Electron-hole liquid - what is next?

The metallic conductivity of exciton liquid is discussed. With increasing of exciton density in many-exciton systems there are two important transitions. They are the gas-liquid and the insulator-metal transitions. The gas-liquid transition, which is the first order phase transition is usually accompanied by the insulator-metal transition. In the case of multivalley semiconductors the electron-hole liquid phase is always a metallic one. Modified Mott criterion for multivalley case is given. In one valley semiconductors with spherical effective masses of both electrons and holes the existence of insulating e-h liquid is possible at least theoretically. (orig.)