Band-to-band tunneling limited ambipolar current in black phosphorus MOSFETs

Black phosphorus (BP) has emerged as a promising channel material for highly scaled transistors with a high mobility (1000 cm2/Vs), tunable band gap (0.3 eV-2.0 eV), and anisotropic effective mass [1]-[4] at ultra-thin body thicknesses. While devices with high performance in the ON-state have been demonstrated by using Schottky contacts, the subthreshold characteristics have been poor [5],[6]. This is caused by large ambipolar carrier injection from the drain due to the relatively small, direct band gap of BP and a low effective mass. Recently, it has been shown that using electrostatically doped (ES-doped) source and drain contacts can improve the subthreshold characteristics in BP MOSFETs [7], however, the potential advantage of this approach was not well quantified. In this work, we directly compare BP MOSFETs with ES-doped and Schottky contacts, and show greatly improved subthreshold and off-state leakage behavior in ES-doped devices, particularly at high drain bias. Using a one-dimensional transport model, we further show that the advantage of the ES-doped devices is due to the ambipolar current being limited by band-to-band tunneling. Experimentally exploring BTB-tunneling in BP is a first step towards developing BP TFETs.