Carrier-density fluctuations and the IGFET mobility near threshold

Abstract

The behavior of the inversion-layer mobility observed in the IGFET at biases close to threshold is explained in terms of small fluctuations in the density of minority carriers. These fluctuations are in response to localized charges at the oxide-semiconductor interface. The theory predicts the abrupt drop of mobility with decreasing carrier density near threshold which is observed in the dc conductance mobility, field-effect mobility, and Hall mobility. The analysis also indicates that the dc mobility drop occurs at higher carrier densities at 4.2 °K than at 300 °K due to the larger fluctuations of the minority carriers at low temperatures (when they are all in a single quantum subband) than at high temperatures (when they are distributed over a large number of subbands). The anomalous threshold shift of the IGFET with temperature for temperatures below 77 °K is attributed to carrier population changes with temperature, i.e., with the freezeout with decreasing temperature of carriers from higher subbands, where fluctuations are small, into the lowest fluctuation-dominated quantum subband. The Chen-Muller mobility step with increasing carrier density is well described. A large number of other experimental facts are qualitatively accounted for by the analysis, which can be made more quantitative with more accurate evaluation of various integrals, etc. In particular, orientation dependence, doping level dependence, and field dependence are all contained in the theory. A striking feature of the calculation is the marked dependence of the dc mobility dropoff upon interface quality as measured, e.g., by the interface charge density. This suggests that the low-field mobility provides a good monitor for process control in device fabrication and that greater control may lead to sharper turn on, lower leakage, or closer threshold tolerances especially for low-voltage complementary metal-oxide-semiconductor (CMOS) applications.