  The bias dependent interface charge is considered as the origin of the
observed non-ideality in current-voltage and capacitance-voltage
characteristics. Using the simplified model for the interface electronic
structure based on defects interacting with the continuum of interface states,
the microscopic origin of empirical parameters describing the bias dependent
interface charge function is investigated. The results show that in non-ideal
metal-semiconductor contacts the interface charge function depends on the
interface disorder parameter, density of defects, barrier pinning parameter and
the effective gap center. The theoretical predictions are tested against
several sets of published experimental data on bias dependent ideality factor
and excess capacitance in various metal-semicoductor systems.
