"Paul Burridge" schrieb im Newsbeitrag
...
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 datasheet:
http://www.fairchildsemi.com/ds/J3/J310.pdf