MOSFET question: Drive voltage vs. Vgs(th)

The threshold voltage is a statistical value. It is typical 0.9V at least 0.4V and at most 1.5V. So if you buy one most likely the threshold will be around 0.9V.

Assuming a simple device behaviour ("square law") the threshold voltage is the value at which the MOSFET starts to turn on. The current you can "drive" is proportional to the square of the difference between gate-source and threshold voltage ("overdrive voltage").

Chris

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.

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.