In this work, how the changing of barrier height in GaAs/AlxGa1-xAs quantum well structures during vertical transport according to applied voltage, sample diameter and number of wells has been experimentally determined. The samples investigated here have 0, 2, 4, 7 and 10 quantum wells. To investigate the effect of sample diameter on barrier height, we used the samples with diameters of 100, 200, 400 and 800 mu m. A voltage, parallel to growth direction, varies between 10 mV and 600 mV is applied. The barrier height values are being different which is determined from the current- temperature curves can be explained by the existence of space charges. The increasing the applied constant voltages reduce the barrier height and occurring a small difference on barrier height for the different diameter and increasing the number of well reduce the effect of the space charge has been determined.