) that we determine the value of x. It is first necessary to compute the number of holes per unit volume (p) using Equation 12.17. Thus | e|
p =
h
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=
2000 ( - m) -1
(1.602 x 10-19 C)(1.0 x 10-5 m 2 / V - s)
= 1.25 x 1027 holes/m 3
Inasmuch as it is assumed that the acceptor states are saturated, the number of vacancies is also 1.25 x 1027 m-3. Next, it is possible to compute the number of vacancies per unit cell by taking the product of the number of vacancies per cubic meter times the volume of a unit cell. This volume is just the unit cell edge length (0.437 nm) cubed: # vacancies = (1.25 x 1027 m -3 )(0.437 x 10-9 m )3 = 0.10 unit cell
A unit cell for the sodium chloride structure contains the equivalence of four cations and four anions. Thus, if we take as a basis for this problem 10 unit cells, there will be one vacancy, 40 O2- ions, and 39 iron ions (since one of the iron sites is vacant). (It should also be noted that since two Fe3+ ions are created for each vacancy, that of the 39 iron ions, 37 of them are Fe2+ and 2 of them are Fe3+). In order to find the value of (1 x) in the chemical formula, we just take the ratio of the number of total Fe ions (39) and the number of total Fe ion sites (40). Thus 39 = 0.975 40
(1 - x ) =
Or the formula for this nonstoichiometric material is Fe0.975O.
Semiconductor Devices
12.D7 We are asked to compare silicon and gallium arsenide semiconductors relative to properties and applications. The following are the characteristics and applications for Si: (1) being an elemental semiconductor, it is cheaper to grow in single-crystalline form; (2) because of its electron band structure, it is best used in transistors; (3) electronic processes are relatively slow due to the low mobilities for electrons and holes (Table 12.2). For GaAs: (1) it is much more expensive to produce inasmuch as it is a compound semiconductor; (2) because of its electron band structure it is best used in light-emitting diodes and semiconducting lasers; (3) its band gap may be altered by alloying; (4) electronic processes are more rapid than in Si due to the greater mobilities for electrons and holes; (5) absorption of electromagnetic radiation is greater in GaAs, and therefore, thinner layers are required for solar cells.
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