Temporal noise analysis and optimizing techniques for 4-T pinned photodiode active pixel sensor

Temporal noise sources in 4-T pinned photodiode (PPD) active-pixel-sensor (APS) are analyzed basing on a typical APS pixel structure working during reset, integration and readout operation. Complete reset operation and incomplete reset operation result in different reset noise power. Incomplete reset can realize lower noise power but cause image lag. To cover the problem a “pre-reset” technique for 4-T PPD APS is proposed. Analysis also indicates that, in most situations, the integration noise dominates the reset noise, even under complete reset operation. So, another method using higher gate-controlling voltage is proposed to implement complete reset to alleviate image lag and the dynamic range is improved as well. Taking reset noise, integration noise and readout noise into consideration, the pixel's output SNR (signal to noise ratio) is analyzed and optimized at C
FD
/C
PH
= kT/(C
PH
V
²
n, in
), in which, C
PH
and C
FD
are the equivalent capacitors of the photodiode and floating-node (FD) in the pixel, V
²
n, in
is the input-referred noise of the in-pixel follower.