undoped, liquid-encapsulated-czochralski (lec)-grown, semi-insulating (si) gaas crystals were electrically characterized by temperature-dependent hall effect and conductivity measurements to establish the mechanism for thermally induced surface degradation. x-ray photoelectron spectroscopy (xps) was used to obtain depth profiles of the ga and as composition, and the nature of the defect complexes and their changes were determined using fourier-transform infrared spectroscopy (ftir). boron-related inhomogeneities were found to occur, and although it is normally considered a passive impurity, the authors conclude that it was in fact responsible for the detrimental p-conversion through the formation of b/sub ga/-ga(i), b/sub ga/-ga/sub as/, and b/sub ga/-v/sub as/ defect complexes. they propose that boron homogenization inhibits the escape of ga from the crystal surface during annealing, leaving an excess of ga in the p-converted layers with ftir they observed changes in the local vibrational-mode absorption of the acceptor defects si/sub as/, c/sub as/, and the donors b/sub ga/, si/sub ga/, and b/sub ga/-si/sub as/ upon annealing. they present a theoretical analysis, which explains the observed changes in electrical characteristics on the basis of variations in deep-donor compensators, less deep donors, and net shallow acceptors, and conclude that higher device yields are achievable in si gaas if boron contamination can be reduced