Capacitive model for integrated PN varactors of cells with N+ buried layer

Abstract

This paper is devoted to analyzing varactors based on PN junction cells in order to obtain a capacitance model for radiofrequency (RF) applications. A cell is the minimum structure that can be considered a varactor, including all necessary layers and connections. Then, a specific capacitance for a RF integrated circuit is obtained overlapping all necessary cells horizontally and vertically. Our model estimates the total capacitance in a varactor, from both ports, considering all relevant internal contributions. Ten varactors based on the same cell have been designed and fabricated in 0.35 µm SiGe technology. These novel structures were also measured on-wafer for frequencies ranging between 0.5 and 10 GHz, and voltages varying from 0 to -5 V. Our model predicts the capacitances in all cases with relative errors smaller than 10%