The spectral performance as a function of the dielectric layer thickness for several CdZnTe Frisch collar devices was investigated. Seven different planar bar shaped detectors were fabricated from Redlen Technologies CdZnTe, and many Frisch collar devices were prepared from each planar device. The optimum dielectric layer thickness was experimentally determined for each device. The result of the optimal thickness study was verified through three-dimensional geometry modeling of the potential and electric field. It is shown that there exists an optimal dielectric layer thickness for best performance for CdZnTe Frisch collar devices with aspect ratios (L/W) greater than 2.5.