Semi-insulating Al0.3Ga0.7AS epitaxial films grown by molecular beam epitaxy at 400°C have been characterised by a recently proposed technique known as zero quiescent bias voltage transient current spectroscopy (ZBTCS). Separated electron trap spectra and hole trap spectra are obtained by ZBTCS. From these spectra, a prominent hole trap with an activation energy of 1.01eV, a prominent electron trap with an activation energy of 0.86eV and a continuum of states are detected. The trap concentrations of the detected hole trap and electron trap are calculated to be about 3.6x1015cm−3 and 9.7x1015cm−3 respectively. The continuum of states is believed to be due to the surface states of the Al0.3Ga0.7AS film. By passivating the surface with (NH4)2S solution, the magnitudes of the continuums in the spectra reduce significantly. With the reduction of the surface states, hole injections that interfere the electron trap spectra are suppressed, and consequently electron trap spectra with more pronounced peaks are produced.