The kinetics of zero-field and field-induced detrapping of electrons trapped in HfSixOy and HfSiON after positive bias stress on n+-polycrystalline silicon (polySi) gate of n-type metal-oxide-semiconductor (nMOS) capacitors are experimentally investigated. The self detrapping follows a simple logarithmic relation with time while field-induced detrapping upon reversing the stress voltage obeys a simple first-order exponential decay suggesting mono energetic shallow traps associated with tunnel emission of trapped electrons. Finally, our investigation raises questions about the validity of the widely used distributed capture cross section model of electron traps to explain the threshold voltage instability in MOS devices with hafnium silicate gate stacks.