A Color-Separable Vertical Pinned Photodiode for Scaled CMOS Image Sensor Pixels

In the complementary metal-oxide-semiconductor (CMOS) image sensor (CIS), a Bayer pattern color filter array (CFA) is used to extract color information. However, this approach reduces spatial resolution and optical efficiency due to signal loss from CFA. In the Bayer CFA, the color signal loss is approximately 50% for the green and 75% for both red and blue. This paper proposes a color-separable pinned photodiode (PPD) structure to enhance optical efficiency while maintaining spatial resolution. In the proposed PPD, two PDs—a shallow and a deep PD—are integrated and separated by a deep transfer gate (DTG). This configuration enables wavelength-dependent separation of incident light, with the shallow PD absorbing short-wavelength and the deep PD absorbing long-wavelength. By adjusting voltage applied to the DTG, the proposed PPD selectively accumulates electrons corresponding to different wavelengths in distinct PDs during the DTG’s off-state, and enables independent readout during its on-state. The effectiveness of the proposed structure was verified through TCAD simulations. The potential profile, photon absorption, and electron transport characteristics were analyzed, confirming that electrons generated by different wavelengths are directed into electrically separated PDs and subsequently transferred to the floating diffusion (FD) node. Simulation results show that the shallow PD absorbs blue light (450 nm) 4.0 times higher than red light (650 nm), while the deep PD absorbs red light 4.1 times higher than blue light, demonstrating the color separation capability of the proposed pixel structure. We believe that the proposed structure contributes to the development of the next generation high-efficiency image sensors with enhanced resolution and efficiency.