n‐type doping of mercury cadmium telluride was achieved using trimethylindium as the dopant source. The layers, grown by the alloy growth technique, were doped to ∼5×1018cm−3. The donor concentration in these layers was found to exhibit a linear dependence on the dopant partial pressure over the carrier concentration range from 5×1016to 3×1018cm−3. Reasonably high electron mobility values were observed in these indium‐doped layers. Typically, layers with a Cd fractionx=0.23, doped to 3.5×1016cm−3, exhibited a mobility value of 7.5×104cm2/V s at 40 K. High electron mobility values, measured over the entire doping regime, suggest a high electrical activity of indium in these layers. The optically measured band edge in these indium‐doped layers was observed to shift to higher energy with increasing doping. The band‐edge energy values measured in 1×1017and 3×1018cm−3doped layers correspond tox=0.23 andx=0.3, respectively. This increase can be due to an increase in the Cd fraction, or to a Burstein–Moss shift of the band edge with doping.