Guard band structure for n-channel MOS transistors

In a depletion mode MOSFET, a conducting channel exists in the region between the source and the drain which is not covered by the gate. This channel appears as a leakage between the source and the drain even when the MOSFET´s are supposed to be cut off and lowers the drain output impedance when the MOSFET is conducting. This situation is aggravated in an integrated circuit when a metallic interconnection with drain potential runs over the oxide. For this reason, it has been very difficult to integrate monolithically the n-channel enhancement mode transistor. The leakage between the source and drain of an n-channel MOSFET can be eliminated by a p⁺guard band. If the conductivity is high, the surface states on the oxide are insufficient to create an inversion layer. The new MOS transistor has an open structure i.e. the drain does not completely surround the source and the source does not completely surround the drain. The metallic gate covers the active channel region between the source and the drain. A p⁺guard band surrounds the active MOS transistor. Two inward extensions of the guard band are inserted into the channel region between the source and the drain and under the gate. If the p-type substrate in the drain compartment is inverted into the n-type surface as does the substrate in the source compartment, the p⁺guard band prevents the two inversion layers from conducting. Thus, the leakage between the source and the drain is avoided. This structure has been used to construct n-channel MOSFET´s of both depletion mode and enhancement mode. For the depletion mode MOSFET´s, output impedance comparable to junction-type FET´s has been achieved. This structure proves to be a practical method for achieving stable enhancement n-channel MOSFET´s. The use of this structure in a practical video amplifier will be illustrated.