A physical model of a CMOS varactor with high capacitance tuning range and its application to simulate a voltage controlled oscillator

A physical model of a CMOS varactor with high capacitance tuning range in multi-finger layout is presented. The model describes the voltage dependent capacitances and resistances and the parasitics due to connecting wires. Simulations of frequency tuning of a LC-oscillator employing the varactor are verified against measurements.

I. Introduction

Integrated voltage controlled capacitances (varactors) are widely used as frequency tuning elements in integrated LCtank voltage controlled oscillators (VCOs) for Radio-Frequency circuits. Together with an integrated inductor, the varactor builds the core of these VCOs. As the inductance is constant, the varactor needs to provide a large capacitance tuning range to allow for a large frequency tuning range of the VCO. Moreover, high quality factors of the inductor and the varactor are necessary to achieve low phase noise of the VCO.