The MOS Varactor: An introduction

In many IC designs frequency based trimming or control is required. For instance a filter may need to to be trimmed for corner frequencies. A PLL VCO needs to be controlled by changing the frequency based on its feedback signal. An adaptive equalizer needs to shift its pole-zero configuration. These and many other related applications need a device to be voltage controllable, and offer a change of reactance. The varactor is a useful component that is used frequently to do this. In general varactors are assumed to be junction type devices where the depletion capacitance can be changed to vary the reactance. In CMOS or BiCMOS processes another type of varactor is available, almost as a byproduct of the MOSFET structure. This is the MOS varactor It seems that every CMOS process has the capability to produce a MOS varactor. However, although the varactor is available it may have some limitations of Q and sensitivity. In addition CMOS technology vendors do not characterize or optimize their MOS varactors. This is left to those specialized technology vendors who offer high performance or RF type processes. A recent report on the MOS Varactor is available as an introduction at http://www.signalpro.biz/ > engineer's corner for interested parties.