Semiconductor light emission is analyzed as a paradigm of a nonequilib
rium quantum mechanical many-body problem. The medium excitations and
the quantized light field inside and outside a semiconductor slab are
treated consistently. Splitting the photon density of states into a me
dium and a vacuum induced contribution the arbitrarily strong semicond
uctor emission is described as spontaneous emission into the vacuum in
duced part. With increasing gain narrowing peaks of growing intensity
evolve for each propagation direction, whereas under laser conditions
one propagation direction is favored by the cavity.