Electrical – Why does this MOSFET switch on so slowly

mosfetswitches

I am learning Tina and drew this simple MOSFET switch:

simple MOSFET switch

Adjusting P1 will at a certain point switch on the MOSFET, which I expected to saturate immediately. However when I look at the DC transfer characteristic, it turns on 'slowly' when the gate voltage moves between 3.5V and 4.7V

DC transfer characteristic

I confirm this by setting P1 to 4.3%, where the MOSFET seems to be half-conducting and thus dissipating 249W:

voltages & currents

How can this be? I thought that this kind of MOSFET was a switch but it seems to behave like a BJT (albeit in a narrow gate voltage range)?

EDIT: Thanks to for the answers, all +1, which thoroughly enlightened me; I accepted Andy's answer simply because it corresponded best to my lack of understanding.

Best Answer

However when I look at the DC transfer characteristic, it turns on 'slowly' when the gate voltage moves between 3.5V and 4.7V

I'm assuming here that you don't mean "slow" in time but that you mean it doesn't instantly turn fully-on as you reach some precise voltage threshold.

This is what a MOSFET does - if you wish to use it as a switch then you apply a suitable voltage level on the gate (relative to the source) in as quick a time as you can. Maybe 12 to 15 volts or, for logic level MOSFETs, only 5 volts is needed. Then it behaves like a switch.

If you wish to use the MOSFET as an amplifier then you apply a linear voltage to the gate to get a (somewhat) linear voltage at the output. Notice the voltage gain in your circuit - For a change in gate voltage of about 1.15 volts you get a change in output voltage of about 100 volts. That's a gain of about 87 if you want it in simple numbers.

If you looked at gate voltage from 4.3 volts to 4.5 volts, the output voltage changes about 25 volts - that's a gain of 125.

it seems to behave like a BJT (albeit in a narrow gate voltage range)?

A biploar transistor might totally switch on with a base-emitter voltage change from 0.6 volts to 0.7 volts. As a range that is 0.1 volts with an offset of 0.65 volts and if you compared this to your mosfet switching on in a range of 3.5 to 4.65 volts (1.15 volts) it isn't that dissimilar.

0.1/0.65 = 15% and 1.15/4 is about 29% so relatively speaking a BJT and a MOSFET are similar.