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We have extensively measured and analyzed the current mismatch of a small-size n-channel MOS transistor of 2 /spl mu/m/spl times/2 /spl mu/m operated in weak inversion with its p-well-to-n/sup +/-source junction forward and reverse biased. The case of slightly forward biasing the well-to-source junction represents the action of a gated lateral bipolar transistor in low level injection. The measured dependencies of the mismatch in weak inversion on the back-gate forward and reverse biases have been successfully reproduced by a new simple statistical model. From the experimental data, we suggest that i) subthreshold circuits should be carefully designed for suppression of mismatch arising from back-gate reverse bias, and ii) a gated lateral bipolar action in low level injection may be utilized as a new method of improving the transistor matching.