Optimizing the match in weakly inverted MOSFET's by gated lateral bipolar action

The on-chip n-type MOSFET current mirror circuit with different drawn gate widths and lengths has been fabricated, and has been characterized across the wafer with back gate slightly forward biased. The weakly inverted MOSFET device with a small back-gate forward bias represents equivalently the high-gain gated lateral bipolar transistor in low-level injection. Experimental results have exhibited a substantial improvement in the match of the drain current in weak inversion due to action of the gated lateral bipolar transistor, especially for the small size devices. The extensively measured mismatch of the weak inversion drain current has been successfully reproduced by an analytic statistical model with back-gate forward bias and device size both as input parameters. The experimentally extracted variations in process parameters such as the flat-band voltage and the body effect coefficient each have been found to follow the inverse square root of the device area. The mismatch model thus can serve as a quantitative design tool, and has been used to optimize the trade-off between the device area and the match with the forward back-gate bias as a parameter.