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Design >> Analog Design >> Distortion in MOS Transistor
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Message started by lv_designer on Oct 6^th, 2006, 1:28pm

Title: Distortion in MOS Transistor
Post by lv_designer on Oct 6^th, 2006, 1:28pm

Hi everybody,
I have a question about distortion in MOS devices. Assume we use fully differential circuits, hence only 3rd and 5th harmonics are of concern. We know that
for a given input signal amplitude (say 100mV pk-pk sine wave) the distortion drops as square of overdrive voltage (Vgs - Vth). But this is assuming ideal MOS
equations. What happens to distortion when vertical electric fields are high (i.e. too much overdrive)? Does the distortion drop as square of overdrive in high field
case when the mobilty is degraded due to high vertical fields ? What is effect on distortion due to velocity saturation caused by high latertal electric fields?
Does BSIM4 model captures these effects accurately?
Thanks.

Title: Re: Distortion in MOS Transistor
Post by loose-electron on Oct 6^th, 2006, 2:38pm

Distrortion issues as a result of the device physics?

I strongly suggest that you don't go there.

To a first order, most of your linearity issues should be seen using linearity tests (two tone IIP3 or similar) as a function of the circuit architecture, transistor model and bias point characteristics.

A lot of what is seen there can be directly observed based upon the bias point, a DC bias curve set, and the input signal amplitude. Dealing with all of the second order effects are generally not too necessary.

Will the BSIM4 (4.5 ??) reflect this stuff accurately? The model is very capable of it. ***However*** you got to remember that a lot of the model parameters are not properly set. The model definition is ok, but the numbers put into the model are not accurate. Need to discern between those two. They are very different things. A foundry can use the BSIM99.9 for 1picometer CMOS (just joking) but if they dont set the model values correctly, all the defaults get you back to a much simpler model.

Oh, and one other comment - Just because the system is differential does not mean that second order harmonics go away. Greatly reduced, yes. Eliminated no.

Jerry

Title: Re: Distortion in MOS Transistor
Post by lv_designer on Oct 6^th, 2006, 3:30pm

Thanks Jerry,
Is it more power efficient to obtain the required THD (say 45dB) by brute force increase of overdrive voltage or by using a degeneration resistor at the source? For nanometer technologies the electric fields could be high. In this case it may be better to use degeneration R.
Thanks.