In the classic transistor, the number of electric charge carriers—and thus the electrical conductivity—is precisely controlled by external voltage, providing electrical switching capability. This simple but powerful feature is essential for information processing technology, and also provides a platform for fundamental physics research1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16. As the number of charges essentially determines the electronic phase of a condensed-matter system, transistor operation enables reversible and isothermal changes in the system’s state, as successfully demonstrated in electric-field-induced ferromagnetism2, 3, 4 and superconductivity5, 6, 7, 8, 9, 10. However, this effect of the electric field is limited to a channel thickness of nanometres or less, owing to the presence of Thomas–Fermi screening. Here we show that this conventional picture does not apply to a class of materials characterized by inherent collective interactions between electrons and the crystal lattice. We prepared metal–insulator–semiconductor field-effect transistors based on vanadium dioxide—a strongly correlated material with a thermally driven, first-order metal–insulator transition well above room temperature17, 18, 19, 20, 21, 22, 23—and found that electrostatic charging at a surface drives all the previously localized charge carriers in the bulk material into motion, leading to the emergence of a three-dimensional metallic ground state. This non-local switching of the electronic state is achieved by applying a voltage of only about one volt. In a voltage-sweep measurement, the first-order nature of the metal–insulator transition provides a non-volatile memory effect, which is operable at room temperature. Our results demonstrate a conceptually new field-effect device, extending the concept of electric-field control to macroscopic phase control.
Figures at a glance
Figure 1: The first-order metal–insulator transition in VO2. 
a, Schematic drawings of the thermally driven first-order metal–insulator transition (MIT) in VO2. b, Temperature dependence of the resistivity of strained 10-nm and relaxed 70-nm VO2 films grown on TiO2 substrates. c, Schematic of an electric-double-layer transistor (EDLT) based on VO2, potentially enabling electrical switching of the MIT between the metallic tetragonal phase and the insulating monoclinic phase.
Figure 2: Effect of electric field on the transport properties of a 10-nm-thick, strained VO2 film. 
a, Temperature dependence of the sheet resistance (Rs) for a 10-nm strained VO2 film with different gate voltages (VG). Inset shows the resulting phase diagram. The transition temperature (TMI) is defined as the average of the two inflection points (for cooling and warming, respectively) in plots of d[ln(Rs)]/d(1/T) versus temperature24. b, VG dependence of Rs, measured at T = 260 K. Sweep rate, 15 mV s−1. c, Schematic energy diagrams of VO2, showing a double-minimum potential as a function of the normal coordinate.
Figure 3: Emergence of the three-dimensional metallic ground state. 
a, Rs versus temperature in 10-, 20- and 70-nm films, showing both initial (VG = 0) and electric-field-induced metallic states. b, Sheet conductance (σs) of the electric-field-induced metallic states at T = 50 K as a function of film thickness. Inset, schematic depiction of the strain situation in the samples.
Figure 4: Electronic phase diagrams of electrostatically and chemically doped VO2 films. 
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