Can an NMOS act as an NPN BJT if Vds is high enough?
As we increase the drain voltage the depletion region around the drain gets wider while the depletion region around the source does not change. If we keep on increasing it further, the depletion region around the drain will finally reach the source side.

This undesired behavior is known as punch-through and can be regarded as an extreme case of channel-length modulation.

For this reason the drain current will then strongly depend on the drain-source voltage.