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The goal of this work is to study the transient electrical features of metal/high-k interface in MOS stacks. To this aim, electrons are injected from the gate of TaN/Al2O3/SiO2/p-Si capacitors in pulsed regime. The displacement current flowing during the pulse front is detected, and the capacitance is extracted. As a result, for the first time, a flat-band instability related to electron trapping in states near the metal/high-k interface is measured on a millisecond timescale. A picture of the time evolution of the charge density inside the oxide bulk is drawn, with the aid of a finite-element simulator. Reliability of the metal/high-k interface is also investigated by performing stress experiments in pulsed conditions. It is shown that after a certain number of pulses, the creation of new traps at the top interface of the MOS stack becomes relevant.