Please help! I have done a lot of pouring over internet searches to try and understand the operation equations for a n type enhancement mode MOSFET for quite some time now. All textbooks limit equations for such devices to currents flowing from drain to source only. But I do know that current can flow from drain to source (reverse from convention). Can anyone help? I am using this type of FET in a level shifting circuit for a I2C bus. This is a very common application I believe, but I can't find good details to explain the reverse current. Here is a example circuit.