[Moved]: Vdsat in weak inversion

I am confused about what is the definition of Vdsat in weak inversion.
As far as I know, Vds > Vdsat = Vgs - Vt and Vgs > Vt then the mos is in saturation.
However, in weak inversion Vgs - Vt < -72mV, then Vdsat don't mean anything to me.
Could you explain it?

Re: Vdsat in weak inversion

The shown equation is just a rather crude approximation.
See here the context of Veff = Vgs - Vth and Vdsat vs. Inversion coefficient:


... derived from data from the book mentioned in the following diagram, which shows the relationship between Vdsat and drain current of a certain nMOS in all regions of its inversion:


For explanation I'd suggest to read the mentioned book.

Re: Vdsat in weak inversion

Thanks for the detailed answer!
I am reading the book now. Could you gave some advice relating to how to study the book?
Btw, could you send me the excel file?

Re: Vdsat in weak inversion

anhnha said:

Could you gave some advice relating to how to study the book?

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Depends on your knowledge (I studied the whole book). If you just want to learn about the Vds,sat behavior, read chapter 3.7.3 Drain–Source Saturation Voltage.

anhnha said:

Btw, could you send me the excel file?

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I (later) found out it's exactly the same as Fig. 3.16 in the before mentioned chapter. If you want the XLS file anyway, just tell me.

Re: Vdsat in weak inversion

Thank you, I am reading it from beginning. There are lot of math here. Do you check/ prove them all? I will post more and ask for help as I read along thr book. Hope you could help.

Re: Vdsat in weak inversion

That's fine. Don't be afraid of the math, very seldom it needs more knowledge than the 4 basic arithmetic operations. On the other hand - for an overview it's not necessary to retrace all the formulas and equations - just believe the author. During a first overview reading it's enough to keep in mind which parameters - i.e which MOSFET properties are related with each other - otherwise you'd probably lose the desire to go on reading - and lay the book away ;-).

Only if you want to really understand the full context of a special property dependency, then dig deeper and try and retrace and understand the equations. If then you have difficulties to understand such a context, you're welcome to return to the forum with your question. Don't forget to clearly describe the problem and what - and perhaps why - you don't understand (this special context).

Re: Vdsat in weak inversion

If you're interested in the physics, sure you can go through all the derivations yourself. But the bottom line is that in weak inversion/subthreshold regions, Vdssat approaches 4Φ_t≈100mV and becomes independent of Vgs.

Re: Vdsat in weak inversion

mtwieg said:

Vdssat approaches 4Φ_t≈100mV and becomes independent of Vgs.

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... and becomes independent of Vgs - with the restriction that Vgs biases the MOSFET in deep weak inversion.