N-type channel mosfet
For an NMOS transistor the channel itself consists of electrons. They are negative mobile charges flowing from the source to the drain.

Before the channel is created the region under the gate is depleted. By applying a small (!) positive voltage between gate and bulk electrons are attracted. These electrons fill the existing holes of the p-type material and the acceptor atoms loose their holes. They are said to be uncovered and act now as fixed negative charges. Since they are fixed, they won't conduct a current.

So first we have fixed negative charges underneath the gate but so far no channel. By further increasing the voltage at the gate more and more atoms become uncovered and the potential at the surface increases. Finally an energy level is reached where a larger number of electrons can exist and a channel is formed.

As soon as we have a channel the situation is as follows. There is a region with fixed negative charges with a layer of mobile charges on top of it. This layer, the channel, connects drain and source and it is possible to have a flow of electrons from source to drain.