Current density—fieldstrength characteristics of MOS structures with thermally grown SiO2 and undoped and with arsenic doped pyrolytic SiO2 are measured. The results show that there exist different ranges of current conduction which could be refered to typical energy distributions of bulk trap density within the insulator. In this way tunnel hopping range at lower fields strengths, Poole‐Frenkel behaviour, and Fowler‐Nordheim range could be understood very well. Looking to de current behaviour, inhomogeneities of the trap density within SiO2 layers are to be taken into account as the normal case of such layers.