Equivalent circuit model for capacitances in PN varactors with buried channel

Abstract

In this work, varactors based on PN junction cells have been studied in order to obtain a capacitance model for radiofrequency applications. These cells are unit cells: the minimum structure that can be considered a varactor, including all necessary layers and connections. Then, a specific capacitance for a radiofrequency integrated circuit (RFIC) is obtained horizontally and vertically overlapping the necessary cells. Our circuit model estimates the total capacitance in a varactor, from two ports, considering all relevant internal contributions. Three varactors based on the same cell have been designed and fabricated in 0.35 mum SiGe technology. These novel structures were also on-wafer measured for frequencies ranging between 0.5 to 10 GHz, and voltages varying from 0 to-5 V. This circuit model predicts the capacitance in all cases with relative error under than 4%