A Technique for Enhancing Varactor’s Quality Factor in Millimeter-Wave Frequencies

Varactors are key devices in the design of voltage-controlled oscillators (VCOs), as their quality factor (Q-factor) at the millimeter-wave frequency range limits the overall LC tank Q-factor. This brief presents a new circuit-level structure, called Q-enhanced varactor (QEV), that is composed of two metal-insulator-metal capacitors (MIMCAPs) and a varactor. Depending on Q-factors and the values of the MIMCAPs, the Q-factor in QEV can be improved more than two times. The QEV parameters are analyzed and a design procedure for QEV is proposed. Also, a QEV with two times Q-factor improvement is employed in the design of a 40-GHz LC-VCO, illustrating more than 6-dB enhancement in the oscillator figure of merit.