A multiple-gate CCD-photodiode sensor element for imaging arrays

The addition of barrier and storage gates between the photodiodes and transfer gates of a CCD imaging array provides improved and versatile operating characteristics compared with present silicon scanners. An experimental scanner, made with multiple-gate photoelements, has new functional capabilities which result from the on-chip structure. These include exposure control in real-time which can compensate for temporal illumination variations during the integration cycle, linearization of the output signal with respect to light intensity, and adaptive level setting to normalize the output based on the whitest portion of the image. Complete charge transfer into the shift register is achieved at reduced shift-register voltages. The use of photodiodes and the absence of polysilicon in the photosensitive region improve spectral response and overall sensitivity. A sixteen element 4-phase, 2-level polysilicon CCD imager was designed, fabricated, and used to test the improved photoelement structure.