In this study, the charge collection efficiencies (CCEs) of a 7.8- × 7.8- × 15.6-mm3 CdZnTe Frisch collar detector and a 2.1- × 2.1- × 4.1-mm3 HgI2 Frisch collar detector were measured and compared. Two Frisch collar devices were designed and fabricated to have identical aspect ratios of 2.0 to maintain similar weighting potential distributions. Pulse-height spectra were acquired from both Frisch collar devices with a standard calibration gamma-ray source of 137Cs, and the results are presented. As known, the Frisch collar alters the weighting potential within the planar device and enhances the CCE distributions. Thus, the parameters affecting these distributions have great impact on the pulse-height spectrum. The device length and mobility-lifetime product have great impacts on CCE. Primarily, crystal (device) length L directly affects CCE because more charge carriers are trapped in longer devices with longer traveling distances. Alternatively, the better mobility-lifetime product of the charge carriers enhances CCE of the fabricated device. It is shown in this study that as a result of similarity in shape for both devices (equal aspect ratio), the weighting potential distributions resemble each other. However, as a result of the trapping effect (due to both length and ), the CCE profiles are not the same, and the CdZnTe detector shows more uniform response to gamma rays and, therefore, better spectroscopic performance (even with a longer device length), which is confirmed through CCE simulations. Finally, by applying the CCE model to the HgI2 Frisch collar device, the mobility-lifetime products e, h e, h of electrons and holes were estimated to be 0.0008 and 0.00003 cm2V-1 , respectively, for the HgI2 crystal.