# A Review of Sharp-Switching Band-Modulation Devices
## 6 Reconfigurable Sharp-Switching Modes
### Esaki Diode  
Esaki tunneling diodes [67] feature a negative differential resistance (NDR), attractive for fast-switching circuits [68] and high-frequency detectors and oscillators. The forward current is carried by the electrons tunneling from the conduction band on the \(N\)-side into the empty states of the valence band of the \(P\)-side. This mechanism is the reciprocal of the Zener effect. BTBT occurs if the conduction and valence bands are properly aligned and the tunneling barrier is as thin as possible. These conditions imply the processing of a very sharp junction between heavily doped N and P regions, which is hard to achieve in ultrathin bodies. Another fundamental aspect that hampers tunneling in Si is the tunneling rate, exponentially higher in materials with lower bandgap \(E_{\text{G}}\) and effective mass. While physical doping with degenerate concentration and sharp junctions is hardly feasible in FD-SOI, electrostatic doping can offer an attractive substitution. The device is biased such as to emulate a virtual \(P\)-\(N\) diode with the junction located at the left corner of the gate. High voltages applied to the front and back gates induce adjacent populations of free electrons and holes, with concentrations above \(10^{19}\) cm\({}^{-3}\). Although the depletion region is depth-dependent, it does not exceed 5-10 nm length. To avoid damaging the gate dielectric, the anode is grounded and the cathode is negatively biased; the BOX being much thicker, it can sustain high electric field without reliability concerns. Measured at low temperature, where the generation current at interfaces is massively attenuated, the electrostatic Esaki diode exhibits a clear NDR region (Figure 26) [69]. The initial current is due to band-to-band tunneling and increases with cathode voltage. A peak current is reached before the tunneling current decays (for \(\mid\)V\({}_{\text{K}}\)\(\mid\) > 0.88 V). Then, the regular diffusion current of the \(P\)-\(N\) diode takes over. The NDR peak-to-valley ratio attains 3.6, a record in our silicon world and close to the best values in Si-SiGe heterojunctions [68]. Interestingly, 2D numerical simulations indicate that BTBT occurs at the front and back interfaces, as well as diagonally between the heavily-doped layers of electrons at the film-BOX interface and holes at the front-gate interface. Figure 25: Average subthreshold swing SS vs. normalized I\({}_{\text{ON}}\) plot, using the [PERSON] et al. benchmarking approach [63]. Reproduced from [7], with additional recent data points from [64; 65] indicated with stars. Green points indicate CMOS-compatible materials. It is worth noting that for low gate voltage ( \(\mid\) V\({}_{\text{FG}}\mid\) \(<\) 0.4 V), the concentration of holes \(P^{*}\) on the anode side is too small to enable interband tunneling. On the other hand, at high V\({}_{\text{FG}}\) the tunneling current increases and gives rise to a plateau that replaces the NDR peak (for \(-2<V_{\text{FG}}<-1.6\) V in Figure 26). This excess current originates from trap-assisted tunneling [69]. The electrons can tunnel either from the conduction band into empty deep traps located in the forbidden gap of the anode or from filled traps into the valence band of the cathode. Similarly, increasing the back-gate voltage is beneficial to tunneling, as long as the hole concentration set by the front gate is not affected. The solution, to avoid this undesirable coupling between the gates, is a virtual diode featuring twin ground-planes, as shown in Figure 3a. The \(P\)-type ground-plane must be located beneath the gated region and negatively biased to reinforce, rather than dilute, the \(P^{*}\) electrostatic doping. This biasing scheme is important for keeping the junction between the two ground-planes reverse-biased and minimizing the leakage current. These preliminary results emphasize the tremendous potential of electrostatic doping: an Esaki diode with a priori degenerate doping can actually be emulated in an undoped body using the front and back gates.