Provided is a field effect transistor (FET) type biosensor including a source electrode, a gate, and a drain electrode. A ligand that can bind to a side of a nucleic acid is added to the surface of the gate. In a conventional FET type biosensor, it is difficult to detect a signal within the debye length because a target nucleic acid is directly fixed to the surface of a gate of the conventional FET. However, in the present invention, this problem can be overcome and the debye length can be increased by treating the surface of a gate of an FET sensor with a ligand that can bind to a side of a nucleic acid. The ligand can be adsorbed onto the surface of the gate. In this case, the nucleic acid is adsorbed parallel to the surface of the gate, not perpendicular to the surface of the gate, thus generating an effective depletion region. In addition, hybridization efficiency can be increased because a hybridized sample can be injected into an FET sensor at high ionic strength.