  We calculate the structure of accretion disk around a spinning black hole for
accretion rates 0.01 - 10 M_sun/s. The model is fully relativistic and treats
accurately the disk microphysics including neutrino emissivity, opacity,
electron degeneracy, and nuclear composition. We find that the accretion flow
always regulates itself to a mildly degenerate state with the proton-to-nucleon
ratio Y_e ~ 0.1 and becomes very neutron-rich. The disk has a well defined
"ignition" radius where neutrino flux raises dramatically, cooling becomes
efficient, and Y_e suddenly drops. We also calculate other characteristic radii
of the disk, including the neutrino-opaque and neutrino-trapping radii, and
show their dependence on the accretion rate. Accretion disks around
fast-rotating black holes produce intense neutrino fluxes which may deposit
enough energy above the disk to generate a GRB jet.
