Cross-view of a dual-gate N-channel depletion type MOSFET is shown in figure. It acts as if two FETs are connected in series, as is obvious from figure. The middle block acts as drain for unit no 1 and source for unit no 2. Thus the current flow through the MOSFET is controlled by the voltage of both the gates. Drain current ID can be cutoff by making either gate sufficiently negative.