Tunneling from Metal to Semiconductors

Abstract

Conductance and differential capacitance have been measured as a function of frequency and of bias on a number of silicon-silicon-dioxide-aluminum sandwiches. The results indicate that information about the density and energy of states near the silicon-silicon-dioxide interface may be deduced from these measurements. Conductance is due to electron tunneling transitions between the aluminum and silicon. Analysis of the tunnel current is given in terms of a single-particle WKB approximation. A simplified formula is derived to explain tunneling between the metal and the intrinsic semiconductor bands. A preliminary analysis of tunneling to impurity bands is discussed. Experimental data are shown which display details of impurity-band spreading near the valence-band edge of silicon samples containing ${10}^{18}$ and ${10}^{19}$ boron atoms/${\mathrm{cm}}^3$. Tunneling from silicon-silicon-dioxide interface states is analyzed by defining a tunneling lifetime and using this concept to explain the observed frequency dependence of the conductance and capacitance measurements. Quantitative experimental values are obtained giving the density of interface states versus energy. These results are in close agreement with those obtained by Statz et al., from inversion-layer conductance studies.