# Using a mosfet instead of a transistor

What is the main difference between using a mosfet and a transistor?

Do you need to use a resistor to drive the gate like you do with a transistor?

This caught my eye because if the reply above was true then half of my circuits wouldn't work. Mosfets don't need a series resistor if used as a switch, can be triggered with the 5 volts on an arduino pin, and do have a linear region.

I had a quick google for a good nontechnical overview that explained the differences between bipolar and mosfet transistors but didn't see one. Perhaps someone else here can point to a good site that covers this topic.

BJTs use a small current flow to induce a larger current flow. FETs use the presence of a voltage to induce current flow.

Last week I posted some info about MOSFETs and some links to good MOSFETs use with an AVR: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1222805771/3#3

Also a FET has a much lower ON resistance than a transistor. Typically a transistor might have 0.4V across it when it is fully on where as a FET presents a resistance of anything between 2 ohms and 0.01 ohms when it is on. This means a FET will burn less current when switching heavy loads, that it it won't get as hot as a transistor switching the same load.

Which do I use to determine the series resistor? Vgss or Vgs(th)?
For one mosfet example I looked at, Vgss was 30v while Vgs(th) was 3-5v

How do you calculate the series resistor needed?

You don't need a series resistor in the gate with a FET. What are you asking?

I'm trying to find out why the mosfet is dissipating so much heat.. I'm only using about .200ma of current at 16.8v to drive some LEDs. The mosfet is getting to ~130F even with a decent sink mounted to it. The switch signal is 9.5v so I was wondering if maybe I have the wrong resistor or need to change the value.

as Mike and I have said, you should not be using a series resistor on the gate.

If the FET is getting hot with such a current then it is not being turned on completely and you are in the liner zone.

This is due to either putting a resistor in the gate or not having a high enough gate voltage for the type of FET you are using.

So where do I look in the spec for the gate voltage?

Is it Vgs(th) which is 3-5v or is it Vgss which is 30v?

Look for the specification of the Ron value. Here it will tell you the condition of the gate voltage for that value of the on resistance. Manufacturers want this parameter to be as low as possible so the voltage shown here on the gate will just sufficient to turn it full on.

Other parameters concern the maximum voltage that the thing will work at, that is values you must not exceed.