《材料科学与工程基础》英文影印版习题及思考(18)

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mm (1 in.) length of crystal?

12.12 An n-type semiconductor is known to have an electron concentration of 3 _

1018 m_3. If the electron drift velocity is 100 m/s in an electric field of 500 V/m, calculate the conductivity of this material.

12.13 At room temperature the electrical conductivity and the electron mobility for

copper are 6.0 _ 107 (_-m)_1 and 0.0030 m2/V-s, respectively. (a) Compute the number of free electrons per cubic meter for copper at room temperature. (b) What is the number of free electrons per copper atom? Assume a density of 8.9 g/cm3.

12.21 (a) Compute the number of free electrons and holes that exist in intrinsic

germanium at room temperature, using the data in Table12.2.

(b) Now calculate the number of free electrons per atom for germanium and silicon (Example Problem 12.1).

(c) Explain the difference. You will need the densities for Ge and Si, which are

5.32 and 2.33 g/cm3, respectively.

12.22 For intrinsic semiconductors, both electron and hole concentrations depend on

temperature as follows:

or, taking natural logarithms,

Thus, a plot of the intrinsic ln n (or ln p) versus 1/T (K)-1 should be linear and

yield a slope of -Eg/2k. Using this information and Figure 12.16, determine the band gap energy for silicon. Compare this value with the one given in Table 12.2.