We report a study of metal–insulator–semiconductor (MIS) structures on InP. The interfacial state density and deep levels existing in MIS structures were measured by deep level transient spectroscopy (DLTS) technique. The electrical insulator properties were measured by current–voltage techniques. MIS structures were fabricated on InP substrates by direct deposition of silicon nitride (SiNx:H) thin films by electron cyclotron resonance chemical vapor deposition. In this work, we show that interfacial state density can be diminished, without degrading electrical insulator properties, by fabricating MIS structures based on a dual layer insulator with different compositions and with different thickness. The effect of rapid thermal annealing treatment has been analyzed in detail in these samples. Interface state densities as low as 3×1011 cm-2 eV-1 were measured by DLTS in some structures. Conductance transients caused by disorder-induced gap states have been observed and analyzed providing some information about interface width. Finally, deep levels induced in the substrate have been investigated. Three deep levels at energies of 0.19, 0.24, and 0.45 eV measured from the conduction band have been found, and their dependence on the rapid thermal annealing process has been analyzed. © 1999 American Institute of Physics.