Fig. 2: Black phosphorus (BP) photo-tunneling transistor.

a Schematic of device structure, showing BP channel, graphene contacts (with source and drain electrodes V_s and V_d), bottom gate (V_bg), and two partial top gates (V_tg1 and V_tg2) as well as the dielectric layer of hexagonal Boron Nitride (hBN). b Scanning electron microscopy (SEM) photograph (with false color) of a representative device in (a), the colored dashed line indicate different layers of the device: yellow for bottom hBN and white for top hBN, while the solide white line indicates the bottom gate, the scale bar is 10 \({{\rm{\mu }}}{{\rm{m}}}\). c, d illustrate the schematic band-diagram profiles for the principal operation of different photo-transistors. Specifically, they depict the dark (dashed blue line) and light-illuminated (red line) cases in a photo-tunneling transistor and a conventional field-effect transistor (FET), respectively. With a band-to-band tunneling (thermionic) charge injection mechanism, the device has a steep (smooth) Sub-threshold Swing (SS) and response (does not respond) to weak light, I_d represents the source-drain current, E_f denotes the fermi level and E_v indicates the valence band energy of the BP channel. e Room-temperature transfer curves of the device in (b) at different modes. Without top gate (V_tg), the device is an ambipolar FET (black curve). With V_tg1 = 6 V (V_tg2 = −6 V), the device is configured as p-type (n-type) tunneling-transistor V_ds = 0.1 V. f, g are enlarged transfer curves for tunneling transistor mode. The dashed triangles show subthreshold slopes of SS = 60 mV/dec for comparison.