Physical reservoir computing has recently been attracting attention for its ability to substantially reduce the computational resources required to process time series data. However, the physical reservoirs that have been reported to date have had insufficient computational capacity, and most of them have a large volume, which makes their practical application difficult. Here, we describe the development of a Li ⁺ electrolyte–based ion-gating reservoir (IGR), with ion-electron–coupled dynamics, for use in high-performance physical reservoir computing. A variety of synaptic responses were obtained in response to past experience, which were stored as transient charge density patterns in an electric double layer, at the Li ⁺ electrolyte/diamond interface. Performance for a second-order nonlinear dynamical equation task is one order of magnitude higher than memristor-based reservoirs. The edge-of-chaos state of the IGR enabled the best computational capacity. The IGR described here opens the way for high-performance and integrated neural network devices.

Diamond is a wide bandgap semiconductor that can work at high temperatures and resist very high electric fields. It endures harsh environments through its physical stability and conducts heat very well. These properties make diamond suitable for the fabrication of unique electronic devices. In particular, diamond field effect transistors (FETs) have promising applications, including high-power converters for trains and electric vehicles and high-power high-frequency amplifiers for telecommunications and radar. Although high mobility is desirable for these applications, it has been difficult to achieve in diamond FETs particularly when the carrier density is high. The low mobility is most probably due to fixed and trapped charges in the non-ideal amorphous gate dielectric and at the dielectric/diamond interface. Here, we report on diamond FETs with monocrystalline hexagonal boron nitride (h-BN) as a gate dielectric. Thanks to the low density of charged impurities in monocrystalline h-BN, we obtained unprecedentedly high mobilities (>300 cm² V⁻¹ s⁻¹) for moderately high carrier densities (>5 × 10¹² cm⁻²). The resulting minimum sheet resistance was exceptionally low (<3 kΩ). Our results show that a heterostructure consisting of monocrystalline h-BN and diamond is an excellent platform with which to manufacture high-performance electronic devices.

Diamond field-effect transistors (FETs) have potential applications in power electronics and high-output high-frequency amplifications. In such applications, high charge-carrier mobility is desirable for a reduced loss and high-speed operation. We recently fabricated diamond FETs with a hexagonal-boron-nitride gate dielectric and observed a high mobility above $300{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ . In this study, we identify the scattering mechanism that limits the mobility of our FETs through theoretical calculations. Our calculations reveal that dominant carrier scattering is caused by surface charged impurities with a density of $\approx 1\times {10}^{12}{\mathrm{cm}}^{-2}$ and suggest that an increase in mobility over $1000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ is possible by reducing these impurities.