Aluminum nitride insulating layers have been grown at room temperature with a film resistivity of 3.3×1016 cm on mercury cadmium telluride substrates. Insulator breakdown fields of 640 MV/m were reached. Capacitance-voltage measurements with Al/AlN/Hg0.76Cd0.24Te metalinsulator-semiconductor devices demonstrate band bending at the semiconductor surface indicating that damage to the substrate was minimized during film growth. A fixed interface charge density (given here as total charge per unit area divided by the electron charge=Qss/q) of +2×1011 cm-2 and a slow interface state density of 4×1010 cm-2 were measured. The procedures for achieving these high quality insulating layers are reported. Frequency dependent dielectric constant and dielectric loss tangent measurements, carried out at room temperature and 100 K, are also presented.