[SI-LIST] Re: Modeling of varactor in CMOS process Upto 10G
- From: "Vadim Heyfitch" <heyfitch@xxxxxxxx>
- To: Neo <neoflash2008@xxxxxxxxx>
- Date: Mon, 7 Apr 2008 12:05:09 -0700
Most likely the RF PDK model of the varactor is a hyperbolic tangent fitted
into a measured data. From my firsthand experience with such fitting (on a
later CMOS process) and also with fitting an NMOS- based model (with
BSIMPro+), I can tell that the hyperbolic tangent does not fit the raw data
very well. But for some reason, TSMC likes it.
NMOS in N-well has both drain and source as an identical second terminal. If
the varactor model is a combo of an NMOS transistor and a bulk-to-well
diode - with various parasitics - then both the drain and the source of the
NMOS are floating. So both should have very large resistances Rs and Rs.
Even if the model has Rds it should not be in series with the gate, I think.
You should request from TSMC the raw data Q(freq) under different biases Vgb
and Vg(d/s). Ideally, you would make test structures and do on-die
measurements. Otherwise, you have no choice by to take TSMC model (and data)
at their face values. That includes trusting their measurements and
de-embedding , both of which are quite tricky.
- Vadim
On Mon, Apr 7, 2008 at 7:19 AM, Neo <neoflash2008@xxxxxxxxx> wrote:
> Hi, folks:
>
> I have a question on the modeling of RF varactors (nmos in nwell) in TSMC
> 0.18um RF PDK. It is related with high-frequency measurement and
> characterization, so it should be helpful to consult SI friends.
>
> If we take a look at the RF model, we will find that Rg and Rds element
> is in series with the variable capacitor. However, I'm not sure whether it
> fully modeled the loss of the varactor.
>
> 1. the Rg and Rds is not related to Vgb of the varactor. This doesn't
> match the reality that changing the Vgd will change the series resistance of
> Nwell.
>
> 2. I'm not sure how well it is correlated with measurement.
>
> Since I will use the varactor in very high frequency VCO (10g), and the
> quality factor of the varactor is very important. Please comment on this.
>
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