We have studied the defect states and electronic transport mechanisms in the mismatched p-InP/i-InGaAs/n-InP photodiode grown by molecular beam epitaxy (MBE) using deep-level transient spectroscopy (DLTS) and current–voltage (I–V) measurements. Two defect states were observed in the p-InP layer, which could have originated from Zn diffusion, and another one detected in i-InGaAs acted as a dislocation defect. The total defect density of the device estimated by DLTS and space-charge limited current (SCLC) was about 4 × 1013, which was much lower than the device grown by other methods. I–V measurements pointed out that the leakage current in this photodiode was caused by the existence of defect states and the small conduction band offset. Therefore, to suppress the leakage current in the devices, it is urgent to reduce the trap density and optimize the thickness of the p-InP layer. The study of defect state and electronic transport mechanism in p-InP/i-InGaAs/n-InP PIN diodes could help to develop a strategy to improve the SWIR sensing technology in the future.