Black Phosphorus Radio-Frequency Transistors (1410.3880v1)

Abstract: 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.

• The paper demonstrates breakthrough performance by achieving 12 GHz cut-off and 20 GHz oscillation frequencies in BP FETs with 300 nm channel lengths.
• It highlights BP’s advantages over graphene, using its 0.3 eV bandgap and high carrier mobility to improve current saturation and operational stability.
• S-parameter measurements validate BP’s potential for multi-GHz operations, underscoring its promise for scalable next-generation RF and optoelectronic applications.

Evaluation and Implications of Black Phosphorus Radio-Frequency Transistors

The research elucidated in this paper concerns the novel exploitation of black phosphorus (BP) as a channel material in radio-frequency (RF) transistors, assessing its potential against traditional materials such as silicon, organic compounds, and oxides. Black phosphorus emerges as a robust contender for RF applications due to its moderate 0.3 eV bandgap and high carrier mobility, rendering it suitable for high-performance nanoelectronics and infrared optoelectronics. Key distinctions are drawn between BP and graphene, wherein the latter's lack of a bandgap results in compromised current saturation, thereby impacting its voltage and power gain properties. In contrast, BP provides a platform to enhance saturation characteristics and operational stability over a broader spectrum.

The paper underscores the pioneering terrestrial gigahertz operation of BP field-effect transistors (FETs), evidencing operational current-gain cut-off frequencies ($f_{T}$) of 12 GHz and maximum oscillation frequencies ($f_{\text{max}}$) of 20 GHz for devices with 300 nm channel lengths. Such specifications denote a significant leap forward in bridging the functional gaps left by graphene and enabling devices that could outstrip the capabilities of existing materials under similar conditions. The achievement of a current density exceeding 270 mA/mm and a DC transconductance above 180 mS/mm for hole conduction speaks to the potential for BP to operate effectively at multi-GHz frequencies.

The fabrication process employs layered BP films measuring around 8.5 nm, which provide an optimal balance between mobility and on-off ratio characteristics crucial for RF applications. Hall mobility recorded along the x-direction surpasses 400 cm²V‾¹s‾¹ at room temperature, indicating BP’s high efficacy for charge carrier mobility. Such figures align with the low output conductance readings, presaging substantial improvements in voltage and power amplification for RF transistors.

The evaluation through S-parameter measurements corroborates the high-frequency aptitude of BP transistors. Outcomes from short-circuit current gains and unilateral power gains exemplify the advancements achieved, marking BP's prospects in delivering more reliable and high-performance RF transistors. The intrinsic cut-off frequency potentially approaches 51 GHz when further exploiting intrinsic properties while minimizing parasitic capacitances. Such data considerably extend the prospects of BP in conventional and emerging applications where high-speed and high-frequency operations are pivotal.

Practically, transitioning the operational capabilities of BP transistors from research environments to commercial applications will necessitate addressing device scalability, variability, and stability concerns. Future research might further explore the scaling laws applicable to BP and its impact on device integration in existing semiconductor processes. The continual development in this domain holds promise for affording next-generation electronics, potentially transforming the landscapes of digital and analog RF systems for civilian and defense applications. The research collectively suggests an important stride towards expanding the operating frequencies and robustness of RF technologies through BP integration, potentially redefining performance benchmarks for semiconductor devices.