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This paper discusses material and device engineering in field-effect transistors (FETs) with HfO2-based ferroelectric gate insulators to attain a precipitous subthreshold swing (SS) by exploiting negative capacitance. Our physical analysis based on a new concept of a negative dielectric constant reveals that fully depleted silicon-on-insulator (FD-SOI) channels with a modest remnant polarization P r (3 µC/cm2 at most) are more suitable for realizing SS < 60 mV/decade than a higher P r of 10 µC/cm2, which is commonly reported for HfO2-based ferroelectric materials. We also confirm SS < 60 mV/decade in more than 5 orders of the subthreshold current in FD-SOI FETs with ferroelectric HfO2 gate insulators by device simulation.

Phonon transport in silicon nanowires (Si NWs) doped with isotopes is investigated theoretically. The ballistic thermal conductance and diffusive thermal conductivity are calculated at room temperature using the phonon dispersion relations derived through a semiempirical atomistic approach. The thermal conductance and conductivity in 28Si NWs randomly doped with 29Si are smaller than those in the corresponding pure 28Si NW, which can be fully explained by the effect of isotope impurities on the dispersion relations. In [001]-oriented 28Si NWs having a square cross section with a side length of 1.086 nm and randomly doped with 29Si, the maximum reduction in thermal conductivity reaches more than 20%. This reduction leads directly to an improvement in the thermoelectric figure of merit by more than 25%. It is also found that the impact of isotope impurities on phonon transport becomes large with increasing mass difference between the constituent and impurity isotopes or with increasing wire cross-sectional area. Phonon transport in isotopic core–shell Si NWs is also investigated. Some of these Si NWs show increased thermal conductance and conductivity although the increase is very small.

We fabricated suspended straight and corrugated Si nanowires (NWs) from 55 nm thick Si-on-insulator and studied their thermal conductivity using Raman mapping. We demonstrate that corrugations induce 60%–70% reduction in NW thermal conductivity at temperatures 300–400 K. This proves the significance of ballistic phonon transport at these temperatures in sufficiently thin Si NWs and the efficiency of corrugations in thermal conductivity reduction for application in thermoelectricity. The experimental results presented here are in agreement with our NW thermal conductance calculation taking into account the effect of corrugations on low-frequency acoustic phonon branches.