MOSFET question: Drive voltage vs. Vgs(th)

Can anyone tell me the difference between the drive voltage and the
gate-to-source threshold voltage (Vgs,th) of a MOSFET, if any?


I was under the impression that these refer to the same voltage - the
voltage that is needed to turn the MOSFET 'on'.
However, if you look at the following datasheet for a Fairchild
MOSFET...


http://www.fairchildsemi.com/ds/FD/FDG311N.pdf


you'll see that the title says "2.5V Specified", which I understand
refers to the drive voltage. However, under the "On Characteristics,"
you'll see Vgs(th) min. = 0.4V and Vgs(th) max. = 1.5V. Why isn't
Vgs(th) max. = 2.5V, as the title suggests? Is there a difference
between Vgs(th) and the "specified voltage"?


Thanks!

 

Threshold is where it begins to conduct. This varies a hell of a lot
between FETs so a much better idea is to "turn on" to a few extra volts,
which also affords you a lower ON resistance if you'll inspect the Ids vs.
Vds, Vgs graph (if printed), or R(on) vs. Vg graph.

Tim

 

Threshold is the voltage where it barely begins to conduct. Drive is
the voltage you elect to use in a given situation. If you want it to
conduct hard, drive it way past threshold.

John