Two-dimensional (2D) semiconductor, such as transition metal dichalcogenides (TMDs) and black phosphorus (BP), have been extensively studied for future transistor applications because of the atomic thin layered structure for excellent immunity to short channel effects. BP has been considered to be promising for future p-type channel material, because of its high hole mobility, up to 5000 cm2/Vs at room temperature demonstrated [1-6]. However, the on-current (ION) of transistors made of 2D semiconductors as channel materials haven't been demonstrated to be comparable to silicon metal-oxide-semiconductor field-effect transistor (MOSFETs).