Analysis of PN Integrated Varactors with N+ Buried Layer Varying P+ Diffusions Contour for RF Applications
- 1. Analysis of PN Integrated Varactors with N+ Buried Layer Varying P+ Diffusions Contour for RF applications J. García, B. González, M. Martín-Marrero, I. Aldea, J. del Pino, and A. Hernández Dto. de Ingeniería Electrónica y Automática (DIEA) & Instituto Universitario de Microelectrónica Aplicada (IUMA). Universidad de Las Palmas de Gran Canaria (Spain) Publicated in XXII Design of Integrated Circuits and Systems Conference (Internacional), Sevilla, España, 2007 Abstract In this work integrated varactors based on PN junction are studied. They are designed and fabricated in 0.35um SiGe standard process for radiofrequency applications between 500 MHz and 10 GHz. The varactor performance can be improved varying the geometry of P+ and N+ diffusions, which minimum separation is estimated by numerical simulations. A buried N type layer under varactors is used, connected to N+ diffusions. Thus, novel structures called crosses, fingers, donuts, and bars are proposed. From measurements of Sparameters the diffusion geometry influence on capacitance, quality factor and tuning range of varactors are studied. Furthermore, a library including varactors with scaled capacitance is provided. Design methodology Measurements & Simulations Figure 8 a) Capacitance vs. reverse voltage at 900 MHz Capacitance vs. reverse voltage at 9.1 GHz Figure 3 Geometry and grid for Figure 2 Layer structure in 0.35 SiGe overlapping lateral depletion with technology for PN varactors design TAURUS DEVICE 0 Figure 10 a) Quality factor for different varactors b) Capacitance Library vs. reverse voltage at 900 MHz Conclusions The influence of P+ diffusion geometry on varactor performance has been analyzed. The selected geometry strongly depends on the varactor parameter to consider. Thus, the maximum tuning range is obtained when cross diffusions are used in basic cells. However, Figure 5 Layout of PN basic cells layout the highest quality factors are produced by bar basic cells. In all cases the ratio capacitance-area is higher than that for the test varactor, being maximum when using donut basic cells. On the other hand, a library with scaled capacitances has been proposed, useful for DVB applications, i. e.. Acknowledgments Figure 7 Photography of the varactors under study This reported work is supported in part by the Spanish MEC under projects TEC-2005-08091- C03-02 and TEC-2005-06784-C02-02. INSTITUTO UNIVERSITARIO DE MICROELECTRÓNICA APLICADA (IUMA) UNIVERSIDAD DE LAS PALMAS DE GRAN CANARIA (ULPGC)