The influence of interface states upon the admittance of metal-semiconductor diodes

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Abstract
The effects of interface states upon the electrical properties of metal-semiconductor diodes are investigated with the purpose of casting some light on current interpretations of capacitance and conductance measurements. The predominance of majority-carrier mechanisms in the ideal diode is quantitatively reassessed. Several kinds of interface state are considered, accounting as well for states in equilibrium with the semiconductor as for states in equilibrium with the metal. Electrostatic and kinetic effects lead to three types of admittance that are evaluated and compared under a small signal voltage superimposed on a forward bias. The major contribution arises from the reaction of interface charge variations with the diode conductance and depletion width. Corresponding conductances and capacitances are analysed as a function of the energy distribution of states, bias voltage, frequency and temperature. Densities about 1013 eV-1 cm-2 or higher and the existence of several types of states lead to non-linear effects as a function of the density of states and even to negative differential capacitance when recombining centres are concerned. Some fundamental and technological implications are discussed.