Low temperature photoconductivity (PC) and photoluminescence (PL) measurements from Si-doped GaN samples reveal evidence for two distinct types of donor. The PC spectra contain two strong features, one of which is very unusual because it is observed at lower energy than the sole feature in PL. The dominant feature in the PC of unetched samples, at 3.484 eV, is associated with deliberately incorporated Si donors. The second feature, 14 meV to lower energy, is associated with a native defect. The density of this defect can be increased by etching with an argon plasma. (C) 2003 Elsevier Ltd. All rights reserved.