Performance Consideration of MOS and Junction Diodes for Varactor Application

The engineering and tradeoffs for quality factor, capacitance tuning ratio, capacitance mismatch, and low-frequency noise for different varactor device structures, geometry sizes, and operations have been compared to provide a comprehensive guideline of an optimized structure for the advanced radio-frequency voltage-controlled oscillator (VCO). Compared with junction varactors, ${\rm n}^{+}$ /n-well (NW) MOS varactors exhibit a higher capacitance tuning ratio ($\sim$ 3) and a superior capacitance mismatch ($≪$ 0.1}% for a drawn size of 25 ${\rm \mu}\hbox{m}^{2}$). However, a noticeable low-frequency noise $(\sim \hbox{10}^{-18}\ \hbox{A}^{2}/\hbox{Hz})$ was observed in the accumulation-mode ${\rm n}^{+}/{\rm NW}$ MOS varactors. A derived figure of merit in selecting a suitable varactor in terms of VCO's phase noise has been proposed for the first time. The measurement was performed by using the 0.18- ${\rm \mu}\hbox{m}$ 1P6M with AlCu backend wafer.