for an n-channel mosfet the body diode is pointed from the source to the drain. so a positive voltage on the drain is not conducted through the body diode normally. if you apply the positive voltage to the source then current is conducted through the body diode to the drain, and there is a voltage drop across the body diode there, called the forward bias.
by measuring the forward bias you can tell that the body diode remains intact and you can compare all the mosfets since the forward bias should be identical on all of them.
the body diode is inherent in the structure of the mosfet because of the way they are manufactured by the doping of the source overlapping the doping of the drain.
so for an n-channel mosfet the drain is always at the higher potential (so the body diode does not conduct) and does not conduct until the channel is turned on by raising the gate voltage and the gate is a conductor, planar in nature, separated from the conducting channel by the gate oxide, and as the potential on the gate is raised then that draws the conducting majority carriers, electrons, into the channel so that current can be conducted through the channel from drain to source. (actually the electrons go the other way). the reason the channel does not conduct when there is no charge on the gate is because there are no available conducting electrons in the channel and whatever ones that accidentally escape are drawn back into the drain by the depletion layer.
anyway that is why you have to use the higher voltage probe, the red probe of the diode tester, and of the ohmmeter too, on the drain to measure to see if the mosfet conducts. so that is why i try to get people to measure the body diode. most just use an ohmmeter.
totally lost.. I did pull apart the controller and all 12 mosfets seems fine (no burn damage or abnormal lumps)
