28-nm FD-SOI CMOS RF Figures of Merit Down to 4.2 K

This work presents a detailed RF characterization of 28-nm FD-SOI nMOSFETs at cryogenic temperatures down to 4.2 K. Two main RF Figures of Merit (FoMs), i.e., current-gain cutoff frequency (ft) and maximum oscillation frequency (fmax), as well as parasitic elements of the small-signal equivalent circuit, are extracted from the measured S-parameters. An improvement of up to ~130 GHz in ft and ~75 GHz in fmax is observed for the shortest device (25 nm) at low temperature. The behavior of RF FoMs versus temperature is discussed in terms of small-signal equivalent circuit elements, both intrinsic and extrinsic (parasitics). This study suggests 28-nm FD-SOI nMOSFETs as a good candidate for future cryogenic applications down to 4.2 K and clarifies the origin and limitations of the performance.