www.seipub.org/aime Advances in Microelectronic Engineering (AIME) Volume 2 Issue 1, January 2014 20 Design of a Current‐Mode CMOS Image Sensor Based on Standard CMOS Process with on Pixel Color Separation Peiman Aliparast *1 , Fatemeh Niknahad 2 , Mohammad Hossein Zarifi 3 1 Astronautics Research Institute, Iranian Space Research Center, 14665‐834, Tehran, Iran 2,3 Department of Electronics Engineering, East Azarbaijan Science and Research Branch, Islamic Azad University, Tabriz, Iran * 1 Aliparast@ari.ac.ir; 2 fniknahad85@gmail.com; 3 Mohammad.h.zarifi@gmail.com Abstract In this paper, the design of a current mode CMOS image sensor pixel in standard 0.18 μm TSMC RF‐CMOS process was presented. The proposed photodiode has been achieved by using Deep NWell layer. Taking advantages of buried NWell, three diode junctions: N+/P‐Well, DNWell/P‐Well, DNWell/Psub have been achieved that are used as blue, green and red detectors, respectively. In the photodiode RGB curve, the peaks have been accrued at wavelengths of 440 nm, 500 nm and 620 nm. In this way, the colors have been well separated in standard CMOS process with no additional cost, compared to other methods. For the pixel circuit of CMOS Image sensor, three high swing cascade current mirrors which are suitable to operate at lower voltages have been used. By using this pixel circuit in CMOS image sensors, there is no need to use costly methods of color detection (for example; color filters on the pixel). In addition, the complexity of the process and the chip area was decreased. Keywords Color Separation; CMOS Image Sensor; Current Mode; Image Pixel; Photodiode Introduction For a long time, CCDs have been unequalled leaders in the field of electronic cameras for all kinds of applications (Lulé et‐al, 2000). Over many years, this concept has been driven by an increasing market demand for ever larger pixel numbers (Wen et‐al, 1999) and better image quality (Hagiwara, 1996) that no other technology could serve better than CCDs. However, interest in image sensors based on standard CMOS technology has increased dramatically. CMOS based imagers offer significant advantages over CCD’s such as system‐on‐chip capability, low power consumption and possibly lower cost (Lulé et‐al, 2000). In order to perform color imaging, the sensor is usually combined with an array of three color filters, often red, green, and blue (RGB) arranged in a Bayer pattern (Bayer, 1975) or alternatively with complementary color filters (Parulski, 1985). Recently, a filter‐less multicolor photodiode was investigated to detect different wavelengths of light in a color sensor (Polzer et‐al, 2011). The design of this novel color detector based on the effect that light with longer wavelengths can penetrate deeper into silicon than light with shorter wavelength. The main obstacle of CMOS‐based imaging systems comes from the low responsivity and unscalability of the photosensor (Wong, 1996; Fossum, 1997). Because most CMOS technologies are not tailored for light sensing, photoactive devices available in a standard CMOS process are mainly photodiodes formed by various PN junctions (Zhang, 2001). For example, in an n‐well CMOS process, the available optical sensors are the n diffusion to substrate diode, n‐well to p‐ substrate diode and p‐channel MOS‐transistors in a floating n‐well externally biased in the subthreshold region or in strong inversion (Schanz, 1997; Sandage, 1995). This paper reports a current mode CMOS image sensor pixel, which integrates three vertically stacked photodiodes together with three high swing cascade current mirrors into the pixel. This kind of pixel can detect three different wavelengths of light without any additional filters. Consequently, the process complexity and also the size of the chip are reduced as well as the total cost of the whole device. This PD and the current mode pixel are designed in a standard 0.18 μm TSMC RF‐CMOS technology without any process modification. The rest of the paper is organized as follows. Section II describes the photodiode structure in standard CMOS