In this paper, a new photodetector, bipolar junction photogate transistor (BJPG), is proposed for CMOS imagers. Due to an injection p^(+)n junction introduced, the photo-charges drift through the p^(+)n junction by the applied electronic field, and on the other hand, the p^(+)n junction injects the carriers into the channel to carry the photo-charges. Therefore this device can increase the readout rate of the pixel signal charges and the photoelectron transferring efficiency. Using this new device, a new type of logarithmic pixel circuit is obtained with a wide dynamic range which makes photo-detector more suitable for imaging the naturally illuminated scenes. The simulations show that the photo current density of BJPG increases logarithmically with the incident light power due to the introduced injection p+n junction. The noise characteristics ofBJPG are analyzed in detail and a new gate-induced noise is proposed. Based on the established numerical analytical model of noise, the power spectrum density curves are simulated.