Mosfet

I have a 2N7000 MOSFET and was wondering if a resistor is needed at the gate.
I'm using 5V and Drain current is only 20mA when transistor is "ON"

Gate resistors can stop oscillations from happening, but they need to be small enough that the driving current can get the gate loaded with electrons quickly. If you're just piddling around with a low speed experiment, you may get away with no resistor at all. It isn't so much about current as it is about speed.

The datasheet for the 2N7000 Mosfet says that it needs a gate-source voltage at least 4.5V to conduct 75mA and a gate-source voltage of 10V to conduct 500mA.
Maybe you are using it as a source-follower with a gate-source voltage of only 3V.

A gate resistor is basically intended to keep the circuit stable and prevent what are called "parasitic oscilations". Bychon is correct that these resistors need to be small in value...

It does sound as if the mosfet isn't turning on all the way, the specs on this device tell me that you probably need to select something that will work on 5V rails as this mosfet really requires a higher voltage to fully snap on or off. Still, don't be afraid to experiment, that's how we all learn!

Mad Soundman

Perhaps he means that the intended load is only 20mA.

drkidd22 said:

I have a 2N7000 MOSFET and was wondering if a resistor is needed at the gate.
I'm using 5V and Drain current is only 20mA when transistor is "ON"

Click to expand...

If you're driving the 2N7000 with the output of a microprocessor or digital chip, no rsistor is needed. For logics sake, if using a uP, then a pull up or pull down resistor of 10K will keep the gate in the desired on or off state until the uP finishes being reset and loads the pin drivers. If the line is long, then use a 1K resistor. The gate C is very small on a 2N7000, so unless you are using it in a high speed circuit, 0 to 1K is a good value.