Field effect transistors (FETs) may respond to charge variations occurring when proteins are adsorbed to the gate surface. The electrolyte electrostatic screening allows a practical detection of charges that are only within a distance of approximately the double layer thickness, which is related to the Debye length in the electrolyte. Hence, operating the FET biomolecular sensors at the highest possible ionic concentration improves reproducibility; however, it may reduce the signal if the concentration is too high. Here, we propose an optimization method of the buffer ionic concentration using a figure of merit "charge number," Z(d), which is defined as the ratio between the effective number of the surface charges and the number of immobilized molecules. The theoretical Z(d) was calculated using a three-dimensional conformation data taken from the Protein Data Bank. The Z(d) was obtained from the total number of four charged amino acids of avidin, arginine (positive), lysine (positive), aspartic acid (negative), and glutamic acid (negative), which are within the double layer length from the surface. To verify this model, we chose avidin-biotin interaction. The experimental Z(d), which was obtained for three buffer concentrations, matched the theoretical Z(d). This method shortens the solution calibration time and reduces the analyte amount. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3491764] All rights reserved.