Environmental Engineering Reference
In-Depth Information
If we apply a negative voltage to the gate, negative charges will be stored
in the polysilicon while positive charges will be attracted to the channel
region thus increasing the channel doping to p+. This situation leads to an
accumulated channel. Source and drain are electrically separated because
they form two back-to-back diodes with the substrate. Even if we positively
bias either the source or the drain, only a negligible current (the leakage
current) will flow from the biased n+ regions to the substrate.
By applying a positive voltage to the gate, positive charges will be stored
in the gate. Below the silicon oxide, if the gate voltage is small, positive free
charges of the p- substrate will be repelled from the surface thus depleting
the channel area. A further increase in the gate voltage leads to negative free
charges being attracted to the channel that thereby becomes an n region. In
this condition the channel is said to be inverted.
