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 (<inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-factor) at the millimeter-wave frequency range limits the overall LC tank <inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-factor. This brief presents a new circuit-level structure, called <inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-enhanced varactor (QEV), that is composed of two metal-insulator-metal capacitors (MIMCAPs) and a varactor. Depending on <inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-factors and the values of the MIMCAPs, the <inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-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 <inline-formula> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula>-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.

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