The electrical characteristics of a hydrogen sensor utilizing a metal/AlN/Si(111) structure, which behaves similarly to a metal-insulator-semiconductor-type capacitor, have been investigated. The AlN layer, which acts as the insulator, was deposited by plasma source molecular-beam epitaxy. The device employed alternating gates of Pd and Al, and an Al back contact, all grown by magnetron sputtering. The Pd gate structures were found to be sensitive to hydrogen, down to parts per million concentrations in the surrounding flow, while the Al gate structures showed no response to hydrogen. Both structures show similar capacitance versus voltage as well as conductance versus voltage curves. Therefore the combination of Al and Pd gates can be utilized in a balanced hydrogen sensor structure. Furthermore, the frequency dependence of the capacitance and conductance curves clearly indicated the presence of series resistance effects, but with multiple time constants, requiring a complex equivalent circuit representation. Evidence was found for the presence of carrier traps at the AlN/Si interface, with energy levels distributed throughout the silicon band gap, and with an estimated surface density ranging between 8×1014 and 1.5×1015 m-2 eV-1. © 2003 American Institute of Physics.