Black Phosphorus Flexible Thin Film Transistors at Gighertz Frequencies

Black phosphorus (BP) has attracted rapidly growing attention for high speed and low power nanoelectronics owing to its compelling combination of tunable bandgap (0.3 to 2 eV) and high carrier mobility (up to similar to 1000 cm(2)/V-s) at room temperature. In this work, we report the first radio frequency (RF) flexible top-gated (TG) BP thin-film transistors on highly bendable polyimide substrate for GHz nanoelectronic applications. Enhanced p-type charge transport with low-field mobility similar to 233 cm(2)/V.s and current density of similar to 400 mu A/mu m at V-DS = -2 V were obtained from flexible BP transistor at a channel length L = 0.5 mu m. Importantly, with optimized dielectric coating for air-stability during microfabrication, flexible BP RF transistors afforded intrinsic maximum oscillation frequency f(MAX) similar to 14.5 GHz and unity current gain cutoff frequency f(T), similar to 17.5 GHz at a channel length of 0.5 mu m. Notably, the experimental f(T) achieved here is at least 45% higher than prior results on rigid substrate, which is attributed to the improved air-stability of fabricated BP devices. In addition, the high-frequency performance was investigated through mechanical bending test up to similar to 1.5% tensile strain, which is ultimately limited by the inorganic dielectric film rather than the 2D material. Comparison of BP RF devices to other 2D semiconductors clearly indicates that BP offers the highest saturation velocity, an important metric for high-speed and RF flexible nanosystems.