Black holes as quantum membranes: path integral approach
  We describe the horizon of a quantum black hole in terms of a dynamical surface which defines the boundary of space-time as seen by external static observers, and we define a path integral in the presence of this dynamical boundary. Using renormalization group arguments, we find that the dynamics of the horizon is governed by the action of the relativistic bosonic membrane. {}From the thermodynamical properties of this bosonic membrane we derive the entropy and the temperature of black holes, and we find agreement with the standard results. With this formalism we can also discuss the corrections to the Hawking temperature when the mass $M$ of the black hole approaches the Planck mass $M_{\rm Pl}$. When $M$ becomes as low as $(10-100) M_{\rm Pl}$ a phase transition takes place and the specific heat of the black hole becomes positive. 
Michele Maggiore