Now let a potential VGS be applied to the gate, making it negative with
respect to the source.The negative gate sets up within the device an electric field
(hence the “field effect”) that tends to repel electrons from the n channel down
into the substrate. This electron movement widens the (naturally occurring)
depletion zone between the n channel and the substrate, at the expense of the n
channel. The reduced width of the n channel, coupled with a reduction in the
number of charge carriers in that channel, increases the resistance of that channel