<div>We formulate a simple, yet accurate, model for a non-uniform mobile charge density ρ(z) giving rise to a mean potential Ψ* across an inversion layer of finite extent, which we measure by means of a novel, sensitive, experimental method involving nulls of harmonic distortion components (D2 ≈ D3 ≈ 0) of the drain current under sinusoidal excitation below saturation. We thus establish analytically and experimentally, that the low-field, "universal" effective mobility µ_eff varies as ~(E*_T)^-1/3for transversal fields E_T*= <b>-</b>(1/ε_si)<b>·</b>[ɳQ_i + Q_b] <b>≤ </b>0.5 MV/cm, wherein ɳ varies continuously between 1/2 and 1/3. We also establish and observe that the higher order, derivative, parameter θ_T quantifying µ_eff’s modulation by E*_T varies as ~(E*_T)^-5/3 under laminar flow conditions, thereby further corroborating the foregoing effects and interpretations thereof.</div>