Ligand-receptor binding site model is used to elucidate the binding affinity between ligands and receptors, with transistor-based sensors. AlGaN/GaN high electron mobility transistors (HEMTs) were immobilised with antibodies and human immunodeficiency virus type 1 reverse transcriptase enzymes to detect peptides and human immunodeficiency virus drugs, respectively. The signals generated by the sensors because of the binding of the ligands to the receptors were fitted into the binding-site models and analysed. The dissociation constants of the ligand-receptor pairs and the number of the binding sites on the receptors were revealed. The results are very consistent with the data reported by the other methods from the literatures. The incorporation of the HEMTs and the binding-site models is demonstrated to be useful for studying the mechanism of the biomolecular interaction and the application for quick and cost-effective drug developments.