The paper reviews advancements in CMOS image sensor technology, including both passive pixel and active pixel sensors. The introduction highlights the diverse applications of electronic cameras, from camcorders to digital still cameras and spaceborne imaging systems. It emphasizes the need for miniaturization, low power consumption, and cost reduction in camera technology. The historical background of solid-state image sensors, from MOS to CCDs, is discussed, noting the challenges and improvements in each generation. The paper then delves into modern CMOS image sensors, describing three main approaches: passive pixel, photodiode-type active pixel, and photogate-type active pixel. Each approach is detailed with its advantages, disadvantages, and performance metrics. The integration of on-chip analog-to-digital converters (ADCs) and timing and control circuits is also explored, highlighting the potential for a single-chip camera-on-a-chip system. The roadmap for camera-on-a-chip technology is outlined, emphasizing the need for improved ADCs and color filter array processes. Finally, the paper concludes by discussing the bright future of CMOS image sensors, driven by scaling trends and technological advancements.The paper reviews advancements in CMOS image sensor technology, including both passive pixel and active pixel sensors. The introduction highlights the diverse applications of electronic cameras, from camcorders to digital still cameras and spaceborne imaging systems. It emphasizes the need for miniaturization, low power consumption, and cost reduction in camera technology. The historical background of solid-state image sensors, from MOS to CCDs, is discussed, noting the challenges and improvements in each generation. The paper then delves into modern CMOS image sensors, describing three main approaches: passive pixel, photodiode-type active pixel, and photogate-type active pixel. Each approach is detailed with its advantages, disadvantages, and performance metrics. The integration of on-chip analog-to-digital converters (ADCs) and timing and control circuits is also explored, highlighting the potential for a single-chip camera-on-a-chip system. The roadmap for camera-on-a-chip technology is outlined, emphasizing the need for improved ADCs and color filter array processes. Finally, the paper concludes by discussing the bright future of CMOS image sensors, driven by scaling trends and technological advancements.