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The optoelectronic characteristic of a p-i-n thin-film phototransistor is analyzed by comparing an actual device with device simulation. It is found using the actual device that the detected current is maximized when the control voltage is equal to the applied voltage. It is also found using the device simulation that a depletion layer is widely formed at that voltage because the electron mobility is higher than the hole mobility and the generated electrons can be rapidly transported and do not accumulate much in the poly-Si film. The behavior in the actual device can be explained by the results in the device simulation. Particularly in this presentation, we will illustrate how the electric current flows using the current flowline.