(color online) (a) Device schematic diagram. (b) The log plotted I_ds–V_g curve was measured under V_ds = 500 mV. The color gradient implies the transition between hole- and electron-doping. Inset shows the data on a linear scale. (c) Color maps of I_ds under different V_lg, V_rg and V_ds which suggest the types of the junctions. (d) I_ds–V_ds characteristics under different illumination wavelength. The inset shows the responsivity (R) under V_ds = −1 V and V_ds = 0.^[40]