he properties of the free-exciton transitions in undoped MOCVD GaAs epitaxial layer of thicknesses varying from 4 to 30 Urn were determined by using the photoconductivity measurements.It was found that as the thickness is increased,the binding energy R.increases,and the free exciton peak for n=1 shifts slightly towards lower energies,but both the intensity of the higher excitonic levels and the lifetime T of the excitons decrease.These effects were attributed mainly to the defects and electrical field near the surface of the epilayer.Finally,the photoluminescence results and the influence of the measurement temperature on them were also discussed.