This paper presents an experimental investigation on the effect of pH sensitivity of the dielectric surface in field effect based sensors on the label free detection of prostate specific antigen (PSA). Two types of electrolyte-insulator-semiconductor (EIS) devices using covalently attached antibodies were used in this study; a device with low pH sensitivity showed similar to 63 mV flatband voltage shift while a device with relatively higher pH sensitivity showed similar to 23 mV shift with a PSA concentration of 10 ng/ml in buffer. These observations validated the reported theoretical models, and reconciled the other reported experimental observations using adsorbed antibodies. The effects of different steps of surface modifications using aqueous solutions to covalently attach the antibodies were investigated to understand governing mechanisms responsible for enhanced PSA detection with the device with low pH sensitivity. The variations in the flatband voltages, contact angles, and pH sensitivities confirmed the tunability of the low pH sensitive devices upon any charge associated with the biomolecules (PSA). The learning from this study could be utilized to enhance the sensitivity of conventional ISFET biosensors and more specifically that of devices with non-silicon based semiconductors as the chemical functionalization processes significantly affect their performance. (C) 2017 The Electrochemical Society. All rights reserved.