Analytical inversion-mode varactor modeling based on the EKV model and its application to RF VCO design

An analytical modeling approach for the CV-characteristics of inversion-mode MOS varactors that is based on the continuous EKV model equations is presented. Based on this approach it is possible to obtain an analytical expression for the effective large signal capacitance of varactors incorporated into a VCO and to calculate and optimize the resulting VCO tuning sensitivity K_VCO. We present a simple but quite accurate hyperbolic tangent approximation for the CV-characteristic of inversion-mode MOS varactors that depends on selected design variables and is therefore well suited to be used in a systematic VCO design flow. In order to verify the validity and accuracy of the modeling approach the CV-characteristics obtained by using the EKV based simulation model are compared with Spectre (Cadence) simulations using a BSIM 3.3 transistor model. As reference semiconductor technology we use a 0.35 μm CMOS process (C35) from austriamicrosytems (AMS).