We have carried out a comparison between flat-band transients displayed in metal-oxide-semiconductor (MOS) structures fabricated on several atomic layer deposited (ALD) high-k dielectric films: HfO2, ZrO2, Al2O3, Ta2O5, TiO2, and Gd2O3. The gate voltage as a function of time is recorded while keeping constant the capacitance at the initial flat band condition (CFB). Since samples are in darkness, under no electric fields and no charge-injection conditions, transients must be due to charge trapping of localized states produced by electrons (holes) coming from the semiconductor by tunnelling. The process is assisted by phonons and it is therefore thermally activated. The temperature-transient amplitude relation follows an Arrhenius plot which provides the thermal activation energy of soft-optical phonons. Finally, we describe the dependencies of the flat-band voltage on the setup bias history (accumulation or inversion) and the hysteresis sign (clockwise or counter-clockwise) of the capacitance-voltage (C-V) characteristics of MOS structures.