Field and Hall effects in semiconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathrm{\delta }}$

The field-effect mobility, Hall coefficient, and conductivity as functions of the oxygen concentration and temperature are reported for ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{\delta }}$ films on the insulating side of the insulator-to-metal transition. The temperature dependence of the conductivity and Hall coefficient indicate that for small δ the excess oxygen introduces acceptor states at about 30 meV above the valence-band edge. The field effect reveals a space-charge layer in which carriers are depleted at the air-${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathrm{\delta }}$ interface. The size of the field effect is limited by localized states at the interface.