The performance of n-MOSFET's with furnace N2O-annealed gate oxides under dynamic Fowler-Nordheim bipolar stress was studied and compared with that of conventional oxide (OX). Time-dependent dielectric breakdown at high frequency was shown to be improved for the N2O-annealed devices compared with that for devices with OX. In addition, a smaller Vt shift for nitrided samples after stress was found. The shift decreased with increasing stressing frequency and annealing temperature. Measurements of both Gm and Dit revealed a 'peak' frequency at which the degradation was the worst. A hole trapping/migration model has been proposed to explain this.