Optimal design of CMOS pseudoactive pixel sensor (PAPS) structure for low-dark-current and large-array-size imager applications

Abstract

In this paper, a pixel structure called the optimal pseudoactive pixel sensor (OPAPS) is proposed and analyzed for the applications of CMOS imagers. The shared zero-biased-buffer in the pixel is used to suppress both dark current of photodiode and leakage current of pixel switches by keeping both biases of photodiode and parasitic pn junctions in the pixel bus at zero voltage or near zero voltage. The factor of photocurrent-to-dark-current ratio per pixel area (PDRPA) is defined to characterize the performance of the OPAPS structure. It is found that a zero-biased-buffer shared by four pixels can achieve the highest PDRPA. In addition, the column sampling circuits and output correlated double sampling circuits are also used to suppress fixed-pattern noise, clock feedthrough noise, and channel charge injection. An experimental chip of the proposed OPAPS CMOS imager with the format of 352 288 (CIF) has been designed and fabricated by using 0.25-m single-poly-five-level-metal (1P5M) -well CMOS process. In the fabricated CMOS imager, one shared zero-biased-buffer is used for four pixels where the PDRPA is equal to 47.29 m 2 . The fabricated OPAPS CMOS imager has a pixel size of 8.2 8.2 m, fill factor of 42%, and chip size of 3630 3390 m. Moreover, the measured maximum frame rate is 30 frames/s and the dark current is 82 pA cm 2 . Additionally, the measured optical dynamic range is 65 dB. It is found that the proposed OPAPS structure has lower dark current and higher optical dynamic range as compared with the active pixel sensor (APS) and the conventional passive pixel sensor (PPS). Thus, the proposed OPAPS structure has high potential for the applications of high-quality and large-array-size CMOS imagers.Index Terms-CMOS imagers, dark current, optimal pseudoactive pixel sensor (OPAPS), photodiode, pn junctions.

A Zero Bias Pixel Sensor and its Zero-Bias Column Buffer-Direct-Injection Circuit

“…8. Its fill factor is 59% which is comparable of PAPS in [26] (with pixel size almost 4 times bigger) but larger than 42% of OPAPS in [27].…”

“…Although each leakage current is only a tiny fraction of the photocurrent, its effect becomes pronounced when there is a large number of pixels connected to a column. In [27], it is suggested that one ZCBDI should be shared with only 4 PAPS pixels, called OPAPS. This results is using more than one ZCBDI per column and effectively reduces the fill factor down to 42% for a 352x288 pixel array.…”

“…In particular, 2 types of pixel structure are reviewed. They are active pixel sensor (APS) [6,[13][14][15][16][17][18][19][20][21][22][23][24][25] in Section 2 and pseudo-active pixel sensor (PAPS) [26,27] in Section 3. Then a new sensor called zero bias pixel sensor (ZBPS), that can eliminate dark current, is proposed in Section 4.…”