Here is a simple explanation of nMOS based on the lecture in the univ which I took.
There’s no argument for chemical process, nor effect of the voltage of gate. It just explains relationship that whether the current passes or not when gate is charged.
In a unit of nMOS transistor, two zones rich in electrons, are placed in a substrate rich in positive holes.
Two zones are placed in a definite distance to each other and between them a chargeable material, called gate, is faced indirectly.
When gate is not charged, two electron-rich zone are surrounded by positive holes and there is no current between them.
When gate is positively charged, electrons from substrate, though not rich but sure to exist, is pulled near to gate, e.g. between two electron-rich zones. Therefore a route for electrons to move between these zones.
Actually, one of the electron-rich zones is connected to the ground, to obtain limitless electrons. Thus it is called source.
The other one is connected to a higher voltage, usually, the same power as that for gate. It is called drain, because it drains electrons from source, when gate is charged.
As above picture, substrate is also grounded for stability.
Therefore in nMOS, when gate is (positively) charged, the current runs from drain to source. When not, there’s no current.