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, n+/n-well (NW) MOS varactors exhibit a higher capacitance tuning ratio (~3) and a superior capacitance mismatch (< 0.1% for a drawn size of 25 mum²). However, a noticeable low-frequency noise (~ 10^-18 A²/Hz) was observed in the accumulation-mode n+/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- mum 1P6M with AlCu backend wafer.