HgCdTe or mercury cadmium telluride (also cadmium mercury telluride, MCT or CMT) is an alloy of CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological importance after silicon and gallium(III) arsenide.
The amount of cadmium (Cd) in the alloy (the alloy composition) can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength. CdTe is a semiconductor with a bandgap of approximately 1.5 eV at room temperature.
HgTe is a semimetal, hence its bandgap energy is zero. Mixing these two substances allows one to obtain any bandgap between 0 and 1.5 eV.
HgCdTe is usually referred to as MerCad Telluride, MerCadTel, or simply MerCaT in the infrared sensors community.