Suppose for a transistor finger width is 'fw', length is 'L', Number of fingers is 'nf' & Multiplier is also 'm', When we write sigmaVt = Avt/sqrt(WL), What is W ? Is W=fw or W=m*nf*fw or W=nf*fw ? It will be helpful if someone can clarify this...

For BSIM models,

W

always equals total width of all fingers (

W=nf*fw

) not counting any multplier

M

. The multiplier is harder to deal with and implementation can differ by foundry. The underlying problem is that

M

is treated differently from other parameters by ngspice and cannot be used inside model equations. The most common method I've seen (which is done in sky130, for example) is to create a "shadow" parameter called, say,

mult

, which is always set to the same value as `M`; then the model equation for area becomes

mult*W*L

.