Fix to negative output conductance problem in BSIM2 model [MOSFET]

The long-standing negative output conductance problem observed in the BSIM2 model, which causes convergence problems in circuit simulation, has been studied and resolved. The problem is identified to be related to both parameter extraction methodology and model equations. A redundant boundary condition in the saturation region for the optimization of output characteristics was found. A problematic channel length modulation equation also contributes to the problem. The fix to this problem not only eliminates the negative conductances in the simulated output characteristics of long-channel transistors but also preserves fits to output characteristics of short-channel transistors. The fix, at the same time, speeds up the parameter extraction process by 20% by eliminating five of the model parameters