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12 Modern Materials Solutions to Exercises 12.65 Analyze/Plan. Follow the logic in Sample Exercise 12.3. Solve. (a) CdS. Both semiconductors contain Cd; S and Te are in the same family, and S is higher. (b) GaN. Ga is in the same family and higher than In; N is in the same family and higher than P. (c) GaAs. Both semiconductors contain As; Ga and In are in the same family and Ga is higher. 12.66 (a) InP. P is in the same family and higher than As. (b) AIP. Al and P are horizontally separated, resulting in greater bond polarity, and Al and P are in the row above Ge. Band gap values in Table 12.4 confirm this order. (c) AgI. The four elements are in the same row; Ag and I are farther apart than Cd and Te. 12.67 Analyze. Given: GaAs. Find: dopant to make n-type semiconductor. Plan. An n-type semiconductor has extra negative charges. If the dopant replaces a few Ga atoms, it should have more valence electrons than Ga, Group 3A. Solve. The obvious choice is a Group 4A element, either Ge or Si. Ge would be closer to Ga in bonding atomic radius (Figure 7.6). 12.68 p-type semiconductors have a slight electron deficit. If the dopant replaces As, Group 5A, it should have fewer than five valence electrons. The dopant will be a 4A element, probably Si or Ge. Si would be closer to As in bonding atomic radius. 12.69 (a) Analyze. Given: 1.1 eV. Find: wavelength in meters that corresponds to the energy 1.1 eV. Plan. Use dimensional analysis to find wavelength. Solve. 1 eV = 1.602 (inside-back cover of text); = hc/E = 6.626 10⁻³⁴ J 3.00 m 1 1eV = 1.128 10⁻⁶ = 1.1 10⁻⁶ m (b) Si can absorb energies ≥ 1.1 eV, or wavelengths 1.1 10⁻⁶ m. The range of wavelengths in the solar spectrum at sea level is 3 10⁻⁶ to 2 m, or 30 to 2 10⁻⁷ m, a span of 28 m. Si can absorb 11 10⁻⁷ to 2 m, a span of 9 m. This represents 28 9 10⁻⁷ 10⁻⁷ = 32% of the wavelengths in the solar spectrum. According to the diagram, these wavelengths represent much more than 32% of the total flux. Silicon does absorb a significant "portion" of the visible light that comes from the sun, whether that "portion" is defined by wavelength range or flux. 12.70 (a) From Table 12.4, the band gap of CdTe is 1.50 eV (or 145kJ/mol). 350