Abstract: An improved model for accumulation-mode MOS varactor RF devices that can describe the characteristics of the device with simple equations valid in all operating regions is presented.Equations of the improved model are continuous and differentiable to any order.Derivative results fit measurements accurately to three orders at least.Drawbacks,such as derivability but with wrong results and noncontinuous voltage dependent resistance,are solved.The model is finally used to model a 30-gate-finger (channel mask length,L=1μm,finger width,W=4.76μm) accumulation-mode MOS varactor,which is fabricated with a 0.25μm RF-CMOS process supplied by Chartered Semiconductor Manufacture Ltd.(CSM) RF-CMOS technology.Comparison between the simulated and measured C-V,R-V,Q-factor characteristics,and S-parameters up to 39GHz demonstrates the excellent accuracy of the model.

Key words: accumulation-mode MOS varactor, RF model, equation, continuous, derivative