Does anybody know why some ngspice sims of

nfet_01v8

return negative junction capacitances?

see section 31.6.9 of the ngspice manual: "_Negative capacitance values may occur, depending on the internal calculation. To compare_ with measured data, please just use the absolute values of the capacitance data. For an explanation of negative values and the basics on how capacitance values are evaluated in a BSIM model, please refer to the book BSIM4 and MOSFET Modeling for IC Simulation _by Liu and Hu, Chapt. 5.2_".