will not affect the microwave frequency comb as the capacitance of the insulation will not attenuate the waveform the dielectric layer of the insulation also acts as a dc block to prevent the flow of electrical charges between the semiconductor 30 and spectrum analyzer 50 as can be seen in the dc circuit diagram fig7 this circuit is essentially the same as in the prior art except that instead of the resistance of the depletion region being represented the spreading resistance r sp is represented the high frequency circuit fig8 however replaces the resistance and capacitance of the depletion layer represented as being in parallel with each other in fig4 with a single spreading resistance if the semiconductor is forward biased then there will be no depletion region so the high frequency equivalent circuit in fig8 yields a power described by the following equation where i sn is the complex value of the current at the nth harmonic the resistance of the semiconductor bulk r b is negligible due to the proximity of the microwave probe 60 and the tunneling junction combined with the fact that the spreading resistance naturally dwarfs this value the resistance of the spectrum analyzer r sa is a known constant quantity and is easily accounted in the equation therefore changes in power are readily attributed to the spreading resistance of the semiconductor sample the spreading resistance from a hemispherical electrode with radius r 0 at the surface of a half space of semiconductor having resistivity.