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 HgTe (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

(ü E = h 2 = 10 um - hc = 10x10-6 m А 6.63x10 10 X10 - 6 X10-34 3x108 Joule 21 T9-89 TO Joute o = -21 T9.89 xlo ev 1.6 X10-19 12.438 10 -2 ev E o. 124 ev (012 S has indiret band-gap of Ege to ter Eg> 0. 124 ev so Si will not able to absorb Photons of wave lingth loum. Hence, it cam not be used tor this purpose.

(iv) Carrier Concentretion is Intrinsic given by No No Expl Eg 2K8T T9 Na 2.8 x 10 cm-3 Nga +9 1:04 X10 cm Egz E ñ 0.12 4 ev KBT = 26 men = 26x10 ev at TE goo. .. m; = - 0.124 38 2.8 X1:04x103 12 exp 2x 26 X10-3 19 0.15 7 X10 cm-3 m: 1.57 x 1018 cm-3

(vi) Hg Te Te i : Eg - 0.3ev cd Te .. Eg 1.7 ev x x Eg + (1-x) Eg E =) sex (-0.3) + (1-x) 1.7 = 1-x) 1.7 = 0.124 -0.3x +1.7 - 1.78 = 0.124 . => - 24 = -1.576 ne 0.788 На Те (1-4) 0.212 - A cd Te So to get the desired band gap we have to mix with 0.788 (or 78.84.) of Hg Te 0.212 Cor 21-29]. ) of cd Te.