Test results of various CMOS image sensor pixels

Today, CMOS image sensors are increasingly used in consumer products. Digital cameras, Web cameras, optical mouses, and smart sensors are some examples. One of the most used CMOS image sensor architectures is based on the active pixel sensor (APS). In order to understand more design tradeoffs, a prototype chip was designed to evaluate different pixel architectures. This paper describes the prototype chip and the test results. The prototype chip is composed of 8 different pixels and a 3-to-8 row decoder. There are two different APS architectures, 3 transistor or 4 transistor. The 8 different pixels are based on the 2 APS architectures with some variations. These variations are 2 types of reset transistor, N_mos or P_mos, and 2 types of photodiodes, n⁺ - p_sub and n_well - p_sub. The prototype chip was manufactured in a standard 3.3 V 0.35 mum 1-poly 4-metal CMOS process. The row decoder was designed using a pass transistor network, inverters, and pull-up transistors. The test results show that for white light, n⁺ - p_sub photodiodes and n_well - p_sub photodiodes have approximately the same response. Also, 4 transistor pixels have better response, but the voltage range for linear operation is smaller. 3 transistor pixels have larger voltage range for linear operation and less dark current. The results also confirm that the output of the pixels is inversely proportional to the distance of the light source