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In this article, we used AlGaN/GaN high electron mobility transistors (HEMTs) to construct the first label-free semiconductor-sensor-based binding assay to our knowledge. Our results suggested that the nucleotide-c terminal domain of SARS coronavirus (SARS-CoV) nucleocapsid protein interaction is a two-step binding event with two dissociation constants (Kd1 = 0.052 nM, and Kd2 = 51.24nM) extracted by using the modified two-binding-site Langmuir isotherm equation proposed here. We found that there were at least two protein binding sites on the specific 41-base SARS-CoV double-stranded DNA (dsDNA) genome (29,580-29621) conjugated with a 20-mer poly-dT tail. This result presented a high binding affinity is comparable with the antibody-antigen reaction, and suggested this designed dsDNA could be treated as an aptamer for SARS-CoV N protein capture.