Enhancement in Capacitance and Transconductance in 90 nm nFETs with HfO2-ZrO2 Superlattice Gate Stack for Energy-efficient Cryo-CMOS

We show from cryogenic RF characterization that the negative-capacitance (NC) mixed-ferroic HfO
2
-ZrO
2
superlattice (HZH) gate stack [1] on SOI nFETs provides dispersion-free capacitance enhancement (< 8 Å EOT) over regular HfO
2
high-k gate stack up to 40 GHz all the way down to 77 K. Furthermore, the electron injection velocity is unaffected by the capacitance enhancement and shows a 40% increase at 77 K, leading to a record-high intrinsic transconductance of 2.03 mS/$\mu$m from $L_{g}=90$ nm SOI nFETs, which is 19% higher than from regular HfO
2
gate stack. By elaborate analysis of the DC characteristics with a newly modified 2-valley Cryo-MVS model, we show that degeneracy statistics is important for the thermal velocity at low temperatures and can be harnessed to boost I
on
with EOT reduction. Projected $I_{on}/V_{dd}$ from 5-nm nanosheet nFETs with $L_{g}=16$ nm can reach 1 mS/$\mu$m under $V_{dd}=0.2V$ with a 6.5 {\AA} EOT demonstrated previously from the HZH gate stack [1], suggesting the low EOT NC gate stack as an attractive technology booster for energy-efficient cryo-CMOS.