SAM_K wrote on Nov 2nd, 2010, 11:20pm:in VeriolgA.
I'm not quite sure how Verilog-A is involved here -- do you have a compact model description of a diode written in Verilog-A?
If you're trying to fit four regions, then you'd better have equations for the four different regions. Diode models built into commercial simulators typically have a couple saturation currents: forward, reverse/breakdown, tunneling.