## 3 Results
### Photoconductive Configuration  
In PC configuration, the PD acts as a 3-terminal device: the output current signal is recorded between the interdigitated S and D electrodes, connected by the SWCNT film, while the G electrode on the bottom side of the Si substrate is used as an external input control providing an electric potential \(V_{G}\) at the ends of the heterojunction. \(V_{G}\) is applied relative to the source electrode, S. In Figure 5a, the \(I_{SD}\)-versus-\(V_{G}\) feature at different values \(V_{SD}\) of the voltage applied to the SWCNT film in dark conditions is shown. The curves are representative of a typical trend of a 3-terminal device: at low \(V_{G}\), the current flowing inside the CNT is constant, but from \(V_{G}\approx 15\)V it starts to increase, due to an avalanche effect, and rapidly intensifies as \(V_{G}\) = 20 V. The beginning of the avalanche region starts off at a noticeable low voltage value compared to commercial PDs which need even a 100 V bias to reach the same operating point [37]. In this configuration, when \(V_{G}\) = 0 and the device is in the dark, the current measured between the interdigitated electrodes is linear with respect to \(V_{SD}\), and the \(I_{SD}\)-versus-\(V_{SD}\) characteristics represent the resistance of the CNT film. If the device is illuminated, a higher current value at the same \(V_{DS}\) has been measured: now, there are more free charges available in the film, as the minority positive charges (holes), photogenerated mainly in the Si region, crossed the potential barrier of the junction and flowed into the CNTs thanks to the junction potential \(V_{bi}\) generated at the nanotubes/n-Si interface. If a \(V_{G}>0\) is applied, the junction is reverse polarized, and a higher injection of holes occurs inside the CNT film, driven in this case not only by \(V_{bi}\), but also by \(V_{G}\), while for \(V_{G}<0\) and \(V_{G}>V_{bi}\), the NSH is directly polarized, and the majority charges (electrons) flow inside the SWCNT film. A detailed explanation of the  
Figure 4: (**a**) Sketch of the devices used: in the top panel the one with the Si active area at a lower level with respect to the metal multifinger electrodes (henceforth, named TF); in the lower panel, the one with the Si active area at the same level of the multifinger metal contacts (hereafter, called HF); (**b**) Current-voltage curves acquired in dark (solid lines) and under 100 mW/cm\({}^{2}\) white light illumination (dot-dashed curves) conditions for the TF (red) and HF (blue) devices; (**c**) responsivity (open circles) and detectivity (solid circles) as a function of the 475 nm LED light power for the TF (red) and HF (blue) PDs; (**d**) noise equivalent power as a function of the 475 nm LED light power for the TF (red) and HF (blue) PDs. voltage doping mechanism in our detectors has been recently reported in [21]. Here, we report only the improvements in the performance of our devices when a positive voltage was applied to the gate electrode because usually in photodiode devices, the operation involves a reverse bias of the junction which is more easily controllable and tunable by the gate voltage. A hole mobility \(\mu_{\text{h}}=5\times 10^{5}\text{ cm}^{2}/\text{V}\cdot\text{s}\) was obtained from the PC I-V measurements [21] resulting in some orders of magnitude higher than other CNT-based fast switching devices [38]. To support the 3-terminal photoconductive proposed mechanism of photodetection, measurements of the EQE collected between S-D terminals were performed in the wavelength range 300 nm-1000 nm. The results reported in Figure 5b point out that for \(V_{G}=0\) the EQE shape is similar to that of a Si p-n junction and very close to the EQE acquired in PV mode (Figure 2a) [39]. This occurs because the current flowing in the CNT film comes mainly from the minority photocarriers generated in the n-Si substrate injected by \(V_{bi}\). It is worth noting that EQE values measured for \(V_{G}=0\) in PV configuration are higher than the ones obtained in the PC mode.