Biosensor having ultra fine fiber

A biosensor device including a biosensor device body and an element section which is attachable to and detachable from the biosensor device body, wherein said element section includes a field effect transistor comprising:

• a substrate having a first insulating film, a second insulating film and a support substrate composed of a semiconductor material or metal, the support substrate being sandwiched between said first insulating film and said second insulating film;
• a source electrode and a drain electrode disposed on the first insulating film;
• a channel electrically connecting said source electrode and said drain electrode, and including an ultra fine fiber, said channel being disposed on said first insulating film;
• a gate electrode capable of causing a polarization due to a movement of free electrons in said substrate and capable of controlling an electric current flowing through said channel, said gate electrode being arranged on said second insulating film;
• and
• molecules immobilized on said second insulating film, said molecules capable of capturing a detection target substance being sandwiched between said second insulating film and said gate electrode, wherein when said element section is mounted to said biosensor device body, the electric current flows through said channel, and said gate electrode controls the electric current flowing through said channel.