Characteristics of fault-tolerant photodiode and photogate active pixel sensor (APS)

A fault-tolerant APS has been designed by splitting the APS pixel into two halves operating in parallel, where the photo sensing element has been divided in two and the readout transistors have been duplicated while maintaining a common row select transistor. This split design allows for a self correcting pixel scheme such that if one half of the pixel is faulty, the other half can be used to recover the entire output signal. The fault tolerant APS design has been implemented in a 0.18 μm CMOS process for both a photodiode based and photogate based APS. Test results show that the fault tolerant pixels behave as expected where a non-faulty pixel behaves normally, and a half faulty pixel, where one half is either stuck low or high, produces roughly half the sensitivity. Preliminary results indicate that the sensitivity of a redundant pixel is approximately three times that of a traditional pixel for the photodiode APS and approximately twice that for the photogate APS.