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Croaker Senior Member Offline Posts: 235	Plotting Vth variations Mar 06^th, 2007, 8:04am Hello all, I'm wondering if anyone knows how to do a plot of how Vth varies due to process mismatch... I guess I'm looking for a graph similar to a bell curve to show what Vth values I'm likely to get. Thanks! :) Oh, I'm using Cadence.
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sheldon

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Re: Plotting Vth variations
Reply #1 - Mar 6^th, 2007, 8:32pm

Croaker,

Not sure I understand exactly what you are looking for. Do you want
to plot distribution of delta Vt? Do you want to plot the correlation between
Vt and mismatch [delta Vt]? Do you want to plot correlation between Vt
and delta L, delta tox, ...? Do you want to plot correlation between delta
Vt and deltaL, delta tox, ...? It should be possible plot the results, just
need a understand what you would like to plot.

Best Regards,

Sheldon

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Geoffrey_Coram

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Re: Plotting Vth variations
Reply #2 - Mar 7^th, 2007, 5:39am

In some processes, the VTH0 parameter is set by calls to a Gaussian distribution function. In this case, you can just take the mean and std dev of the Gauss call and plot the distribution directly.

In some cases, though, TOX and VFB (which indirectly change Vth) are varied, and then you'd need to define what you mean by "Vth" ... and probably run some M-C simulations to "measure" it for your devices.

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SATurn Community Member Offline Posts: 53	Re: Plotting Vth variations Reply #3 - Mar 18^th, 2007, 7:00am The question is that how it is possible to plot (or print) the VTH for example in Cadence? Saturn
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Geoffrey_Coram

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Re: Plotting Vth variations
Reply #4 - Mar 19^th, 2007, 4:23am

SATurn wrote on Mar 18^th, 2007, 7:00am:

The question is that how it is possible to plot (or print) the VTH for example in Cadence?

What is VTH? With subthreshold current, Id != 0 for Vgs < VTH, so you need to decide what you want to use as a definition of VTH: is it based on the value of drain current, the slope of the Id/Vg curve, ... BSIM3/4 have a VTH output parameter, which is computed from VTH0 and other parameters, but it does not include things like GIDL that might actually change where a circuit designer would say the "threshold voltage" is based on the Id/Vg curve.

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Croaker Senior Member Offline Posts: 235	Re: Plotting Vth variations Reply #5 - Mar 19^th, 2007, 8:59am SATurn wrote on Mar 18^th, 2007, 7:00am: The question is that how it is possible to plot (or print) the VTH for example in Cadence? Saturn I do know how to plot Vth...or are you looking to find out?
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Croaker

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Re: Plotting Vth variations
Reply #6 - Mar 19^th, 2007, 9:03am

sheldon wrote on Mar 6^th, 2007, 8:32pm:

Croaker,

� Not sure I understand exactly what you are looking for. Do you want
to plot distribution of delta Vt? Do you want to plot the correlation between
Vt and mismatch [delta Vt]? Do you want to plot correlation between Vt
and delta L, delta tox, ...? Do you want to plot correlation between delta
Vt and deltaL, delta tox, ...? �It should be possible plot the results, just
need a understand what you would like to plot.

� Best Regards,

Sheldon

Hi Sheldon, I want to have something like a pdf so I can see what my chances of getting certain Vths are.

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Croaker

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Re: Plotting Vth variations
Reply #7 - Mar 19^th, 2007, 9:04am

Geoffrey_Coram wrote on Mar 19^th, 2007, 4:23am:

SATurn wrote on Mar 18^th, 2007, 7:00am:

The question is that how it is possible to plot (or print) the VTH for example in Cadence?

What is VTH? �With subthreshold current, Id != 0 for Vgs < VTH, so you need to decide what you want to use as a definition of VTH: is it based on the value of drain current, the slope of the Id/Vg curve, ... �BSIM3/4 have a VTH output parameter, which is computed from VTH0 and other parameters, but it does not include things like GIDL that might actually change where a circuit designer would say the "threshold voltage" is based on the Id/Vg curve.

For the purposes of this thread, I will say Vth is the operating point value that spectre spits out.

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Geoffrey_Coram

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Re: Plotting Vth variations
Reply #8 - Mar 19^th, 2007, 11:15am

Croaker wrote on Mar 19^th, 2007, 9:04am:

For the purposes of this thread, I will say Vth is the operating point value that spectre spits out.

If that's what you want, and you know how to access (plot) Vth, then I would expect you could do a Monte-Carlo and set your expression to be that Vth. See "spectre -help montecarlo"

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If at first you do succeed, STOP, raise your standards, and stop wasting your time.

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Croaker

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Re: Plotting Vth variations
Reply #9 - Mar 19^th, 2007, 1:46pm

Geoffrey_Coram wrote on Mar 19^th, 2007, 11:15am:

Croaker wrote on Mar 19^th, 2007, 9:04am:

For the purposes of this thread, I will say Vth is the operating point value that spectre spits out.

If that's what you want, and you know how to access (plot) Vth, then I would expect you could do a Monte-Carlo and set your expression to be that Vth. �See "spectre -help montecarlo"

Yes, I've done that but it just shows a bunch of Vth values. I was looking for something more histogram-ish. Undecided

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ACWWong Community Fellow Offline Posts: 539 Oxford, UK	Re: Plotting Vth variations Reply #10 - Mar 19^th, 2007, 3:19pm in the cadence monte-carlo GUI, histogram ploting of your outputs (i.e. vth expression) is available under results, as are loads of other useful plot types.
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Croaker Senior Member Offline Posts: 235	Re: Plotting Vth variations Reply #11 - Mar 20^th, 2007, 5:44am OK thanks, I'll look into that! Sounds like what I wanted.
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