[HH] n channel mosfet questions

[Greg London]

I'm looking for an N channel mosfet that can be turned on/off
with 5v/0v on the gate and can sink a hundred milliamps or two
of load.

I found one example here:

http://search.digikey.com/us/en/products/SI2312BDS-T1-E3/SI2312BDS-T1-E3CT-ND/1656844

Rds On (Max) @ Id, Vgs   is listed as
31 mOhm @ 5A, 4.5V

[question 1]
I'm I making this too hard?
I can't seem to find very many of these at all.
Most datasheets list the "on" gate voltage at around 10v.
Are there a bunch of mosfets I can use to switch 5v that
I'm just not seeing?


On a related n channel mosfet topic,

[2] Can I simply hook a 5v TTL logic output to the gate of a mosfet?
A lot of schematics show a resistor from gate to source,
but usually say this is to drain the capacitance on the gate pin.
If I use the output pin of a 7408 AND gate and tie it directly
to the gate of the mosfet, won't the 7408 drain the capacitance
when it outputs zero volts? It seems like the resistor is for
when the thing driving the mosfet gate is either 5v or tristated, and
doesn't actually drive to 0 volts.

[3] Some schematics also show a resistor between whatever is driving
the mosfet and the gate pin of the mosfet. If the gate to source
impedance on most mosfets is 6.28 giga-watts, I mean giga-ohms,
then I'm not sure why bother adding another resistor in there.
A resistor into the base of a NPN BJT transistor will limit the
current into the base, but that's because the base is just a diode
drop away from the emitter.

Thanks for any help,
Greg