Using the Laplace transform method, the specific experimental conditions are given under which the extraction of a Gaussian distribution of barrier heights, in the Si–SiO2heterojunction, is based. Specifically described are: (1) the maintenance of a constant oxide field despite oxide charging; (2) the determination of the substrate field produced by the backgate bias; (3) the extraction of the short‐circuit source–drain photocurrent collected under the gate from the total short‐circuit photocurrent which is the sum of the photocurrents collected under the gate and by the source–drain junctions; (4) the minimization of inversion layer free‐carrier absorbtion to make the amount of incident light that penetrates the substrate independent of both the gate and backgate bias; and (5) the implementation of the measurement system to extract the barrier height distribution. ©1995 American Institute of Physics.