So I'm currently studying power MOSFETs and I'm getting a bit confused about when and how it conducts current.
Let's start with the normal MOSFET functioning. I'm considering a n-enhancement type MOSFET.
So, for \$ v_{gs} <0 \$ (or less than the threshold voltage), there is no inversion layer and therefor no channel. The MOSFET does not conduct. Now for \$ v_{gs} >0 \$ the channel is formed and the MOSFET can conduct through if a positive \$ v_{ds} \$ is applyed. Everything ok until here.
Now my doubt is how the internal diode takes place here. I know why the internal diode existes (shorting the p substract to the n+ terminal through the metalization) but what I don't know is when does it conduct.
For the situation we already checked: \$ v_{gs} >0 \$ and \$ v_{ds} \ >0 \$ it definitely does not conduct since it's a reversed junction (it has just a small parasitic current). The same can be said if \$ v_{gs} <0 \$ and \$ v_{ds} \ >0 $.
But now what if \$ v_{gs} >0 \$ and \$ v_{ds} \ <0 \$. I was told that until \$ v_{ds} \$ reaches a certain negative limit, the conduction still occurs essentially through the inversion layer. Then, after that limit, the conduction is taken by the diode. Is this correct?
And what about when \$ v_{gs} <0 \$ and \$ v_{ds} \ <0 $. In this case there is no channel but the diode is forward biased therefore, will there be a current? Is this why the internal diode is useful? So that the MOSFET can also conduct when there is no channel and therefore, apply it to reversible converters.
I'm a bit confused if someone can clarify me all this situations it would be very helpful.
Thanks in advance!