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The width of the QHE plateaus ΔB is investigated in GaAs/-AlGaAs using DC and microwave (MW) techniques. ΔB is narrower at MW frequencies than at DC and saturates below 2.4K. No fractional plateaus were observed at MW frequencies where they appeared at DC.

The observation of integer quantum Hall, effect (IQHE) behavior of σxv at microwave frequencies (~30 GHz) is reported. This clears the discrepancy concerning the socalled low-frequency breakdown of the IQHE, It was observed in a.c, experiments in the MHz range whereas 100 ns pulse experiment did not show a deviation from the d.c. behavior. In this investigation, for the observation of σxy at microwave frequencies, a crossed waveguide arrangement is used. The common part of the cross-sections of the two waveguides is much smaller than the sample area. This is expected to minimize edge effects. From the observed quantized behavior of σxy we conclude that the mechanism of an effective delocalization at low frequencies has to be ruled out.

Quantum Hall effect (QHE) behavior of the Hall conductivity σxy is observed in multiple (MHS) and single heterostructures (SHS) of GaAs/AlxGa1-x As at microwave frequencies (Ka band). Integer plateaus with even filling factor i are observed in σxy of the MHS as well as of the SHS with 2D electron and hole gas layers. In the case of the 2DEG in a SHS also the i=3 plateau is resolved. With this SHS it is clearly shown that the σxy plateaus occur equidistant on a 1/B scale and that the σxy values on the plateaus are multiples of a fundamental value.