A MOS Model 9 Extension for GHz CMOS RF Circuit Design

Abstract

This paper presents an extension to the compact MOS model 9 that enables accurate simulation of CMOS RF circuits in the GHz range. The MOS model 9 is generally accepted for low frequency design, but it is quite inaccurate at GHz frequencies if device parasitics are not considered. The presented model is based on a MOS model 9, extended by a network of parasitics, consisting of six resistors, five capacitors, and two JUNCAP diode models. A model developed for a 0.25 µm CMOS technology shows good accuracy in the measured frequency range up to 12 GHz and over a wide bias range. By applying simple rules for scaling of parasitics and a unit transistor layout approach, the model also shows scalability with respect to device width. The model also predicts third-order intercept point with good accuracy, and simulations and measurements on a 2 GHz CMOS amplifier shows also good agreement. INTRODUCTIONWith today's sub-micron CMOS technologies, it is now possible to implement CMOS RFICs for operation in the GHz. Although CMOS is notorious for its inferior performance as an RFIC technology, it certainly has very interesting properties for realizing competitive radio transceiver solutions. Among the most important advantages is the possibility of integrating RF analog circuits together with large-scale digital circuits, and also the important potential for low cost. A major obstacle in designing CMOS RFICs is the lack of adequate models that predict device behavior at GHz frequencies. For low frequency analog and digital circuit design, compact MOS models such as MM9 (MOS model 9) [4], BSIM3v3 [5] and EKV [6] are widely accepted, but applying these models at GHz frequencies without parasitic effects gives inaccurate results [7]. A clear example of this is given by the s-parameter plots shown in Fig. 1. This paper describes an extension to the compact MM9 by a network of parasitic elements, which enables accurate prediction of NMOST (nchannel MOS transistor) s-parameters in a wide bias range and for frequencies up to 12 GHz.