Noise response in a CMOS active pixel sensor due to the radiation effects

In a viewpoint of electric noise, the main provider of the CMOS APS is the pixel among other components and generally the radiation response to it makes additional noise source leading to change of leakage current. For the change of leakage current, the main mechanism of total ionization dose (TID) effect on CMOS APS had been investigated in other research activities. However, it is difficult to distinguish dominant noise among several other noise sources, affecting the performance of active pixel since that is composed of photodiode and three MOS transistors, reset, source follower, and selection transistor. In this paper, we have performed characterization of dominant noise source after measuring total noise and dominant source in each operation step. To derive each source we designed and fabricated the combination set of discrete components as follows: (1) only photodiode with two styles n-well/p-sub and n+/p-sub, (2) only MOS transistor, (3) photodiode and reset transistor, (4) photodiode, reset transistor, and source follower, finally (5) photodiode, reset transistor, source follower, and select transistor. The size of the designed pixels is 100 mum and the fill factor is approximately over 73%. The test chip was fabricated using ETRI 0.8 mum (2P/2M) CMOS standard process. These test samples were irradiated using DC X-ray source to the 4 krad dose. It was found that reset noise and readout noise is not changed, but only number of dark charge is changed to that exposure dose.