Contact current distortion due to tunnel effect

The third harmonic distortion ratio of the tunnel current between two electrodes separated by a thin insulating film, is evaluated by analyzing the formula derived by Simmons (J. Appl. Phys. vol. 34, no. 6, pp. 1793-803, 1963) into the Fourier series in this paper. The results show that the theoretical third harmonic distortion ratio is a function of s/spl phi//sub 0//sup 1/2/ (where s is the thickness of the film, and /spl phi//sub 0/ is the work function). The numerical evaluation results in -40 to -80 dB, when s is in the practical range of 0.4/spl sim/2.0 nm and /spl phi//sub 0/ is 1/spl sim/4 eV. Experimental results produce readings of -50 to -130 dB, when using practical contact materials (95Au-5Ni). The reasons for the discrepancy may involve the effect of the nonhomogeneous contact area, which was observed using a scanning tunneling microscope. When the contact resistance of copper with a thin insulating film is measured, the resultant small and negative thermal coefficient of contact resistance proves that tunnel resistance does exist. It is also shown that in this tunnel film, a contact deterioration caused by corrosion can clearly be detected using harmonic measurement.