DISPERSION OF MOS CAPACITANCE-VOLTAGE CHARACTERISTICS RESULTING FROM THE RANDOM CHANNEL DOPANT ION DISTRIBUTION

The random nature of the channel dopant ion distribution in silicon MO S devices is shown to cause a stretch-out of the capacitance-voltage c haracteristics. This nonideal behavior is caused by surface potential fluctuations which can be accurately modeled by assuming that the thre shold voltage varies laterally over the area of the device with a norm al distribution. The standard deviation of the threshold voltage distr ibution extracted from n- and p-channel MOS devices is presented for a wide range of substrate doping levels at both room and liquid-nitroge n temperature. The observed standard deviation of the threshold voltag e is accurately predicted by a three-dimensional model based on the me thod of images which includes only the contribution from a random dist ribution of dopant ions in the depletion region. The contribution to t he surface potential fluctuations from other sources is shown to be in significant at high channel doping levels.