## 3 Results and Discussion
### Photoresponse  
After the electrical characterization of a p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction transistor, its photoresponse measurements were also conducted. The device was measured under spot illumination with a 633 nm laser at different power intensities to investigate its photoresponsivity. The scanning photocurrent microscopy (SPCM) method was used to obtain a spatial-resolved photoresponse of the device. SPCM is performed at V\({}_{\rm{DS}}\) = 0 V, V\({}_{\rm{G}}\) = 0 V and an incident power of 1.94 \(\mu\)W. As shown in Figure S8, the photocurrent of the p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction is small because the photogenerated carriers cannot be effectively collected using source and drain electrodes under no bias (V\({}_{\rm{DS}}\) = 0 V) conditions. To obtain more obvious photocurrent responsivity, SPCM was performed at V\({}_{\rm{DS}}\) = 5 V, V\({}_{\rm{G}}\) = \(-\)10 V and an incident power of 1.94 \(\mu\)W. As shown in Figure 4a, the I\({}_{\rm{DS}}\) at a p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction is much stronger than that at nearby areas, owing to the stronger capability of separating the photogenerated electron-hole pairs in the built-in electric field of the p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction. Figure 4b shows the transfer curves of a p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction transistor without and with laser-beam illumination. The I\({}_{\rm{DS}}\) of the p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction increases with the laser-beam illumination intensity. The increasing laser-beam illumination intensity can produce more photoexcited electron-hole pairs after separating through the built-in electric field of the p-doped WSe\({}_{2}\)/MoS\({}_{2}\) heterojunction, more electrons can transfer to MoS\({}_{2}\) and then can be effectively collected with the drain electrode, eventually forming the increasing photocurrent. The photoexcited holes are transferred to p-doped WSe\({}_{2}\), modulating the conductance of the MoS\({}_{2}\) channel, resulting in the photogating effect, which induces the threshold voltage leftshift. In such a heterostructure phototransistor configuration, the photodiode separates electron-hole pairs under light illumination and then cumulated holes in p-doped WSe\({}_{2}\), modulating the current in the channel in the phototransistor. It maximizes the electrical response to light in terms of photogain. To quantitatively assess the device's photoresponse performance, two crucial parameters, photoresponsivity (\(R\)) and detectivity (\(D^{\star}\)), can be calculated according to the following formulas:  
Figure 4: Photoresponse of the phototransistor. (**a**) Spatial map of the photocurrent measured by conducting scan over the surface of the device using a focused laser beam at V\({}_{\rm{DS}}\) = 5 V, V\({}_{\rm{G}}\) = \(-\)10 V and incident power of 1.94 \(\mu\)W. (**b**) I\({}_{\rm{light}}\) and I\({}_{\rm{dark}}\) (I\({}_{\rm{DS}}\) with and without illumination) measured under focused laser-beam irradiation for V\({}_{\rm{DS}}\) = 5 V and V\({}_{\rm{G}}\) = \(-\)20 to 20 V of 633 nm laser-beam illumination intensity from 1 nW to 3.28 \(\mu\)W.  
\[R=\frac{I_{light}-I_{dark}}{P} \tag{3}\]  
where \(I_{light}\) is drain current, \(I_{dark}\) is dark current and \(P\) is incident laser power. \[D^{*}=\frac{\sqrt{A}\cdot R}{\sqrt{2 qI_{dark}}} \tag{4}\]  
where \(q\) is the element charge and \(A\) is the effective illuminated area. Figure 5a,b present photoresponsivity (\(R\)) and detectivity (\(D^{*}\)) under the illumination of a 633 nm laser with different power intensities from 1 nW to 3.28 uW, respectively. As shown in Figure 5a, the photoresponsivity increases as gate voltage. The increasing gate voltage induces more electrons accumulated in the MoS\({}_{2}\) channel.