The tunable, coherent terahertz wave generation in a variety of isotropic semiconductor nonlinear crystals, such as Ⅲ-Ⅴ and Ⅱ-Ⅳ compounds, by difference frequency mixing is studied theoretically based on the theory of the cross-Reststrahlen band dispersion compensation phase-matching. An angle-tuned, dual-wavelength KTP-OPO operating near the degenerate point is used as the pump source for THz wave generation. The theoretical analysis of phase-matching conditions and coherence length for different materials mentioned above is presented according to their optical dispersion properties, and their gain characteristic and nonlinear figure of merit are also described and calculated. Through the theoretical analyses and calculations, it is shown that ZnTe is a promising DFG crystal for THz wave generation under the collinear phase-matching condition, while InP is the one under the noncollinear phase-matching condition.