Integrated circuit varactor having a wide capacitance range

Abstract

Integrated circuit varactor structures that include either an P-gate/N-well or N-gate/P-well layer configuration formed on an SOI substrate. The varactor structure is completely electrically isolated from the substrate of the IC by an oxide layer of the SOI substrate and by oxide-filled trenches formed on both sides of the varactor structures. The isolation trenches preferably extend to the oxide layer of the SOI substrate. The P-gate/N-well varactor structure includes N,implant regions formed in an N-well implant layer of the varactor. The N,implant regions comprise the source and the drain of a varactor. A LOCOS layer may be formed over the N-well layer where the P-gate is formed over the LOCOS layer. The P-gate may be formed of polysilicon. The N-gate/P-well varactor structure includes P,implant regions formed in a P-well implant layer of the varactor. The P,implant regions comprise the source and the drain of a varactor. A LOCOS layer may be formed over the P-well layer where the N-gate is formed over the LOCOS layer. The N-gate may be formed of polysilicon. The P-gate/N-well varactor is ideally suited for use as a binary or digitally-controlled varactor because it abruptly changes from a first lower capacitance of C,to a second higher capacitance of C,as the D.C. control voltage is varied from a first to a second threshold level. In contrast, the N-gate/P-well varactor finds utility as an analog timing varactor of a fully integrated VCO device, for example, because it slowly changes from a first lower capacitance of C,to a second higher capacitance of C,as the D.C. control voltage is varied from low to high threshold voltage levels.