We describe experimental and theoretical studies to determine the effects of phosphorous as a passivating agent for the SiO2/4H-SiC interface. Annealing in a P2O5 ambient converts the SiO2 layer to PSG (phosphosilicate glass) which is known to be a polar material. Higher mobility (approximately twice the value of 30-40 cm(2)/V s obtained using nitrogen introduced with an anneal in nitric oxide) and lower threshold voltage are compatible with a lower interface defect density. Trap density, current-voltage and bias-temperature stress (BTS) measurements for MOS capacitors are also discussed. The BTS measurements point to the possibility of an unstable MOSFET threshold voltage caused by PSG polarization charge at the O-S interface. Theoretical considerations suggest that threefold carbon atoms at the interface can be passivated by phosphorous which leads to a lower interface trap density and a higher effective mobility for electrons in the channel. The roles of phosphorous in the passivation of correlated carbon dangling bonds, for SiC counter-doping, for interface band-tail state suppression, for Na-like impurity band formation and for substrate trap passivation are also discussed briefly. Published by Elsevier Ltd.