Varactors are essential component of LC VCOs. Varactors also occasionally serve to tune the resonance frequency of narrowband amplifiers.

A varactor is a voltage dependent capacitor. Two attributes of varactors become critical in oscillator design:

1. the capacitance range i.e the ratio of the maximum and minimum capacitances that the varactor can provide

2. the quality factor of the varactor which is limited by the parasitic series resistances within the structure.

A regular MOSFET exhibits a voltage dependent gate capacitance but the nonmonotonic behavior limits the design flexibility. For example, a voltage controlled oscillator (VCO) employing such a varactor would generate an output frequency that rises and falls as Vgs goes from negative to positive values. This nonmonotonic frequency tuning behavior becomes problematic in PLL design.

A simple modification of the MOS device avoids the above issues called an accumulation mode MOS varactor as shown in figure (a) this structure is obtained by placing an NMOS transistor inside an n-well. If Vg < Vs, then the electrons in the n-well are repelled from the silicon/oxide interface and a depletion region is formed in figure (b). Under this condition, the equivalent capacitance is given by the series combination of the oxide and depletion capacitances. As Vg exceeds Vs, the interface attracts electrons from the n+ source/drain terminals, creating a channel in figure (c). The overall capacitance therefore rises to that of the oxide, behaving as shown in figure (d).

The C/V characteristic of MOS varactors has scaled well with CMOS technology generations, approaching its saturated levels of Cmax and Cmin for Vgs = 0.5V in 65nm devices. These varactors therefore operate with low supply voltages better than their pn-junction counterparts.

Another advantage of accumulation mode MOS varactors is that, unlike pn junctions, they can tolerate both positive and negative voltages. In fact, the characteristic suggests that MOS varactors should operate with positive and negative biases so as to provide maximum tuning range.