I have another dumb question about MOSFETS. I was working on an RF Power output stage the other day, and I suspected one of the power MOSFETS were faulty. I looked up it's datasheet and it's an N-type Enhancement mode component. My understanding of an Enhancement mode MOSFET is that when you apply bias to the gate, you attract majority carriers which "enahnce" the channel - in other words make it wider. In so doing, you decrease the channel resistance, which causes more current to flow. With a depletion mode MOSFET, the opposite would apply. If you apply bias to the gate you attract majority carriers which depletes the channel - in other words makes it smaller and so increases the resistance between drain and source and causes less current to flow.
I measured the resistance between drain and source with the suspected component removed, and it was an open circuit. Going on the understanding I have above, I figured this to be correct but I've often had trouble diagnosing FETs and I've found that the only real way to check for sure if an FET is faulty is to replace it. So I did that, but before fitting the new one in I measured the resistance between drain and source and it was low resistance! Without any bias applied, I'd have expected an open circuit.
This has thrown my entire understanding of MOSFETS into the bin. Am I confusing something here?
Brian