High-Q SOI gate varactor for use in RF ICs

Summary form only given. A high spectral purity voltage-controlled oscillator is known to be the crucial component in achieving an all-CMOS RF transceiver. Full-scale integration of complete transceivers often does not include the VCOs. One of the problems in achieving a good quality VCO is in obtaining a high quality varactor. Gray has formulated that phase noise is inversely proportional to the power dissipation and quality factor product term of the varactor (ISSCC, pp. 45-46, 1997). This paper reports a thin-film SOI gated varactor based on a gated pin diode structure which achieves a nominal capacitance of 7 pF with a sensitivity of 5 pF/V through the anode-cathode voltage. A third terminal (gate) provides additional tuning flexibility and offers 1 pF/V sensitivity. S-parameter measurements reveal a Q of 14. The process is conducted without any modification to an industrial SOI CMOS process. The nominal capacitance can be varied with different gate bias.