Mobility and high electric fields

Although the mobility constant of an ionic species can be easily measured at low electric potentials, at high potentials the constant often appears to be dependent on the potential or apparent electric field. Clearly, if the mobility varies with the applied potential, then the concept of a mobility constant becomes ambiguous. This paper examines the assumptions normally made to convert raw data to a mobility. The examination shows the ambiguity arises when the space-charge effect is ignored in the analysis at high potentials, Using the charge-injection equation, the result shows the mobility at high electric fields can still be considered a constant. The analysis predicts the nonlinear rise in the measured current density at high field, without the need to impose a nonconstant mobility. Conversely, the result shows the true mobility can be obtained by extrapolating the apparent mobility to zero electric field