Nanoscale CMOS devices display very high peak transit frequency Ft of several hundreds of GHz. This feature can be exploited for reducing power consumption by shifting the operating point towards moderate or eventually weak inversion where the Ft still reaches tens of GHz, high enough for many modern RF applications. This necessitates the use of compact models that are accurate even at such low current densities. The recently standardized charge-based bulk MOSFET compact model BSIM6 has been developed with the aim to provide an accurate description of advanced CMOS processes, including low levels of inversion and RF operation. This paper compares BSIM6 against measurements of a commercial state-of-the-art 40 nm CMOS process, mainly focusing on very low bias conditions, at RF operation. The results validate the accuracy and suitability of BSIM6 for very low-power RF IC design.