Re: [SI-LIST] : Idsat variation

D. C. Sessions (dc.sessions@vlsi.com)
Fri, 20 Aug 1999 15:41:58 -0700

Mike Degerstrom wrote:
>
> Jim,
>
> Thanks for your reply. I'm no device physicist but I don't
> think Idsat should be a function of voltage.

Of course it is. You even point it out, below.

> Certainly voltage
> variation affects a CMOS buffer drive strength. However
> I was asking for Idsat for the following equation relating drain current
> to the gate-to-source voltage for a CMOS device in the ohmic region:
>
> ID=IDsat(VGS-VT)^2
-------------^^^

This doesn't work. The dimensions are wrong. Perhaps you meant

Id=Idsat*Vds/(Vgs-Vt)

which is equivalent to

Id=K*Vds*(Vgs-Vt)

But in all cases Id depends on Vgs which is generally dependent on Vdd.

> Also, I haven't been able to find the relationship between Idsat
> and temperature in my text books.

It's not simple because Vt and transconductance have opposite
temperature responses, so the effect on Idsat depends on the
bias point. Clever designers (this should come as no surprise)
take advantage of this to produce low tempco current references.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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