We have extended our previous investigation of the electrical characteristics of a Pd/AlN/Si thin film sensor for varying thicknesses of AlN, from 300–2000Å. The capacitance vs. voltage, C(V), and conductance vs. voltage, G(V), measurements were utilized to investigate the presence of surface states within the Si gap at the AlN/Si interface. Our previous experiments on 500Å AlN did show the presence of interface traps, with an estimated surface density between 8×1014 and 1.5×1015 m−2eV−1 . In our present work we've examined the effect of AlN thickness on the density of these interface traps. The density is dependent on AlN thickness. The thinner devices, 300Å, showed an interface trap density of 20–30×1015 m−2eV−1. The interface trap density decreased with increasing thickness up to 500Å, where the density remained relatively constant at about 1–5×1015 m−2eV−1 for thicknesses up to 2000Å. We have also shown that the interface trap density is independent of annealing.