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 (WO). Compared with junction varactors, n(+)/n-well (NW) MOS varactors exhibit a higher capacitance tuning ratio (similar to 3) and a superior capacitance mismatch (< 0.1}% for a drawn size of 25 mu m(2)). However, a noticeable low-frequency noise (similar to 10(-18) A(2)/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-mu m 1P6M with AlCu backend wafer.