Dual gate black phosphorus velocity modulated transistor

The layered semiconductor black phosphorus has attracted attention as a 2D atomic crystal that can be prepared in ultra-thin layers for operation as field effect transistors [1–3]. Despite the susceptibility of black phosphorus to photo-oxidation [4], improvements to the electronic quality of black phosphorus devices has culminated in the observation of the quantum Hall effect [5]. In this work, we demonstrate the room temperature operation of a dual gated black phosphorus transistor operating as a velocity modulated transistor [6], whereby modification of hole density distribution within a black phosphorus quantum well leads to a two-fold modulation of hole mobility. Simultaneous modulation of Schottky barrier resistance leads to a four-fold modulation of transconductance at a fixed hole density. Our work explicitly demonstrates the critical role of charge density distribution upon charge carrier transport within 2D atomic crystals. Black phosphorus (bP) is an elemental allotrope and a direct bandgap semiconductor with a puckered, honeycomb layer structure [7, 8] that can be exfoliated down to atomic few-layer thickness [1–4, 9, 10]. Although bP is the most thermodynamically stable allotrope of phosphorus, photo-oxidation in the presence of water, oxygen and visible light is known to degrade bP with a reaction rate that increases as bP layer thickness decreases [4]. Several materials have been used to encapsulate bP in order to protect it against photo-oxidation, including hexagonal boron-nitride [11–13], aluminum oxide [14], parylene [4], and poly-methylmethacrylate [15]. Recent works have also shown that 2D hole transport can be achieved in a single 2D sub-band within an accumulation layer of many-layer bP [11, 13, 15], effectively combining 2D transport characteristics with the increased chemical stability of many-layer bP. These advances have culminated in the observation of the quantum Hall effect in bP [5]. Nonetheless, further understanding and control of transconductance, carrier mobility and contact resistance in bP field effect transistors (FETs) is desired. We report here an experimental investigation of the transport characteristics of bP FETs with an asymmetric dual gate geometry consisting of top and bottom gate electrodes. The top gate is found to be effective in modulating the back gate FET transfer characteristics, including both field effect mobility and Schottky barrier contact resistance. The mobility modulation effect enables operation of the dual gate bP FET as a velocity modulated transistor (VMT), first proposed by Sakaki [6] to overcome the limitation on transistor switching frequency imposed by the channel transit time of charge carriers. Mobility modulation has since been demonstrated in GaAs/AlGaAs heterojunctions [16], wide GaAs/AlGaAs quantum wells [17], silicon-on-insulator FETs [18] and the LaAlO3/SrTiO3 interface [19]. Room temperature VMT operation in silicon-on-insulator FETs has been demonstrated with up to 1.4-fold mobility modulation [18]. Asymmetric dual-gate bP FETs exhibit a two-fold mobility modulation at room temperature, and the underlying mechanism is modulation of hole density distribution with the naked bP quantum well channel of the bP FET, and a resultant modulation of scattering by charged impurities within the gate oxide, surface roughness, and other spatially dependent scattering mechanisms. Simultaneously, bP FETs exhibit strong Schottky barrier modulation. First conclusively observed in carbon nanotube FETs [20], Schottky barrier modulation has recently been shown to dominate off-state conductance of bP FETs [21]. The combined effects of mobility modulation and Schottky barrier modulation of dual-gate bP FETs enables four-fold transconductance modulation at a fixed carrier density of 4 × 1011cm−2.