We perform THz generation by optical rectification in ZnTe, KTP and RKTP with femtosecond laser pulses whose wavelength ranges from UV to IR. We observe a strong THz emission when the pump photon energy reaches the crystal bandgap. This phenomenon is explained by the resonance of material nonlinear susceptibility around its bandgap. Additionally, for the first time, we report of a two-photon enhanced nonlinearity in KTP/RKTP, which leads to an enhanced THz emission near half the bandgap.