Why does the voltage at the source side change even though it has no connection to the drain on this NMOS?

Question

I was playing around with transistors and stumbled upon something I dont understand. I have the following circuit below:

Where the NMOS used has a threshold voltage of 5V. This means there will be no connection between drain and source if the gate is connected to ground. I thought this meant the voltage at the source side still would be zero if you switch the drain voltage from 0V to 5V. But when I do this:

The potential at the source side also changes to 5V.

Why does the potential change even though the source side has no connection to the drain?

Answer 1

In your simulation you don't have a load to ground on the source side. Without a load, leakage through the MOSFET channel will cause the source to float up to the voltage of the drain.

However, assuming the MOSFET is off hard, the leakage current is a very small, and a load in the 10MΩ+ level should be more than sufficient to keep the source very near ground potential.

Answer 2

The simulator treats floating nodes as a connection to the ground through very high resistance – I don't know the actual value but let's say 500 GΩ.

The behaviour also depends on the MOSFET model you use. It's unclear what model you selected (although you mentioned a gate-source threshold voltage) but if you select a different one the measurements will be different. LTspice gives me 5V with default model, and 2V with 2N7002. So the internal parameters such as gate resistance, W/L ratio, parasitic capacitances etc. and a virtual resistance across the MOSFET source and ground come into play.