Anomalous Ionic Gating Effect in Degenerate Semiconductors

The insulated-gate field effect, a key feature of semiconductor devices used as switches or rectifiers in power electronics, induces the accumulation or depletion of charge carriers to govern the electrical properties of active layers. In particular, as demands for the miniaturization of electronic devices increase, various attempts have been made to achieve higher carrier density and lower operating voltage in field effect transistors (FETs). Notwithstanding, FETs with silicon dioxide (SiO₂), a typical gate dielectric material, require a high gate voltage (&gt;20 V) to form an active channel by accumulating charge carriers between semiconductor/dielectric layers, resulting in high power consumption. To address this, various high-κ materials, including electrolytes, are desirable as the insulated-gate layer due to their high areal capacitance regardless of the dielectric thickness. Interestingly, studies related to ionovoltaic phenomena show that the electric double layer (EDL)-induced electrolytic gating effect enables ultra-high charge carrier accumulation at the semiconductor channel/electrolyte interface compared to the dielectric (<i>e.g.</i>, SiO₂) gating effect. However, as elucidating the cause of these interesting phenomena remains a challenging task, numerous studies related to the electrolytic gating effect have focused on practical applications rather than interpretation of the fundamental principle. Indium tin oxide (ITO) is one of the most widely used transparent conducting oxides in optoelectronics because of its electrical conductivity and optical transparency, but its degenerate doping (more like a metal) limits utilization as a semiconductor device. Nevertheless, in the ITO-based channel layer with high initial electron density, hidden non-degenerate semiconductor characteristics can be realized using dramatic electrolytic (ionic) gating effects. In this work, we observe an anomalous ionic gate field effect (gating off) that conversely reduces the conductivity of ITO (<i>i.e.</i>, depletion), resulting in a deactivated channel layer. In brief, we newly introduce a groundbreaking gate-off effect opposite to the principle of activating channels by exceeding the threshold voltage, which is the dielectric gating effect commonly used in FETs. Therefore, to implement the same principle as closing an electric faucet by the ionic gating effect, we propose a combination of the degenerate semiconductor (<i>e.g.</i>, ITO) and 21 mol of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), a water-in-salt electrolyte with relatively wide operating voltage window. To clarify the causality of ionovoltaic-related anomalous gating effect, we quantitatively analyze an accumulated/depleted electron density in the ITO channel layer through a sophisticated Hall measurement with applied electrolyte-gated voltage, bridging the fundamental correlation between solid-liquid interfaces and semiconductor principles. Hence, although the dielectric gating effect (I_on/I_off ~ 1) cannot dominate the degenerate semiconductor properties due to its high conductivity, the ionic gating effect (I_on/I_off ~ 10³) operating at low voltage (&lt;2 V) induces EDL formation at the solid-liquid interface, strongly governing the inherent properties of metal-like ITO. The anomalous ionic gating effect observed here has the significance of being able to realize degenerate semiconductors, which have limitations in controlling electrical conductivity, in an on/off state. Furthermore, we believe that this study serves as a stepping stone for revealing the unestablished principle of interfacial phenomena-dependent application devices based on various emerging semiconductor materials such as graphene and other two-dimensional materials besides ITO.