High- $Q$ Thick-Gate-Oxide MOS Varactors With Subdesign-Rule Channel Lengths for Millimeter-Wave Applications

Thick-gate-oxide (TK) MOS structures with subdesign-rule (SDR) channel lengths located in n-wells are used as accumulation mode varactors to improve Q-factors to higher than 100 at 24 GHz, which is about five times that of conventional thin-gate-oxide (TN) MOS varactors. The varactor capacitance ratio (xi = C_max/C_min) is ~1.6. The structure was implemented using the United Microelectronics Company 130-nm logic CMOS technology. The varactors should be particularly useful in circuits for narrowband systems operating near 76 and 94 GHz. Use of TK MOS varactors also makes the C-V characteristics monotonic and mitigates the expected loss associated with the leakage current of TN layer at the 45-nm technology node and beyond.