Cadmium telluride energy bands

The hosts that we have discussed to date have been ionic in nature, and dielectric as well (they are non-conductive). There is also another class of phosphor hosts which are covalent and semi-conductive in nature, namely the zinc and cadmium sulfides and/or selenides. The criterion for selection of a semi-conducting host for use as a phosphor includes choice of a composition with an energy band gap of at least 3.00 ev. This mandates the use of an optically inactive cation, combined with sulfide, selenide and possibly telluride. The ojq gen-dominated groupings such as phosphate, or silicate or arsenate, etc. are not semi-conductive in nature. And, none of the other transition metal sulfides have band gaps sufficiently... 

The monotonic change in mobility, the low effective mass of the electrons ( 0.05m ), and the linear change in the energy gap with the composition, enable us to suggest that the band structures of indium arsenide and its alloys with cadmium telluride in the region of indium arsenide are similar. 

Cadmium telluride is a direct-gap semiconductor with the smallest energy gap at the center of the BrUlouin zone (F). The topmost valence band is split owing to spin-orbit coupling into a fourfold (Fg) and a twofold (F7)... 

In quantum detectors, IR radiation causes electrons to be excited to a higher energy level. In an n-type semiconductor, electrons in the valence band are unable to increase the conductivity. If IR radiation excites the electrons to the conduction band, they can act as current carriers. For good sensitivity, it is necessary to cool the detector. For certain detectors, such as PbS and PbSe, it may be sufficient to use a thermoelectric cooler to maintain their temperature just below ambient. Mercury cadmium telluride detectors must usually be maintained at liquid-nitrogen temperature (77 K), whereas others may require cooling to the temperature of liquid helium (4.2 K). 

---