A field effect transistor sensor includes: a source-drain channel, a semiconductor layer on said source-drain channel, a first gate electrode arranged above said semiconductor layer, a first well enclosing said source-drain channel, said semiconductor layer and said first gate electrode, the first well being configured to be filled, in use, with a first liquid, particularly a gating electrolyte, a second gate electrode arranged above the first gate electrode and exposed to an interior of the first well. Also disclosed is an array device including an array of field effect transistor sensors according to the above.