Recently, negative differential resistance devices have attracted considerable attention due to their folded current–voltage characteristic, which presents multiple threshold voltage values. Because of this remarkable property, studies associated with the negative differential resistance devices have been explored for realizing multi-valued logic applications. Here we demonstrate a negative differential resistance device based on a phosphorene/rhenium disulfide (BP/ReS2) heterojunction that is formed by type-III broken-gap band alignment, showing high peak-to-valley current ratio values of 4.2 and 6.9 at room temperature and 180 K, respectively. Also, the carrier transport mechanism of the BP/ReS2 negative differential resistance device is investigated in detail by analysing the tunnelling and diffusion currents at various temperatures with the proposed analytic negative differential resistance device model. Finally, we demonstrate a ternary inverter as a multi-valued logic application. This study of a two-dimensional material heterojunction is a step forward toward future multi-valued logic device research.

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Black phosphorus (BP) is appealing as a next-generation two-dimensional (2D) van der Waals (vdW) material, but research into doping BP to optimize device performance is still deficient. Here, we study n- and p-doping of variously thick (2, 4, 7, and 10 nm) black phosphorus (BP) films in terms of the performance of the corresponding BP-based transistors and photodetectors. N- and p-doping were respectively achieved with 3-amino-propyltriethoxysilane (APTES) and octadecyltrichlorosilane (OTS). The changed concentrations of BP were between approximately −2.1 × 1011 and −4.82 × 1011 cm–2 for APTES (n-doping) and between 1.06 × 1011 and 1.96 × 1011 cm–2 for OTS (p-doping). In the transistor devices formed on a 2 nm thick BP film, n-doping negatively shifted the threshold voltage from 28.3 to 19.5 V. Conversely, after p-doping with OTS, the threshold voltage was positively shifted from 20.6 to 23.7 V. In the BP photodetectors (2 nm thick devices), responsivity (R) was reduced by −16% (520 nm) and −9% (850 nm) after n-doping, whereas p-doping improved the responsivity by 40% (520 nm) and 20% (850 nm). Through this doping study, the very high photoresponsivity of 1.4 × 104 A/W under 520 nm laser exposure was achieved in 10 nm thick BP/OTS photodetectors. In addition, the n- and p-doping effects were more obvious in thin BP films.

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