Two-Dimensional Impurity States in an <i>n</i>-Type Inversion Layer of Silicon

Abstract

Shallow impurity states associated with the electric subband in the n-type surface inversion layers have clearly been observed on the Si (100) surface. Measurements of the conductivity and the transconductance have been made at temperatures from 4.2 to 300 K, especially at low gate voltages near the threshold for the surface inversion, whose values are given in terms of effective mobility as well as field effect mobility. When the surface carrier density is less than 5×1010 cm−2, the surface conductance varies as exp(−εa/kT) and the activation energy εa has been determined to be 1.2 meV. In this case, the peak of the transconductance, the differential conductance with respect to the gate voltage, disappears at 12 K, which agrees with the activation energy. When the surface carrier density is more than 1×1011 cm−2, the effective mobility is independent of temperature below 20 K and the electron statistics might be degenerate.