Wide-band-gap semiconductor detectors are recently in spotlight for various applications because of their good performances, such as the high energy resolution, the compactness in array geometry, and the room temperature operation. The performance of these detectors, for example, CdZnTe, is mainly limited by the charge transport properties. Especially, the dispersive nature of trapping and detrapping process affects on the detector performance resulting in random fluctuations in the current flowing. Based on the spectroscopic measurement, in this study, a simple analytical model is developed to investigate the charge transport characteristics for planar semiconductor detectors, especially for CdZnTe of m-i-m (metal-intrinsic-metal) diode structure. The model can take the input variables of material properties, as well as the operation parameters, such as the applied bias voltage, the pulse shaping time, the incident direction and the energy of gamma-rays. The measured gamma spectra from CdZnTe for Co57 showed excellent agreement with the simulation results from our model, and the parameters governing detector performance were analyzed. We expect that this model will be very useful to understand the charge transport mechanism in the wide-band-gap semiconductor detectors, and to optimally design the detector geometry for various applications.