Electric-double-layer-gated Transistors based on Two-dimensional Crystals: Recent Approaches and Advances

Electric-double-layer (EDL) gated transistors use ions in an electrolyte to induce charge in the channel of the transistor by field-effect. Because a sub-nanometer gap capacitor is created at the electrolyte/channel interface, large capacitance densities (~µF/cm2) corresponding to high sheet carrier densities (1014 cm-2) can be induced, exceeding conventional gate dielectrics by about one order of magnitude. Because it is an interfacial technique, EDL gating is especially effective on two-dimensional (2D) crystals, which - at the monolayer limit - are basically interfaces themselves. Both solid polymer electrolytes and ionic liquids are routinely used as ion-conducting gate dielectrics, and they have provided access to regimes of transport in 2D materials that would be inaccessible otherwise. The technique, now widely used, has enabled the 2D crystal community to study superconductivity, spin- and valleytronics, investigate electrical and structural phase transition, and create abrupt p-n junctions to generate tunneling, among others. In addition to using EDL gating as a tool to investigate properties of the 2D crystals, more recently efforts have emerged to engineer the electrolyte to add new functionality and device features, such as synaptic plasticity, bistability and non-volatility. Example of potential applications include neuromorphic computing and non-volatile memory. This review focuses on using ions for electrostatic control of 2D crystal transistors both to uncover basic properties of the 2D crystal, and also to add new device functionalities.