Varactors are provided which have a high tunability and/or a high quality factor associated therewith as well as methods for fabricating the same. One type of varactor disclosed is a quasi hyper-abrupt base-collector junction varactor which includes a substrate having a collector region of a first conductivity type atop a subcollector region, the collector region having a plurality of isolation regions present therein; reach-through implant regions located between at least a pair of the isolation regions; a SiGe layer atop a portion of the substrate not containing a reach-through implant region, the SiGe layer having an extrinsic base region of a second conductivity type which is different from the first conductivity type; and an antimony implant region located between the extrinsic base region and the subcollector region. Another type of varactor disclosed is an MOS varactor which includes at least a poly gate region and a well region wherein the poly gate region and the well region have opposite polarities.