Traditional GaN-based metal-semiconductor-metal (MSM) photodetector (PD) features a symmetric structure, and thus a poor lateral carrier transport can be encountered, which can decrease the photocurrent and responsivity. To improve its photoelectric performance, we propose GaN-based MSM photodetectors with an AlGaN polarization layer structure on the GaN absorption layer. By using the AlGaN polarization layer, the electric field in the metal/GaN Schottky junction can be replaced by the electric fields in the metal/AlGaN Schottky junction and the AlGaN/GaN heterojunction. The increased polarization electric field can enhance the transport for the photogenerated carriers. More importantly, such polarization electric field cannot be easily screened by free carriers, thus showing the detectability for the even stronger illumination intensity. Moreover, we also conduct in-depth parametric investigations into the impact of different designs on the photocurrent and the responsivity. Hence, device physics regarding such proposed MSM PDs has been summarized.