Surface potential fluctuations in MOS devices induced by the random distribution of channel dopant ions

The AC split CV technique has been used to measure the gate-channel capacitance of n- and p-channel MOSFETs with channel doping densities in the range 3×10¹⁶-4×10¹⁸ cm^4-3 at 295 and 77 K. Comparison to simulation has revealed a stretch-out of the measured characteristics that can be modeled by assuming a lateral threshold voltage nonuniformity with a Gaussian probability distribution. The observed variance of the threshold voltage has been accurately predicted by a three-dimensional model based on the method of images, which includes only the contribution from a random distribution of dopant ions in the depletion region. In addition to affecting the charge-voltage characteristics, surface potential fluctuations also affect carrier transport in MOS devices by introducing potential barriers in the channel. The resulting thermally activated transport process causes a reduction in the effective carrier mobility at low inversion charge densities, which has been characterized for both n- and p-channel devices at room and liquid-nitrogen temperature. The results of this study clearly demonstrate the presence of surface potential fluctuations in MOS devices, resulting from the random channel dopant ion distribution. While the effects of the surface potential fluctuations do not become significant until doping densities approach the 10¹⁸ cm^-3 level at room temperature, measurable departures from simple theory occur over the entire doping density range studied