Hydrogen gas sensors employ an epitaxial layer of the thermodynamically stable form of aluminum nitride (AlN) as the "insulator" in an MIS structure having a thin metal gate electrode suitable for catalytic dissociate of hydrogen, such as palladium, on a semiconductor substrate. The AlN is deposited by a low temperature technique known as Plasma Source Molecular Beam Epitaxy (PSMBE). When silicon (Si) is used the semiconducting substrate, the electrical behavior of the device is that of a normal nonlinear MIS capacitor. When a silicon carbide (SiC) is used, the electrical behavior of the device is that of a rectifying diode. Preferred structures are Pd/AlN/Si and Pd/AlN/SiC wherein the SiC is preferably 6H-SiC.