  Few-layer and thin film forms of layered black phosphorus (BP) have recently
emerged as a promising material for applications in high performance
nanoelectronics and infrared optoelectronics. Layered BP thin film offers a
moderate bandgap of around 0.3 eV and high carrier mobility, leading to
transistors with decent on-off ratio and high on-state current density. Here,
we demonstrate the gigahertz frequency operation of black phosphorus
field-effect transistors for the first time. The BP transistors demonstrated
here show excellent current saturation with an on-off ratio exceeding 2000. We
achieved a current density in excess of 270 mA/mm and DC transconductance above
180 mS/mm for hole conduction. Using standard high frequency characterization
techniques, we measured a short-circuit current-gain cut-off frequency fT of 12
GHz and a maximum oscillation frequency fmax of 20 GHz in 300 nm channel length
devices. BP devices may offer advantages over graphene transistors for high
frequency electronics in terms of voltage and power gain due to the good
current saturation properties arising from their finite bandgap, thus enabling
the future ubiquitous transistor technology that can operate in the multi-GHz
frequency range and beyond.
