Methods for photodiodes bias voltage optimization in scanning and staring FPA’s
To find the optimal bias voltage, a differential resistance distribution was examined in 6´576 LWIR MCT FPA with 25¸5 µm pitch and long wave cut-off at 10.5 µm. To calculate differential resistance of each photodiode, we measured their I-V characteristics. For each bias voltage, we calculate differential resistance as dU/dI. Then, we can calculate the working bias voltage for each photodiode as bias voltage with maximum differential resistance. It allows for minimum photodiode noise due to leakage current and great stability of photodiode characteristics over time and fluctuations of temperature and supply voltage. At the working bias voltage, photodiodes achieve their best SNR. To optimize bias voltage for maximum mean SNR, we want a maximum count of photodiodes to be near their working bias voltage. In our test FPA, we found that the maximum count of photodiodes near working voltage was at bias voltage of 1025 mV. To optimize bias voltage for minimum defective photodiodes, we want to minimize the count of photodiodes with low differential resistance. To do so, we first need to define a criteria for “low differential resistance”. We will count a photodiode as having low differential resistance at a certain bias voltage if R < R<sub>max</sub>×0.7, where R<sub>max</sub> is the maximum differential resistance for this photodiode. This criteria was developed from our measurements at the optimal SNR bias voltage and 30 % fall of differential resistance doesn’t degrade SNR performance for a considerable amount. In our test FPA, for bias voltages from 987 to 1012 mV we have low count of low differential resistance photodiodes, with the minimum being at 994 mV.