Capacitance estimation of PN varactors based on unit cells

Integrated varactors are key elements in radio frequency integrated circuits. In this paper, several structures of varactors, based on p-n junction cells, are considered. A capacitance model is required to use these structures in a CAD environment. We have developed a model which considers area and perimeter capacitances of adjacent unit cells so as the reverse voltage dependence. Tuning ranges and quality factors are all well estimated. This model is applied to four varactors and their respective unit cells fabricated with AMS SiGe 0.35 mum technology. Modeled values are compared with measurements and the error between our estimations and measurements are smaller than 7% in all cases.