BCMD-An improved photosite structure for high-density image sensors

The author describes a new improved photosite structure and its use in a new image sensor that has been developed for applications requiring high-density pixel integration. The new photosite employs a buried-channel MOS transistor with a specially designed storage well located under the transistor channel in the silicon bulk. The photogenerated carriers accumulate in this well and modulate the transistor threshold. The resulting bulk charge modulated device (BCMD) has a high-sensitivity low-noise high-blooming overload capability, no detectable smear, and no image lag. The BMCD photocell has been integrated into an image-sensing array that has an 8-mm diagonal, 499 lines, and 687 pixels in each line. The individual cells are shaped into closely packed hexagons and are offset by a half pixel width between the neighbouring lines. The half pixel offset together with the dual-time readout capability gives the new sensor its superior resolution in color-sensing applications. The high light sensitivity throughout the entire visible spectrum is achieved by using a very thin polysilicon gate. The peripheral circuits needed for the array addressing and sensing functions are integrated on-chip using CMOS technology and require only the TTL driving pulses. The author further describes the basic steps of the sensor fabrication process and the results of testing and evaluation of completed units