AlGaN/GaN high electron mobility transistors (HEMTs) were immobilized with various receptors, including antibodies, duplex DNA, and HIV reverse transcriptase (RT) enzymes to detect ligands, including peptides, SARS proteins, and HIV drugs, respectively. Signals generated by the sensors were fitted into binding-site models and analyzed. The dissociation constants of the ligand-receptor pairs and the number of binding-sites on receptors were resolved. The HEMTs and the models were demonstrated to be useful for drug developments and for elucidating SARS virus replication.