Analysis and Parameter Optimization of High Dynamic Range Pixels for Split Photodiode in CMOS Image Sensors

Abstract

To achieve high dynamic range and define parameter tradeoff of split photodiode pixel based on lateral overflow integration capacitor (LOFIC) technology, the mechanism of extending dynamic range with split photodiode, dual conversion gain and LOFIC technology was studied. A mathematical model of LOFIC technology combined with split photodiode at different pixel sizes is established, which describes responsivity ratio of dual photodiode, LOFIC, conversion gain, and voltage swing affecting dynamic range by equivalent number of random noise electrons and full-well-capacity (FWC) of pixel. Dynamic range of pixel increases from 131.1dB to 133.6dB as responsivity ratio rises from 12 to 16 with voltage swing of 1V and LOFIC node of 25fF, whereas signal-to-noise ratio (SNR) at dual photodiode signal switching point drops from 28.65dB to 27.23dB for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mu \text{m}\,\,\times 3~\mu \text{m}$ </tex-math></inline-formula> pixel. And dynamic range can reach 141dB when LOFIC node rises to 50fF with responsivity ratio of 18 and SNR at small-size photodiode signal switching point decreased by 0.2dB for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10~\mu \text{m}\,\,\times 10~\mu \text{m}$ </tex-math></inline-formula> pixel. The model shows that dynamic range increases with responsivity ratio and LOFIC, whereas is restricted by SNR at signal switching points. This paper provides a design idea and data reference for the design of high dynamic range (HDR) CMOS image sensors based on split photodiode and LOFIC technology.