## 3 Results
### Photoconductive Configuration
This is due to the fact that in the PV mode, the \(e\) and \(h\) photocharges are both separated by \(V_{bi}\) at the SWCNT/Si interface, while in PC mode, the collected photocharges are just holes driven through the junction by the same built-in potential. However, as shown in Figure 5b, for \(V_{G}>0\) the EQE hugely increases, exceeding 100%, since the number of holes collected by the CNT in the PC mode are enhanced by the action of \(V_{G}\) driving the junction in the avalanche regime, thus giving rise to a voltage doping of the PD. The NSH PD ability to detect ultra-fast signals in PC mode was tested lighting the device with the fs laser source. Figure 6a reports the photocurrent response for a single laser pulse (\(\lambda=275\) nm) with incident photon energy of 100 nJ in absence of voltage doping  
Figure 5: **(a)** Source-drain current \(I_{DS}\) vs gate voltage \(V_{G}\) acquired in dark conditions for different values of \(V_{DS}\); **(b)** External quantum efficiency measured between S and D terminals as a function of the wavelength for different values of the gate voltage \(V_{G}\). (\(V_{G}=0\)). The measured rise time is \(\tau_{\rm r}\) = (4.7 \(\pm\) 0.1) ns. In Figure 6b the rise time vs. \(\lambda\) at \(V_{G}=0\) and E = 100 nJ is shown. As in the case of the PV mode, \(\tau_{\rm r}\) increases with increasing wavelength ranging from 4.7 ns in the UV region to about 10 ns in the IR region, suggesting a similar mechanism in the two different configurations [40]. Nevertheless, the PDs operating in the PC mode appear faster in their response to the light pulse with respect to the PV configuration. In the PC mode, in fact, the signal measured by the external circuit is composed only of the holes mostly photogenerated in the Si substrate close to the SWCNT/Si junction and injected into the CNT film. This hole current takes less time to reach the top metal contacts than the current of electrons and holes generated in the PV mode. In this last case, indeed, electrons have to move through all the Si substrate's thickness to be collected at the G electrode [22]. The PD response can be further improved by applying a \(V_{G}>0\) which means directly polarizing the junction. This is shown in Figure 6c, where \(\tau_{\rm r}\) is reported as a function of \(V_{G}\) for \(\lambda\) = 275 nm: for \(V_{G}=20\) V, \(\tau_{\rm r}\) reaches the minimum measured value of about 1.5 ns. Similar results are obtained for all the other wavelengths investigated from UV (\(\lambda\) = 240 nm) to IR (\(\lambda\) = 1600 nm). This means, therefore, that voltage doping, induced by \(V_{G}\), not only increases the photocurrent thanks to an avalanche mechanism, but also increases the drift velocity of the photogenerated charges which take less time to reach the electrodes and be collected. Figure 6: (**a**) Response to a single 35-fs laser pulse of NSH PD for incident light energy of 100 nJ and wavelength of 275 nm. The arrows indicate the 10% and the 90% of the pulse height used as a criterion for the rise time estimation; (**b**) rise time, \(\tau_{\rm r}\), versus wavelength under 35 Lfs pulsed laser source for incident light energy of 100 nJ; (**c**) rise time, \(\tau_{\rm r}\), measured at \(\lambda\) = 275 nm as a function of the gate voltage, \(V_{G}\).