Question

(a) Name two properties of Si that would make it unsuitable for specific device applications and suggest an alternative semiconductor that can be used in each application. (b) You need to make a detector that is very sensitive to light with wavelength of 10 um. (i) In what part of the electromagnetic spectrum will this device need to operate ? (ii) Calculate the optimum energy gap for the detector material ? (iii) Explain why you would or would not use Si to make such a detector. (iv) Use the value of the energy gap calculated in (ii) above and calculate what the intrinsic carrier concentration of a material with such an energy gap would be at room temperature (300 K). As an approximation assume the same values of the constants Nc and Ny as you would use for Si. (v) Based on this value calculated for the intrinsic concentration in (iv) can you identify a potential problem in making these types of detectors ? Can you suggest a way in which this problem could be solved in a practical device? (iv) One option for making such a detector is to use a ternary compound semiconductor. A possible material is to produce a ternary compound (HgCd)Te from the binary compounds Hg Te (Eg = -0.3 eV) and CdTe (Eg = 1.7 eV). Explain how a ternary compound can be formed from these two binary compounds. If we assume that the energy gap will vary linearly between the binary endpoints, calculate the desired composition of the ternary alloy that will give the optimum bandgap for detection of light at 10 um wavelength.

Answer 1

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