Admittance studies of surface quantization in [100]-oriented Si metal-oxide-semiconductor field-effect transistors

The behavior of charge carriers in the inversion layer under surface-quantized conditions was observed by small-signal magnetoadmittance ($Y$) and manetoconductance ($G$) experiments on $p$-type (100) Si metal-oxide-semiconductor field-effect-transistor (MOSFET) structures. The two-dimensional character of the surface-quantized electron gas was further substantiated. The density of states was studied directly by the surface-capacitance technique. The dependence of $Y$ on a number of variables was investigated, including (1) $T\le 4.22$ °K, (2) magnetic field ($B\le 12$ T), (3) electric field excitation, (4) frequency, and (5) substrate doping (${10}^{19}\le p\le 2\times \frac{{10}^{20}}{m^3}$). Comparative studies of small-signal surface conductance and admittance were useful in establishing the advantages and limitations of each technique. It was observed that $Y$ becomes frequency independent at frequencies below 500 Hz. Both the line shape and fine structure observed at $Y$ at threshold are attributed to a bound-state band overlapping the conduction band resulting from surface-potential fluctuation. Circuit modeling of the MOSFET structures was found to be important in the interpretation of the admittance.