In this paper we present a simplified analysis of the Einstein relation for the diffusivity-mobility ratio of the carriers in n-channel inversion layers on non-linear optical materials under both the weak and strong electric field limits in the presence of a parallel magnetic field at low temperatures. It is found, taking n-channel inversion layers CdGeAs2 as an example, on the basis of a newly formulated 2D generalized electron energy spectra within the framework of k·p formalism incorporating various anisotropies of the energy band constants, that the diffusivity-mobility ratio increases with increasing surface electric field for both the limits together with the fact that crystal field splitting also diminish its numerical values. The same ratio increases with decreasing alloy composition for 2D systems of ternary and quaternary materials respectively. We have suggested an experimental method of determining the Einstein relation in optical materials having arbitrary dispersion laws. In addition, the corresponding well-known results for two-band Kane model in the absence of magnetic field have further been obtained as special cases of our generalized analysis under certain limiting conditions.