  A single-photon CMOS image sensor design based on pinned photodiode (PPD)
with multiple charge transfers and sampling is described. In the proposed pixel
architecture, the photogenerated signal is sampled non-destructively multiple
times and the results are averaged. Each signal measurement is statistically
independent and by averaging the electronic readout noise is reduced to a level
where single photons can be distinguished reliably. A pixel design using this
method has been simulated in TCAD and several layouts have been generated for a
180 nm CMOS image sensor process. Using simulations, the noise performance of
the pixel has been determined as a function of the number of samples, sense
node capacitance, sampling rate, and transistor characteristics. The strengths
and the limitations of the proposed design are discussed in detail, including
the trade-off between noise performance and readout rate and the impact of
charge transfer inefficiency. The projected performance of our first prototype
device indicates that single-photon imaging is within reach and could enable
ground-breaking performance in many scientific and industrial imaging
applications.
