I have a mosfet question..

Willen said:

logic level means?

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In the old days; Logic ran at 5 volts. ( 74ls04 ) and MOSFETS needed 8 or 10 volts to be turned on good. There are "gate drives" that take logic level signals and boost them up to 12 volts. Digital ICs had 0/5V power and the gate drivers had 0/12V power.

There is a class of MOSFETs that have "logic level" in their title. They are designed to be driven from logic ICs with out gate drive buffers.

Willen said:

I used to think that if a mosfet has 3V V(th)/Vgs then it turns ON fully at 3V and turn OFF fully at 2.9V. Clear me please!

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If you look on a datasheet at V(th) which is the gate-source threshold voltage rating it will say "ID= 0.25mA" because that is the Vgs where the Mosfet begins to conduct the 0.25mA low current. The "3V" is the maximum Vgs and "0.8" is the minimum Vgs to produce a current of only 0.25mA for that Mosfet. Some Mosfets with that part number have minimum, some have typical and some have maximum spec's. Most Mosfets turn on fully when Vgs is 10V but a logic-level Mosfet is made to turn on pretty well with a Vgs of 4.5V. The maximum on-resistance is stated in text on the datasheet for one or two Vgs voltages. The graphs on a datasheet are for a "typical" device but when you buy one it might have minimum, typical or maximum spec's.

Willen said:

logic level means?

Then the 'Rds' also depends to gate voltage? Eg. if Vgs/Vth is 3V then Rds= 0.3R, if we applied 2.5Vgs then the Rds= around 0.5R and if we applied 1.5Vgs then the Rds= around 1 or 2 ohms, like this? (And the increment of Rds makes output voltage drop too.)

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"Logic level" means that the MOSFET will fully turn on with minimum drain-source resistance when Vgs is at a voltage equal to typical high logic levels (3.3V or 5V depending upon the specific MOSFET).
"Standard" MOSFETs typically rated at a Vgs of 10V to fully turn on and have minimum Rds).

Yes, in the MOSFET linear region the drain-source resistance varies with Vgs.
But due to manufacturing variation in Vgs(th) the resistance value will vary from unit to unit for a given Vgs.

What is a low voltage system .001 micro volt?

If you switching rapidly, ie not just on/off you could use a gate drive transformer to step up the drive voltage, a torroid ring isnt difficult to design.

Some more info here: **broken link removed**