Deep hole traps in p-type Al0.5Ga0.5As grown by molecular beam epitaxy have been studied by the deep-level transient-spectroscopy method applied to samples with a Schottky diode configuration. Five hole traps, labeled as H0 to H4, were found. For traps H1, H3, and H4 the activation energies for emission were ET1=0.14 eV, ET3=0.40 eV, and ET4=0.46 eV, respectively. Hole emission from trap H2 was dependent on the external electric field. The emission rate obeyed the Poole–Frenkel relation. When extrapolated to zero electric field, the thermal activation energy for hole emission was ET2,0=0.37 eV. Capture cross sections for traps H1 and H4 were thermally activated with energy barriers EB1=0.04 eV and EB4=0.18 eV, respectively. © 1999 American Institute of Physics.