Improved On-Off Ratio in Black Phosphorus Field-Effect Transistors with True Ohmic Contacts

Two-dimensional (2D) materials have emerged in recent years as a candidate for use in electronic and optoelectronic applications due to their unique properties. One such layered 2D material that has attracted much interest is black phosphorus (BP). BP possesses a thickness-dependent band gap of 0.3 - 2.0 eV, making it a suitable material for field effect transistor (FET) applications. Recent studies of BP FETs have shown ambipolar transport characteristics with very high anisotropic hole mobility. However, several issues still remain. Rapid oxidation in ambient conditions and a relatively low ON/OFF ratio of few-layer BP FETs makes them unsuitable for use in low-power digital electronics. In this work, we present a strategy to fabricate BP FETs that simultaneously overcomes these limitations. The channel is fully passivated by hexagonal boron-nitride layers, protecting it from degradation. We also demonstrate that the relatively low ON/OFF ratio of few-layer BP FETs with Schottky contacts is largely caused by the OFF-state leak-current injected from the drain side, which can be suppressed by the achievement of true ohmic contacts while maintaining the large ON-state current. Consequently, we have realized a high ON/OFF ratio exceeding 10⁶ in p-type few-layer BP FETs.