An integrated array of field-effect transistor structures is used to detect two oppositely charged biopolymers: poly(L-lysine) and DNA. Local deposition of polymer solutions on part of the array induces sizeable variations in the dc current-voltage characteristics of the transistors exposed to the molecular charge. The whole transistor array is measured in the presence of a common electrolyte. Differential signals are studied as a function of electrolyte salt and polymer concentrations. The measurements provide information on the interface electrostatic potentials of the (semiconductor/biopolymer/electrolyte) system and the experimental data are compared to an analytical model which accounts for screening of the adsorbed charge by mobile ions.