Beyond Band-Edge Effective Mass: Ultrahigh On-State Current and Electron-Hole Transport Asymmetry in Monolayer PbS MOSFETs

Two-dimensional (2D) semiconductors are commonly evaluated for ultrascaled transistors using band-edge descriptors such as carrier effective mass. Here, first-principles quantum-transport simulations reveal pronounced electron-hole transport asymmetry in monolayer PbS MOSFETs despite their similarly small electron and hole effective masses (<0.23 m0). At a gate length of 9 nm, the n-type PbS MOSFET achieves an ultrahigh on-state current exceeding 7300 μA/μm along the x direction, over 5500 μA/μm higher than that of the p-type device. Position-resolved local density of states and spectral-current analyses reveal that the suppressed hole current originates from a broad low-DOS region within the valence-band transport window, which reduces hole injection. These results demonstrate that transistor performance cannot be fully predicted by band-edge effective mass alone, but is strongly governed by the electronic-state distribution within the transport window. At sub-5 nm gate lengths, enhanced source-to-drain tunneling leads to severe short-channel effects, which can be effectively mitigated through optimized underlap configurations. This work establishes monolayer PbS as a promising channel material for ultrascaled electronics and highlights the importance of beyond-band-edge electronic-structure considerations for evaluating and designing 2D semiconductor transistors.