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Modeling >> Semiconductor Devices >> RF Transistors
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Message started by Berti on Jun 26^th, 2009, 7:07am

Title: RF Transistors
Post by Berti on Jun 26^th, 2009, 7:07am

Hi all,

I have seen that many technology-kit also provide dedicated RF transistors. Apart from an optimized layout p-cell I don't see any difference to the regular transistors, but they gives somewhat different simulation results. Can somebody tell me what typically is the difference between dedicated RF transistors and the regular devices?

Regards

Title: Re: RF Transistors
Post by pancho_hideboo on Jun 26^th, 2009, 11:12pm

Berti wrote on Jun 26^th, 2009, 7:07am:

but they gives somewhat different simulation results.
Can somebody tell me what typically is the difference between dedicated RF transistors and the regular devices?

I can't understand a meaning of your question correctly.

RF transistors have special models for RF application.
For example, models of RF transistor are composed of macro model using regular transistor to include various parasitic effects.

Title: Re: RF Transistors
Post by raja.cedt on Jun 26^th, 2009, 11:36pm

hi berti,
i think in RF they will concern about less gate resistance so they would having more contacts on gate and good silicidation on poly...

Thanks,
Rajasekhar.

Title: Re: RF Transistors
Post by Berti on Jun 28^th, 2009, 10:45pm

Quote:

For example, models of RF transistor are composed of macro model using regular transistor to include various parasitic effects.

I agree on that. However, the DC simulation already gives different results for RF and regular transistors. This makes me wondering what is different.

Regards

Title: Re: RF Transistors
Post by vivkr on Jun 28^th, 2009, 11:14pm

Berti wrote on Jun 28^th, 2009, 10:45pm:

Quote:

For example, models of RF transistor are composed of macro model using regular transistor to include various parasitic effects.

I agree on that. However, the DC simulation already gives different results for RF and regular transistors. This makes me wondering what is different.

Regards

Hi Berti,

I cannot think of much that would change the DC operating points except for the inclusion of source/drain contact resistance and perhaps the source/drain diffusion resistance being different due to a different p-cell layout. But these should not make a big difference. Maybe you have more terms for leakage and impact ionization in the RF model. Perhaps you can try probing the various terminal currents including the bulk current and gate current.

There is also some difference in the use of NQS (nonquasistatic) parameters in some RF models but I don't believe that this will cause a change in DC behavior. In fact, many RF models simply include a series resistance with the gate to capture some NQS effects.

Finally, it could simply be due to the fact that your RF device model was not updated as recently as the standard device model or was not centered correctly for the DC case.

Regards,

Vivek

Title: Re: RF Transistors
Post by Berti on Jun 29^th, 2009, 1:56am

Probably your are right.

It just confuses me that the layout of the RF transistor also contains an additional layer, which makes me wondering whether that has any influence on process steps or just serves as a marker to indicate RF devices in the layout.

Cheers

Title: Re: RF Transistors
Post by ACWWong on Jun 29^th, 2009, 2:49am

Hi Berti,

Yes the RF device layer is typically just a CAD layer, although it can be used by the extraction tools (i.e. not to double count fixed pcell metalisation during RCX) or metal fill process (for metal fill exlusion purposes) or physical verification.

I agree the DC parameters should be very similar for both the RF and normal device, I've never seen an RF device which is not just a fixed layout. Having said that, its always good to look at the model card/equivalent circuit for the device. There are two common ways the RF devices are made:
1) normal FET model with added parasitics (like Rg, Rd, Rs, Rb, etc.)
2) complete extraction, normally BSIM. In this case you'll see that the RF devices have a different BSIM level to the standard devices. This could lead to discrepancies in DC behaviour, but one would hope these discrepancies would be small.

Cheers

aw

Title: Re: RF Transistors
Post by Berti on Jun 29^th, 2009, 3:21am

Thanks a lot for the useful answers!