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A new sensing metric is proposed for a field-effect transistor (FET)-based biosensor. As proof of concept, a nanogap-embedded FET is studied to reduce data fluctuations that originate from process variations during FET fabrication and environmental variations stemming from bioexperiments. The new sensing metric utilizes a crucial gate voltage (VG@Isub,max), which induces the maximum substrate current. The new sensing metric shows higher immunity against variations of the nanogap length, compared with the commonly used metric that relies on threshold voltage or drain current. The proposed metric also shows smaller fluctuation, which is caused by environmental variation coming from biotreatment steps. This analysis is verified experimentally and proved by device simulations. For simple analysis, the effect of external charge of the biomolecules is eliminated by using peptide nucleic acid, which is an electrically neutral biomolecule. Thus, by using such biomolecules, the permittivity effect rising from the biomolecules within the nanogap of the gate dielectric is investigated.