Deep defects in undoped c- and a-planes GaN layers grown by MOVPE on sapphire substrates were investigated by photoluminescence (PL), photocurrent (PC) and thermally stimulated current spectroscopy (TSC). In a-plane GaN the PL spectra are dominated by basal stacking faults and donor–acceptor pair recombinations correlated signals, whereas the near band-edge emission is the dominating in c-plane GaN layers.In TSC, no evidence is found for a strong trap emission of a-plane GaN correlated with the basal stacking fault-related PL signal. But, the high concentration of stacking faults in a-plane GaN layers induces a high concentration of deep defects with activation energies of 220 and 320meV causing the DAP transition in Pl, the broad peak B in PC and TSC trap emissions in the temperature range T2. The trap at 220meV is well known from c-plane GaN and related to dislocations, which support the assumption that this deep trap is introduced by the stacking faults in a-plane GaN.