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Modeling >> Semiconductor Devices >> Capacitance based model vs charge based model
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Message started by kumar on Sep 20^th, 2010, 2:13pm

Title: Capacitance based model vs charge based model
Post by kumar on Sep 20^th, 2010, 2:13pm

Hi,
Could somebody tell me the difference between capacitance based model and charge based model and its effect on simulation accuracy. My opinion is that both capacitance and charge are inter-related. And also the charge based models use capacitance in their equations.

Title: Re: Capacitance based model vs charge based model
Post by Ken Kundert on Sep 20^th, 2010, 8:18pm

Take a look at http://www.designers-guide.org/Modeling/varactors.pdf. You can also find this described in http://www.designers-guide.org/Books/dg-spice/index.html.

-Ken

Title: Re: Capacitance based model vs charge based model
Post by c1000 on Jan 18^th, 2011, 3:00pm

Most of transistor models, including table-based models use charge for charge conservation. By using charge, the derivatives of charge must be evaluated during simulation.

capacitance-based models, although I haven't read a lot about it, shortens simulation time. However, the charge conservation couldnt be guaranteed.

Hope someone will come up to say sth. about accuracy effects.

Title: Re: Capacitance based model vs charge based model
Post by Geoffrey_Coram on Jan 19^th, 2011, 6:09am

c100 - your explanation sounds fishy. It's like saying one could use gm-based models instead of drain current-based, so you don't have to compute the derivatives of drain current.

In fact, the capacitance-based models have to compute the charge -- because it's the change in charge that gives you the current -- so they do this by numerical integration within the model, which is ugly since the model doesn't control the timestep (at least not directly) and can't guarantee accurate numerical integration.

In the beginning, people used capacitance-based models because they could measure the capacitance and construct equations to fit the measurements. It was "hard" to analytically integrate the partial differential equations to get the charge: you have Cgs, Cgd, Cgb, which are supposed to be -dQg/dVs, -dQg/dVd, -dQg/dVb, but it might be that integrating Cgs with respect to Vs gives you a different function than integrating Cgd with respect to Vd. Then what?

So, now, all the "modern" MOS models write the charge equations as the fundamental equations.

Title: Re: Capacitance based model vs charge based model
Post by c1000 on Jan 20^th, 2011, 6:45am

the cap-based transistor model I mentioned is the model of transistor which just use several caps and a current source. Just like the composite current source (CCS) model for a gate, no matter how complicated the gate is. In this model, the transistor is regarded as a black-box.

For a model with current source and caps, the equation is like:
Ax'+Bx+D=0. where x is the nodal voltages and A consists of caps. It's true that in order to solve it, numerical integration method must be used.

There are tables based transistor models, some use charge tables, some use charge and capacitance tables, some use capacitance tables.