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Design >> Analog Design >> NF / M BSIM parameter
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Message started by mkaragou on Jun 9^th, 2006, 4:56am

Title: NF / M BSIM parameter
Post by mkaragou on Jun 9^th, 2006, 4:56am

What is the difference between the parameters M and NF of the BSIM model?

My spectre documentation states:

M=Multiplicity factor(number of MOSFETs in parallel)
NF=number of device fingers

When I have two device fingers I have two MOSFETS in parallel, or not?

So what is the difference?

Is it the number of drain and source contacts?

Title: Re: NF / M BSIM parameter
Post by bernd on Jun 9^th, 2006, 8:25am

m is a bsim instance parameter which is defined as
'Multiplicity factor (number of MOSFETs in parallel)'.

nf is not a bsim parameter it might be a parameter of
a transistor subcircuit in your environment, (for number of fingers) .

If both are available for one MOS it's confusing,
maybe nf is used to calculate drain and source perimeter
and area for multifinger devices.

Bernd

Title: Re: NF / M BSIM parameter
Post by mkaragou on Jun 9^th, 2006, 11:55am

NF is a BSIM4 parameter!!!!

At least it is listed in the "spectre user guide" documentation!

Title: Re: NF / M BSIM parameter
Post by mikki33 on Jun 11^th, 2006, 8:55am

NF and M are interchangable paraneters. You may use or one or another.
It is generally used for circuit driven layout and depends on the company methodology.

NF: you are defining Total Width and NF, and stating where you want the lowest (drain or source) capacitance.
In this case width per finger is calculated.
M: you are defining Width and M, Total Width is calculated.

Title: Re: NF / M BSIM parameter
Post by bernd on Jun 12^th, 2006, 3:26am

Quote:

NF is a BSIM4 parameter!!!!

You're right I'm still thinking bsim3v3 ;-).

I think m is for parallel MOSs, whereas nf relay intents to be
more layout specific for parallel MOSs with shared diffusion (S/D) areas,
This is not required for m.

Have a look at my attacment
A.11 Layout-Dependent Parasitics Model Parameters
So I strongly believe its layout style depended.

Title: Re: NF / M BSIM parameter
Post by Geoffrey_Coram on Jun 12^th, 2006, 4:15am

mikki33 wrote on Jun 11^th, 2006, 8:55am:

NF and M are interchangable paraneters. You may use or one or another.

Yikes! That is a dangerously untrue statement, and it's a little hard to decipher the truth from what you wrote later.

"m" is generally defined as the "number of (identical) parallel devices" -- and implemented in the simulator, rather than in the model source code that one might get from Berkeley.

"nf" is, as Bernd wrote, a way of specifying some geometry information pre-layout.

But it's important to know that
m1 d g 0 0 nch w=1u l=0.18u m=100
is definitely not the same as
m1 d g 0 0 nch w=1u l=0.18u nf=100

The first will have 100 devices, each with a drawn width of 1u. The second will have 100 poly gates each with a width of 0.01u.

Title: Re: NF / M BSIM parameter
Post by mikki33 on Jun 12^th, 2006, 8:24am

It was actually, what I intended to say. I didn't write they are the same, because they are not.
Sorry for the confusion.

Title: Re: NF / M BSIM parameter
Post by mkaragou on Jun 12^th, 2006, 8:28am

Quote:

nf relay intents to be more layout specific for parallel MOSs with shared diffusion (S/D) areas

This makes sense to me, because the number of shared diffusion would for example affect the
amount of drain/source junction capacitance or the drain/source resistance that without the parameter nf
would not be taken into consideration by simulation.

But this means that I have to make layout decisions already during my early simulation phase.