Quantum efficiency improvements in active pixel sensors

DETAILED DESCRIPTION The present invention describes a technique that improves quantum efficiency in a CMOS sensor.
This is done according to the present invention using different techniques.
A first technique describes improving the QE in a photodiode pixel by reducing, rather than increasing, the amount of photosensitive area in the pixel.
A second technique describes improving the QE response of a photogate structure.
An aspect of the present invention involves making the photodiode smaller, instead of larger as was suggested by the prior art.
This is done by using the inventor's understanding of a new technique of operation.
Photons are captured in the substrate of the photodiode and are converted to electron-hole pairs.
These electron-hole pairs diffuse into the diode to form current.
The diode size according to this embodiment is optimized for obtaining diffusion from the substrate, rather than for obtaining the light directly.
This requires that the diode be smaller than the overall open area in the substrate.
Another aspect of the present invention includes the special shapes for these photodiodes which reduce their capacitance and further increase their gain.
Another aspect of the present invention is using a thinned polysilicon layer overlying a photogate structure.
This thinned polysilicon layer avoids certain photon attenuation.