A method of detecting a presence of bio-molecules, or a concentration of the target bio-molecules using a field effect transistor, includes allowing a first sample including a first target bio-molecule to contact a sensing surface of the field effect transistor and measuring a change in an electric signal of the field effect transistor, the field effect transistor including a substrate, a source region and a drain region, the source region and the drain region formed apart from each other on the substrate, the source region and the drain region each doped to having an opposite polarity than a polarity of the substrate, a channel region disposed between the source region and the drain region and an insulating layer including the sensing surface, the insulating layer disposed on the channel region.