Analysis and Comparison of L-Valley Transport in GaAs, GaSb, and Ge Ultrathin-Body Ballistic nMOSFETs

摘要

We systematically analyze the L-valley ballistic transport of (111) GaAs ultrathin-body (UTB) nMOSFETs by changing the body thickness from 5.79 to 2.86 nm. The $Gammahbox{-}{rm L}$ energy offset is 0.2 eV in a 5.79-nm-thick GaAs, and the single $Gamma$-valley subband primarily governs the transport. This energy offset reduces to 0.054 eV for a 2.86-nm-thick GaAs, and the L-valley transport becomes prominent. For a 10-nm gate, a 0.5-nm effective oxide, and a 0.6 V $V_{rm DD}$, we observe an order of magnitude improvement in the sheet density, a 159% improvement in the on-state current, a 15% improvement in the subthreshold slope, and a 54% improvement in the switching delay from the enhanced L-valley transport. The 3-nm (111) GaSb and Ge channels show the $Ihbox{--}V$ characteristics similar to the 2.86-nm GaAs device. The subthreshold current is primarily tunneling while the on-state current is thermal. Performance comparison of GaAs, GaSb, and Ge MOSFETs against the channel thickness shows a significant improvement of on-state current in the GaAs transistor due to enhanced L-valley transport. We also benchmark the L-valley design against a silicon UTB ballistic nMOSFET of a similar dimension.