Gate oxide scaling in NMOSFETs causes electrons to tunnel from the conduction and valence bands of the silicon substrate in the direct-tunneling regime. In NMOSFETs, the tunneling of electrons from the substrate's valence band is a source of the substrate current IB and contributes to the gate current IG. Oxide thickness scaling leads to an increase in the substrate current IB and in the ratio IB/IG of substrate to gate current. In this paper, we report the trends in the IB/IG ratio due to oxide thickness scaling in ultrathin SiO2 and SiO2/Ta2O5 composite gate dielectrics.