In this study, we investigated the potential drift and sensitivity in an extended-gate (EG) field-effect-transistor (FET)-based potentiometric sensor with junctionless (JL) and inversion-mode (IM) gate-all-around (GAA) nanowire (NW) FETs. An EG electrode (EGE) coated with Al2O3 via plasma-enhanced atomic-layer deposition exhibited high stability. Both JL and IM GAA NWFETs were prepared in the same batch and exhibited high electrical performance, including a steep subthreshold swing of approximately 63 mV dec-1 and a high ON/OFF current ratio of approximately 3 ×107 at V DS = 0.1 V. However, JL GAA NWFETs exhibited higher drift stability than did IM GAA NWFETs under various working regions over a long period of operation. Furthermore, JL GAA NWFETs with EGEs (JL GAA NWFETs w-EGEs) exhibited the highest sensitivity when operating under the subthreshold region in pH sensing (ΔI/I 0 = ∼4410%). The readout sensitivity reached S out = 82%. A JL GAA NWFET w-EGE was also demonstrated as a potentiometric biosensor for streptavidin real-time detection at concentrations of 42 fM to 4.2 nM.