Current mismatch due to local dopant fluctuations in MOSFET channel

A microscopic multitransistor model is developed to analyze the impact of local dopant fluctuation on the intrinsic mismatch of long-channel MOSFET. A closed analytical formula for current mismatch is derived to show a nonscaled and self-consistent form ∼[4+Log(L/L_min)]/WL. This is in contrast to the global fluctuation model, in which the current mismatch has a universal scaling form ∼1/WL but is not self-consistent if a MOSFET is modeled as an equivalent two-transistor system. The weak violation of scaling law results from the local fluctuation that has more impact on longer channel devices than on shorter ones. Our new model is consistent with recent experimental observation and can explain the discrepancies between the experimental data and the existing models. The analysis indicates that the local dopant fluctuation is the major cause and accounts for about 60% to 80% of total current mismatch when operated at lower gate voltage, a usual regime for higher output impedance.