# A Review of Sharp-Switching Band-Modulation Devices
## 1 Introduction  
The search for sharp-switching devices compatible with modern complementary metal-oxide-semiconductor (CMOS) digital technology has been a central preoccupation of the device physics community for the past two decades. With the continuous downscaling of CMOS technology, the switching limit of standard CMOS transistors--the 2.3 kT \(\approx\) 60 mV of the controlling gate voltage V\({}_{\text{G}}\) required to reduce the drain current I\({}_{\text{D}}\) by a decade below the transistor threshold voltage V\({}_{\text{T}}\) at room temperature--is the ultimate obstacle to reducing the supply voltage of large-scale high-performance digital circuitry. This is due to the power consumption issue schematically illustrated in Figure 1. As long as the 60 mV/decade subthreshold slope (SS) limit remains in force, achieving an acceptable I\({}_{\text{ON}}\) for switching downstream transistors and wiring parasitics, while maintaining a sufficiently low I\({}_{\text{OFF}}\) to prevent static power consumption, requires a power supply of roughly V\({}_{\text{DD}}\approx\) 0.5 V [1]. Reducing the threshold voltage and V\({}_{\text{DD}}\) leads to a catastrophic exponential increase in I\({}_{\text{OFF}}\). Only a hypothetical sharper-switching device exempt from the 60 mV/dec constraint could provide a high I\({}_{\text{ON}}^{*}\) at a low V\({}_{\text{DD}}^{*}\), opening the possibility of many additional generations of low-power downscaled CMOS circuitry. Physically, the 60 mV/decade limit arises from the fundamental Boltzmann carrier distribution in the source electrode and cannot be overcome in the standard source-gate-drain transistor structure, where V\({}_{\text{G}}\) shuts off the source-drain current by creating a potential barrier in the channel [2]. Whereas various second-order effects that led to even worse SS have been overcome through a sustained research effort into FinFET and fully-depleted SOI (FD-SOI) device technologies [3], true sharp switching requires a modified V\({}_{\text{G}}\)-controlled carrier transport mechanism. While there has been no shortage of alternative device candidates, in this review we will restrict ourselves to devices with realistic CMOS integration prospects; even if beyond the current state-of-the-art. We will also limit ourselves to room temperature operation, as cooling standard CMOS to cryogenic temperatures--while certainly providing sharper switching and possibly interesting for applications, such as quantum sensing or computing at sub-1 K dilution temperatures [4; 5; 6]--does not address the needs of mainstream digital technology, which is evolving towards portable, miniaturized, and, ideally, low-power devices. With these provisos, a number of fascinating sharp-switching devices will fall outside the scope of this review article. For example, we will not discuss nanoelectromechanical switches based on voltage-controlled gate positioning or even direct physical contact in a cantilever geometry; these devices provide truly sharp switching, but appear unlikely to provide the necessary reliability and immunity to sticking. We will also omit ferroelectric negative capacitance devices, where the hypothetical V\({}_{\text{G}}\) amplification by the polarization in the ferroelectric gate material has not produced experimentally validated sharp switching together with competitively fast operation. The interested reader is referred to other, more complete, sharp-switching device reviews [7]. Instead we will focus on a recently developed class of band modulation devices, where reconfigurable gate-controlled electrostatic doping, combined with multiple gate geometries developed in an industrially-validated FD-SOI process, allow for ultra-sharp switching via a feedback mechanism. These devices have potential applications as logic switches, memories, electrostatic discharge (ESD) protection, and sensing devices; without introducing nonstandard materials, nonstandard processing steps, or cryogenic temperature operation.