This paper presents the results of a study of charge transfer time in long doped semiconductor regions. These regions are used to collect and store charge in high performance image sensors. The effect of dopant concentration on charge transfer time was studied using a novel two-dimensional device simulation tool. It was found that the delay associated with the long storage region only becomes significant for doping concentrations that are not degenerate. The effect of storage diffusion length on charge transfer time was also studied for degenerately doped structures. For these structures, it was found that the delay is much less than the conventional belief that the delay is proportional to the square of the diffusion dimension the electrons traverse. It was also found that the diffusion dimension affects the charge transfer time indirectly through the back biasing of the transfer metal oxide semiconductor field effect transistor (MOSFET). Shorter diffusions initially cause a larger back biasing of the transfer MOSFET, decreasing the maximum current flow through the device. On the experimental side, novel image sensor devices were designed that incorporate some of the results discussed above. Experimental image sensor structures were analyzed to study charge transfer time and relate the results to the computer simulations.
A CMOS Image Sensor With In-Pixel Two-Stage Charge Transfer for Fluorescence Lifetime Imaging