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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!
 
C

CWatters

Jan 1, 1970
0
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,

FETs are imperfect devices. Sure they turn on at Vgs but the higher the
ACTUAL gate drive voltage the lower the on-resistance. "2.5V Specified"
probably means that the on resistance and other parameters were
measured/specified with the gate at 2.5V.

Many FETs are designed for particular applications. For example you will
sometimes see FETs called "logic level" devices. This means that Vgs is low
enough for them to be driven by 3V or 5V CMOS logic families.
 
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"?
See how they define threshhold voltage. They state Vds=Vgs, meaning
the gate is tied to the drain. Then to get .00025 amps Ids with the
mosfet configured this way you have to apply about .9 volts, more or
less, but it could actually be anything between .4 and 1.5. Make of it
what you will. For a lot of applications it's irrelevant.
Now look at Figure 1. That information is a lot more likely to be
useful.
 
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