Hints, Tips and Solutions
Q: How can I get a CV curve of a MOSFET? And how can I get the current on all terminals?
A: There are two methods to get the CV curve of MOSFET, Small Signal analysis and Large Signal analysis.
From the charge conservation, the sum of charge in the device must meet the charge neutrality relationship given by:
Qg + Qd + Qs + Qsub = 0
so the Capacitance should be
Cij = dQi/dVij i ≠ j,i j=G,D,S,SUB
Cij = dQi/dVij i=j
The small signal analysis method applies AC sinusoidal bias to a DC condition.
For example, if the small signal is only applied to gate electrode, then
solve vgate=2.0 vstep=0.1 vfinal=2.0 ac freq=1e6 aname=gate
The results can be seen in Figure 1.
Figure 1. CV Curve of MOSFET by Small Signal Analysis. 
The large signal analysis method consists of a Transient simulation with DC offset condition.
To define a sinusoidal bias offset of 0.01V to the gate bias the following syntax should be used:
set vg=2.0
solve vgate=$vg local
log outf=vg$”vg”ac.log
solve trans.analysis freq=1e6 vgate=$vg+0.01
tstop=2e6 tstep=1e8 cycle=2
log off
the resulting currents are shown in Figure 2.


Figure 2. Current of MOSFET by Transient anlaysis at gate bias 2.0V. 
To convert current to capacitance (i=C*dv/dt), the extract statement was used as follows:
extract name=”Cdg” y.val from curve(time,
i.”drain”/(2*3.14*1e4)) where x.val=5e7
extract name=”Csg” y.val from curve(time,
i.”source”/(2*3.14*1e4)) where x.val=5e7
extract name=”Cdg” y.val from curve(time,
i.”gate”/(2*3.14*1e4)) where x.val=5e7
extract name=”Csubg” y.val from curve(time,
i.”substrate”/(2*3.14*1e4)) where x.val=5e7
The capacitance is then extracted at t=5e7s using the following formula:
Cdg=idrain*1/(2*pi*freq)*(vss)=idrain/
(2*3.14*1e6*0.01)
To get the CV curve as in Figure 1., the internal simulator was used as follows:
go internal
load infile=LargeCap.in (1)
sweep param=vg type=linear range=”2.0,
2.0, 41” (2)
save outfile=”largesigcap.dat”
quit
(1) LargeCap.in is a file that contains the transient simulation described before.
(2) The sweep command will vary vg from 2.0V to 2V with 41 points.
The results are shown in Figure 3.
Figure 3. CV Curve of MOSFET by Large Signal Analysis. 