Vacuum instability in external fields

We study particle creation from the vacuum by external electric fields, in particular, by fields, which are acting for a finite time, in the frame of QED in arbitrary space-time dimensions. In all the cases special sets of exact solutions of the Dirac equation (in and out solutions) are constructed. Using them, the characteristics of the effect are calculated. The time and dimensional analyses of the vacuum instability are presented. It is shown that the distributions of particles created by quasiconstant electric fields can be written in a form which has a thermal character and which seems to be universal, i.e., is valid for any theory with quasiconstant external fields. Its application, for example, to particle creation in an external constant gravitational field reproduces the Hawking temperature exactly.