Re: measuring Vgs(off)

From: Helmut Sennewald (helmutsennewald_at_t-online.de)
Date: 09/21/04 

Date: Tue, 21 Sep 2004 20:56:00 +0200

"Paul Burridge" <pb@notthisbit.osiris1.co.uk> schrieb im Newsbeitrag
news:0th0l094c5c3f8mk0ikgcr4dp3uglb4umv@4ax.com...
> Hi guys,
>
> I saw in Malvino's Electronic Principles that it is stated that Idss
> and gfs (the transconductance/gain) are easy to measure, whereas
> Vgs(off) is not and that manufacturers calculate it from this formula
> (hope I've remembered it right)
>
> Vgs(off) = -2*Idss/gfs
>
> I've just checked out this assertion by measuring Vgs - v - Id for a
> bunch of assorted FETs and found that I could easily establish the
> pinch off voltage to within about 0.1V either way. Contrary to what
> the book says, I personally have found it a simple matter to measure
> Vgs(off). So why do they make out it's a big deal?

Hello Paul,
Vgs_off seems to be often specified at Id=1nA. The measurement
at such low current levels takes a lot of time and it requires a
very clean test fixture.

How have you measured at exactly Id = 1nA +/-0.1nA ?

Best Regards,
Helmut

A typical data***:
http://www.fairchildsemi.com/ds/J3/J310.pdf

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