Fig. 1. (Color online) (a) Optical microscope images of fabrication steps of BP/β-Ga₂O₃ heterojunction device. The channel length and width of the β-Ga₂O₃ were 16 and 6 μm, respectively. (b) SEM image of an as-fabricated BP/β-Ga₂O₃ heterojunction device. (c) Schematic illustration of the JFET device fabricated on a Si/SiO₂ (285 nm) substrate. (d) Energy band diagram of multilayer p-type BP and n-type β-Ga₂O₃ heterojunctions with a vdW gap. Scale bars are 10 µm.

Fig. 2. (Color online) (a) AFM image of the BP/β-Ga₂O₃ heterojunction. (b, c) Height profiles of the exfoliated BP and β-Ga₂O₃ flakes in (a). The thicknesses of the nanoflakes are 32.6 and 123.5 nm, respectively. (d) Raman spectra of the BP, β-Ga₂O₃ and the BP/β-Ga₂O₃ overlapped regions obtained under a 532 nm laser. The black and green curve demonstrated typical multilayer BP flake and β-Ga₂O₃ flake. The red curve shows the peaks of the overlapped region. (e) SEM image of the BP/β-Ga₂O₃ heterostructure device (left) and corresponding EDS element mappings for Ga and P (right). Scale bars are 5 µm.

Fig. 3. (Color online) (a)Transfer characteristics for back-gate BP FET. Back gate voltage V_bg swept from –60 to 60 V with a fixed source–drain bias voltage V_ds = 0.1 V. (Inset: output characteristics for back gated BP FET. V_bg ranging from –60 to 60 V with steps of 30 V under V_ds swept from 0 to 50 mV.) (b) Transfer characteristics for back gate β-Ga₂O₃ FET. V_bg swept from –80 to 80 V with a fixed V_ds = 5 V (Inset: output characteristics for back-gate β-Ga₂O₃ FET. V_bg ranging from –80 to 80 V with steps of 40 V under V_ds swept from 0 to 5 V.) (c) I_ds−V_ds curve of BP/β-Ga₂O₃ PN heterojunction. It shows a typical rectifying behavior. (Inset: the circuit schematic diagram of the PN heterojunction.) (d) I_ds−V_ds semi-log plot of the BP/β-Ga₂O₃ PN heterojunction.

Fig. 4. (Color online) (a) Circuit schematic diagram and optical image of the BP/β-Ga₂O₃ JFET. (b) Band diagram of β-Ga₂O₃ along the channel length direction. The red and blue curve shows the band bending at zero and negative gate voltage, respectively. (c) Output characteristics (I_ds−V_ds) of the JFET. V_gs ranging from –15 to 2 V under V_ds swept from 0 to 25 V. (d) Transfer characteristics (I_ds−V_gs) of the JFET. V_ds ranging from 2 to 20 V under V_gs swept from –25 to 2 V. (e) Semi-log plot of the transfer characteristics of the JFET. It shows a high on/off ratio beyond 10⁷. (f) Transconductance curves (estimated from transfer curves of (d)) of BP/β-Ga₂O₃ JFET as function of V_gs with V_ds sweeping from 2 to 20 V.

Fig. 5. (Color online) (a) Output characteristics curves of the BP/β-Ga₂O₃ JFET under different temperatures (ranging from 300 to 450 K with steps of 50 K) at V_gs = 1 V. (b) Transfer characteristics curves of the BP/β-Ga₂O₃ JFET under different temperatures (ranging from 300 to 450 K with steps of 50 K) at V_ds = 10 V.