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 μm. 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 currenttemperature 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.