Modeling study of ultrathin gate oxides using direct tunneling current and capacitance-voltage measurements in MOS devices

Using both quantum mechanical calculations for the silicon substrate and a modified WKB approximation for the transmission probability, direct tunneling currents across ultra-thin gate oxides of MOS structures have been modeled for electrons from the inversion layers in p-type Si substrates. The modeled direct tunneling currents have been compared to experimental data obtained from nMOSFET's with direct tunnel gate oxides. Excellent agreement between the model and experimental data for gate oxides as thin as 1.5 nm has been achieved. Advanced capacitance-voltage techniques have been employed to complement direct tunneling current modeling and measurements. With capacitance-voltage (C-V) techniques, direct tunneling currents can be used as a sensitive characterization technique for direct tunnel gate oxides. The effects of both silicon substrate doping concentration and polysilicon doping concentration on the direct tunneling current have also been studied as a function of applied gate voltage.