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VDSAT more than VGS-VTh (Read 315 times)
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VDSAT more than VGS-VTh
Mar 27th, 2018, 1:55pm
Simulating sub 28nm planner technology node IO 1.8V devices.
Device Sizes: W=1u; L=1.5u
Operating points from Spectre sims.

VGS= 730mV
VDS= 400mV
VDsat= 330mV

Clearly,  VGS-Vth is 275mV whereas Vdsat is 330mV
I have a very little margin on VDS/VDsat.
If my Vdsat is lower (~275mV as expected), I have enough margin.

I have read and heard that due to channel length modulation effect and/or velocity saturation effect Vdsat is lower than expected VGS-Vth.
however I see VDSAT more than Vgs-Vth !!!

Any explanation for Vdsat > than Vgs-Vth?
Could it be due to metal gate technology?
Could it be some error in the technology file setup since its a relatively new setup for us?

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Re: VDSAT more than VGS-VTh
Reply #1 - May 9th, 2018, 12:42am
Saturation condition for short channel devices isn't going to match square law.  I suggest you look at whatever small signal parameters are of interest to you and make sure those are still OK at your biasing point.
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Re: VDSAT more than VGS-VTh
Reply #2 - Jul 10th, 2018, 5:11pm

I think of VDSAT and VGS-TH as two different parameters. VGS-VTH is the overdrive voltage available to me while VDSAT is the voltage that signals the onset of saturation.

Having VGS-VTH < VDSAT means you are operating your transistor in weak inversion/moderate inversion region.

As a rule of thumb, you could also annotate your gmoverid to see if it is > 15. This value might differ for your technology, but it gives a feel for you on where you are operating your transistor.

Can you post the picture of the circuit, you are trying to simulate ?
Also, May I know why the W > L ?
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