Semi-insulating CdZnTe radiation detectors from five leading crystal growers and universities were characterized by thermally stimulated conductivity (TSC), thermoelectric voltage spectroscopy (TEVS), dark conductivity, current- voltage, and variable temperature time-resolved and spatially-resolved photoluminescence (PL). By TEVS, which is an extension of the hot-probe method, all of the samples were found to have n-type electrical conductivities at room temperature and this implied that the dominant deep level is a donor level. The TSC and TEVS spectra showed that all of the samples had a dominant deep electron trap, a series of shallow electron traps, and a deep hole trap. Some of the samples showed large concentrations of shallow hole traps. A two level model of compensation is proposed which is consistent with the observed resistivities, electrical conductivities at room temperature, observed trap level energies, and observed trapping behavior. It consists of a dominant deep donor level compensating a smaller concentration of a hole acceptor level that may be shallow or deep. The model showed that the electrical conductivity type of the stably compensated materials at RT is determined by the dominant level of the compensation, which is a deep donor level for CdZnTe. Preliminary results from the variable temperature time-resolved and spatially-resolved PL showed that the emission from the traps dominate the photoluminescence spectra from these materials and that there is much spatial variation in the trap concentrations.