We present time-, temperature- and intensity-dependent photoluminescence me
asurements of undoped, n-type and p-type GaN epilayers. In the nominally un
doped samples we observe at low temperatures the trapping of free excitons
by neutral donors and subsequent radiative recombination. In the n-type Si-
doped samples bound-exciton luminescence is dominant over a wide range of t
emperatures. The luminescence from p-type Mg-doped samples is dominated by
shallow-donor-shallow-acceptor pair recombination and by a deep blue centre
s at 3.0 eV. These two emission bands show identical temperature and linear
intensity dependence.