Bandgap cutoff wavelength

Prediction of the bandgaps, cutoff wavelengths, and operating temperatures by linear interpolation between the two components of an alloy is not accurate enough... 

Table 5. Variation of bandgap and cutoff wavelength of Hgi- Cd Te as a function of the concentration, x, of cadmium.

Operating wavelength of the detector should be as close to the cutoff wavelength (Ico = hc/Eg) as possible. This requirement is easiest to meet in three-compound semiconductor materials with continually adjustable bandgap, e.g., mercury cadmium telluride (Hgi- cCd cTe) [8], mercury zinc telluride (Hgi- cZn cTe) [69-71], lead tin telluride Pbi Sn Te [72, 73], and indium arsenide antimonide (Ini - cAS cSb) [74] which for x = 0 reduces to indium antimonide, InSb. 

Any chemical element whose characteristic bandgap corresponds to a cutoff wavelength slightly greater than the spectral content of our targets is suitable for our purposes. The elements Si and Ge work well for wavelengths up to 1.1 and 1.9 pm, respectively. Ge and Si can be used in a pure form to make PC devices, or doped and combined as diodes to make PV detectors. They are simple to make and use, but they each provide only one specific cutoff wavelength. 

Figure 5.2 Doped germanium and silicon detectors operating temperature, cutoff wavelength, and bandgap.

This formula is correct, but it is common to encounter inconsistencies when using it. Sometimes the only available value for the bandgap is at room temperature, but we might want the cutoff at the much lower operating temperature. In addition, real detectors do not have a perfectly sharp cutoff we normally use cutoff to mean the wavelength at which the spectral responsivity reaches 50% of its maximum value. 

---