## 3 Results
### The Photodetection Characteristics and Discussion
With a high reverse V\({}_{\text{BG}}\) of \(-\)10 V, the responsivity presented a significant increase over the broad excitation spectrum range, which is consistent with the diode characteristics described in Figure 3. From the equation above, the value of responsivity is proportional to photoconductive gain, which depends on the carriers' lifetime, mobility, and source-drain bias voltage setting on the device [19]. In our carbon-based photodiode,  
Figure 4: The energy level diagram at the graphene/s-SWCNT junction under different bias and illumination condition. (**a**) V\({}_{\text{SD}}\) = 0, V\({}_{\text{BG}}\) = 0, dark (**b**) V\({}_{\text{SD}}\) < 0, V\({}_{\text{BG}}\) = 0, light (**c**) V\({}_{\text{SD}}\) < 0, V\({}_{\text{BG}}\) < 0, light (**d**) V\({}_{\text{SD}}\) < 0, V\({}_{\text{BG}}\) > 0, light. the high trapped holes lifetime and ultra-high mobility in the graphene channel co-contributed to the high photoconductive gain and a high responsivity. Figure 5b plots the responsivity curve as a function of illumination power under the excitation wavelength of 1000 nm (V\({}_{\text{SD}}\) = \(-\)5 V, V\({}_{\text{BG}}\) = \(-\)5 V). The responsivity decreased exponentially with increasing power. This may correlate to the decrease of unoccupied states in the conduction bands of graphene and s-SWCNTs as the power increases [35]. At a 0.5 uW illumination, the device exhibited a drastically enhanced responsivity of 1.75 A/W, as compared with a single CNT-based or bare graphene-based device of 10\({}^{1}\)-10\({}^{3}\) uA/W [10; 37; 38], which could be further improved by shrinking the size of devices and optimizing fabricating technology. Response time is another key quality factor for photodiodes. Figure 6a shows the time trace of the photocurrent for our devices under periodic illumination (1000 nm wavelength, 100 uW, V\({}_{\text{SD}}\) = \(-\)5 V, and V\({}_{\text{BG}}\) = \(-\)10 V). All the fabricated devices were arrayed in a wafer substrate (there were nine devices in a transistors array, the inset of Figure 2b), which present a uniform and stable performance. The single carbon-based device tested the time trace of the photocurrent under periodic illumination of 5 ms (Figure 6a), and after longstanding on-off cycles of one hour the photocurrent exhibited excellent reproducibility. The high-resolution temporal response of the device is shown in Figure 6b and a significant temporal photoresponse was observed in which we estimated the rise response time to be 15 us, 7 times faster than a hybrid graphene/CNT photodetector based on photogating effect [7]. The high-quality graphene and s-SWCNT together with the effective Schottky junctions in the interface, which resulted in fast transfer of photocarriers within the s-SWNT/graphene junction, co-contributed to the excellent temporal photoresponse. When the light was off, the trapped carriers in s-SWCNT film released slowly from different surface states, which led to a slow decay of the temporal photoresponse (Figure 6b). Figure 5: (**a**) Responsivity as a function of excitation wavelength for as-fabricated carbon-based photodiode. (**b**) The dependence of responsivity on excitation power under the 1000 nm illumination. Figure 6: (**a**) The time trace of the photocurrent under periodic illumination. (**b**) The temporal photoresponse of as-fabricated device.