Re: 2n7000 spice models

From: Winfield Hill (hill_a_at_t_rowland-dotties-harvard-dot.s-edu)
Date: 03/26/05 

Date: 26 Mar 2005 09:38:32 -0800

Jim Thompson wrote...
> Jim Thompson wrote:
>>
>> [snip]
>> I was looking thru "pwrmos.lib" in the new OrCAD/PSpice release...
>> everything is STILL Level=3.
>> So I think the problem is the manufacturers don't care about
>> spending money on quality modeling.
>
> It's literally been just shy of 20 years since I designed anything
> with a power-FET, and _most_ of my I/C foundry CMOS models do seem to
> work subthreshold (for small-signal analog).
>
> Win, WHAT is important to you in power-FET modeling, subthreshold?
>
> Maybe we can mutually determine the "fix" for the models?

 OK, here's what I wrote early in the discussion, last December 31st,

> Well, then surely you'll want viable Spice MOSFET models, because
> you'll be using the FETs throughout their useful linear range, and
> the output-stage crossover region is critical.
>
> Sadly most standard Spice library VMOS models simply don't do the
> subthreshold linear region. For example, see the 10-decade plots
> on page 123 of our book. A jellybean 2n7000 is rather similar to
> the VN01 that we show in figure 3.14, and certainly a proper Spice
> model should be able to make that plot. But I'd be surprised if
> your standard Spice libraries work properly below say 5 to 20mA,
> which is not that far below the FET's maximum current. Keep in
> mind that linear power FET circuitry always operates well below
> the maximum rated FET switching current, to keep power dissipation
> junction heating under control.

 I since confirmed this with some 2n7000 Spice-model testing...
 the SQRT-Id plot goes to zero at 2.4V (wrong answer), an abrupt
 transition occurs at 9mA (wrong), and another at 2mA (gross), to
 a 5-decade subthreshold exponential transconductance (correct) at
 Vgs = 2.4V (wrong). Rather useless for linear-circuit modeling.

> The bottom line is you'll have to start your FET-amplifier design
> exercise by designing some decent FET models. Let us know what you
> come up with.

 With respect to the Id-vs-Vgs curves on page 123 of our book, and
 the g_m plots on page 132, the spice models should be able to show
 this performance. At subthreshold currents a FET acts very much
 like a transistor with respect to transconductance, etc., and when
 a power FET is used in linear audio amplifiers, e.g. in class AB,
 it may go through this region during each cycle. So in using Spice
 to determine distortion and evaluate various design configurations,
 surely it's necessary for the FET model to smoothly simulate the
 subthreshold region, and properly progress to the current-saturated
 regions that are normally accurately modeled. I'm going to go in
 to the lab and take some detailed 2n7000 measurements later today. 

-- 
 Thanks,
    - Win
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