High-Q MOS Varactor Models for Quasi-Millimeter-Wave Low-Noise LC-VCOs

Nobuyuki ITOH

IEICE TRANSACTIONS on Fundamentals of Electronics, Communications and Computer Sciences   Vol.E97-A    No.3    pp.759-767
Publication Date: 2014/03/01
Online ISSN: 1745-1337
DOI: 10.1587/transfun.E97.A.759
Print ISSN: 0916-8508
Type of Manuscript: Special Section PAPER (Special Section on Analog Circuit Techniques and Related Topics)
MOS varactor,  quasi-millimeter wave,  modeling,  LC-VCO,  

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This paper presents the strategy of MOS varactor's high-Q optimization, a novel scalable model for the quasi-millimeter-wave MOS varactors, and confirmation results by discrete MOS varactors and VCO measurements. To realize a high-Q MOS varactor in the quasi-millimeter-wave region, low MOS varactor capacitance and low series resistance of unit cell are essential. Downsizing is a key to realize both low capacitance and low resistance. However, it is induced by Cmax/Cmin reduction, simultaneously. Therefore, scalable MOS varactor model is necessary to use optimum MOS varactor to cover various application requirements using same process. Decreasing the MOS varactor's size of W/L =2µm/2µm to 0.5µm/0.26µm, the Q factor increased sevenfold at f =20GHz but Cmax/Cmin is reduced by 60%, by using conventional PSP model, an error of approximately 20% is shown. Proposed model has been improved its accuracy from 18.9% to 0.2% for N+ MOS varactor and from 22.1% to 0.8% for P+ MOS varactor, for minimum size of MOS varactor even if model covers wide dimension range. Also, it has been confirmed this model is covered in two types of layouts. Oscillation frequency and phase noise also have been confirmed by three types of 22GHz VCOs. The accuracy of oscillation frequency is less than 2.5% and that of phase noise at 1MHz offset from carrier is less than 5dB.