How to calculate Cdepletion

[antaryami.mt.er09]

Hi.....
how can i calculate the parameter for Cdep....
as i know Cdep=εsi/xd; where xd is the depletion width
and xd=√[{2.εsi(Φs-Vbsx)}/q.Nch] in BSIM3V3,
Also the parameter Vbsx is not there ,but Vbx is there and Vbx is showing NaN when i simulate the simple MOS ckt

 

[erikl]

xd=√[{2.εsi(Φs-Vbsx)}/q.Nch] in BSIM3V3

Vbsx is the bulk-source reverse voltage (taken negatively, i.e. adds to Φs). You could use the convenient expression shown here: View attachment depletion-region-width_p41.pdf from: David M. Binkley "Tradeoffs and Optimization in Analog CMOS Design", Table 3.1, p. 41 . tdep is your xd (depletion thickness).