We present calculations for the effect of fixed charge density (Qss) due to passivation on responsivity (Rv), noise (Vn) and specific detectivity (D*) for the n-HgCdTe photoconductor (PC) overlap structure. The standard expressions of the PC overlap structure have been modified appropriately to include the effect of Qss, present near the HgCdTe passivant interface, on the important parameters of the detector, namely resistance, responsivity, noise, detectivity and minority carrier lifetime. Our objective in this paper is to evaluate the tolerable limits of Qss for given performance specifications. The symmetric and asymmetric PC overlap structures are studied and compared to standard non-overlap structures. Our results show that Rv versus Qss shows a peak in both symmetric and asymmetric overlap structures when Qss is in the range of (2–3) × 1011 cm−2, and this optimum Qss does not shift significantly when one moves from standard to overlap structures. Besides, this optimum Qss is almost independent of bulk minority carrier lifetime (τ). However, noise voltage versus Qss shows a somewhat flat region for low Qss, a sharply decreasing trend for high Qss and a peak in the intermediate region. Furthermore, there is a stronger dependence on Qss in overlap structures than in standard (non-overlap) structures. However, the parameter to be maximized, i.e. D*, shows that a value of Qss of 2 × 1011 cm−2 is optimum for both symmetric and asymmetric overlap structures, whereas a value of Qss in the range of 2 × 1011 to 1 × 1012 cm−2 is optimum for standard structures. Additionally, we study the dependence of Rv, Vn and D* on detector length (a parameter increased for obtaining overlap design) and frequency for various values of Qss in constant power mode. The predictions of the model fit reasonably well with the experimental data.