Sony patent application US20150021461 "Image sensor and electronic device" by Toshiyuki Nishihara and Hirofumi Sumi proposes a buried channel TG and binary detector to reduce influence of TG surface traps in very low light image sensors:
"since a pixel signal is extremely weak when weak light is detected, it is desirable to reflect electrons generated from photoelectric conversion on intensity of the pixel signal while losing as few electrons as possible. In general, however, when electrons generated in a photodiode are transferred to a floating diffusion, carriers (electrons) are doped at an interface level generated due to a defect present on a gate oxide film interface (interface defect) of a transfer transistor."
"there is provided an image sensor including pixels each configured to include a transfer transistor configured as an embedded channel type MOS transistor and to output a pixel signal based on a charge transferred to a floating diffusion from a photodiode by the transfer transistor in an on state, and a determination unit configured to convert the output pixel signal to a digital value, then compare the converted digital value to a threshold value, and thereby make a binary determination on presence or absence of incidence of a photon on the pixel that has generated the pixel signal. Accordingly, the image sensor that makes a binary determination on presence or absence of incidence of a photon on a pixel exhibits the effect of reducing influence of an interface state on the transfer transistor."