Active pixel sensor (APS) imaging devices are described in U.S. Pat. No. 5,471,515. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, readout control and reset control, in addition to producing the electric signal output from the cell.
APS devices can be operated in a double-sampling mode, in which both a signal voltage and a reset voltage are sampled during pixel readout. The difference between the signal and reset voltages determines the brightness of the pixel. The dominant source of noise in conventional APS is photodetector kTC noise, or reset noise, where k is Boltzmann's constant, T is the absolute temperature, and C is the junction's parasitic capacitance. The kTC noise is the thermal noise resulting from the reset after each pixel's readout. The noise is contributed twice, one with the signal, i.e., the kTC noise from the previous reset, and a second time with reference reset, which follows signal read.