# A Review of Sharp-Switching Band-Modulation Devices
## 5 Alternative Applications of Sharp Switching
### Photo-Detection  
The operation of the \(Z^{2}\)-FET as a photodetector takes advantage of the same nonequilibrium mechanism used in 1T-DRAM. The generic principle is to bias the device such that it stays in blocked mode ('0'-state) in the dark and turns on ('1'-state) during illumination. Light is detected by the abrupt switch from low to high anode current. The turn-on voltage under illumination is the sensing parameter [54; 55]. In order to enhance the light absorption efficiency, rather thick top silicon film (\(\text{t}_{\text{si}}=200\) nm) is used in this simulated device. The gated and ungated regions are 0.5 \(\upmu\)m and 1.5 \(\upmu\)m long and the BOX is 500 nm thick. Sharp voltage pulses of +5 V and \(-\)5 V are applied on the front and back gate, respectively, to induce injection barriers in the Si film (Figure 20a). Under a constant anode bias (\(\text{V}_{\text{A}}=1\) V), the device is exposed to light with intensity of 5 \(\upmu\)W/cm\({}^{2}\) and wavelength \(\lambda=520\) nm. Figure 20b shows the evolution of the potential barriers and anode current with the exposure time. The device is initially in the I\({}_{\text{OFF}}\) state, due to the high potential barriers formed by the gate pulses. In the dark, the potential barrier height does not change appreciably in the tens of ms time scale, so that the device remains off. On the contrary, under illumination, the electrons photo-generated in the ungated region are attracted by the positively biased \(\text{V}_{\text{FG}}\) and accumulate under the top gate oxide. The accumulated photoelectrons reduce the injection barrier and \(\text{V}_{\text{ON}}\), which eventually switches the \(Z^{2}\)-FET into the conduction mode with high \(\text{I}_{\text{ON}}\). Figure 19: (**a**) Schematic ESD protection with a \(Z^{2}\)–FET. (**b**) Output characteristics from TLP tests with long and short current pulses. (**c**) Anode voltage response in the very first nanoseconds after ESD strike showing the voltage overshoot (1 mA/\(\upmu\)m current pulse with 55 ps rise time, adapted from [27; 53]). Two operation modes of the \(Z^{2}\)-FET photodetector can be envisioned. Figure 20c shows the \(Z^{2}\)-FET used as a switch triggered by a threshold light exposure. The device turns on after a few ms, with the delay arising from the time needed to photo-generate enough carriers to decrease the turn-on voltage and equal the reference anode voltage \(\mathrm{V_{ON}}\) (\(t\)) = \(\mathrm{V_{A}}\). The response time measures the light intensity. With a \(\mathrm{V_{FG}}\) pulse, the non-equilibrium condition is reinforced and the potential barrier is taller. Hence, for constant light intensity, a longer exposure is needed to generate enough photoelectrons to lower the barrier and trigger the \(Z^{2}\)-FET. For low \(\mathrm{V_{FG}}\) bias, fewer photoelectrons are required and the response is faster. Figure 21a shows how the response time can be flexibly tuned by the gate voltage. In an alternative operation mode, the \(Z^{2}\)-FET is used as a sensor to monitor the light intensity. Once the gates are sharply pulsed into deep depletion, a short light flash with a constant duration (5 ms) generates photocarriers that lower the potential barrier and \(\mathrm{V_{ON}}\). The anode voltage is ramped from 0 V up to 2.5 V and the current is monitored to detect the switching voltage (see Figure 21b). With higher light intensity, more photoelectrons are available and the \(\mathrm{V_{ON}}\) reduction is more pronounced. Thus, a lower \(\mathrm{V_{A}}\) is needed to turn on the device. The relation between exposure dose and \(\mathrm{V_{ON}}\) is nearly linear, which facilitates the light-to-voltage conversion. Not only does the \(Z^{2}\)-FET detect light, but it also integrates the photogenerated charge, that is the number of photons, and provides exceptional amplification through the positive feedback mechanism. These two operation modes have multiple potential applications.