Large-Velocity Saturation in Thin-Film Black Phosphorus Transistors.

A high saturation velocity semiconductor is appealing for applications in electronics and optoelectronics. Thin-film black phosphorus (BP), an emerging layered semiconductor, shows a high carrier mobility and strong mid-infrared photoresponse at room temperature. Here, we report the observation of high intrinsic saturation velocity in 7 to 11 nm thick BP for both electrons and holes as a function of charge-carrier density, temperature, and crystalline direction. We distinguish a drift velocity transition point due to the competition between the electron-impurity and electron-phonon scatterings. We further achieve a room-temperature saturation velocity of 1.2 (1.0) × 107 cm s-1 for hole (electron) carriers at a critical electric field of 14 (13) kV cm-1, indicating an intrinsic current-gain cutoff frequency ∼20 GHz·μm for radio frequency applications. Moreover, the current density is as high as 580 μA μm-1 at a low electric field of 10 kV cm-1. Our studies demonstrate that thin-film BP outperforms silicon in terms of saturation velocity and critical field, revealing its great potential in radio-frequency electronics, high-speed mid-infrared photodetectors, and optical modulators.