Page "Field-effect transistor" Paragraph 14

Conversely, in an n-channel enhancement-mode device, a positive gate-to-source voltage is necessary to create a conductive channel, since one does not exist naturally within the transistor.

The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel.

But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET ; this forms a region free of mobile carriers called a depletion region, and the phenomenon is referred to as the threshold voltage of the FET.

Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain ; this process is called inversion.