An analytical model of a nanogap-embedded field-effect transistor, which is termed here simply as a biotransistor, is developed in this study. A surface potential model is attained by solving a 2-D Poisson equation with approximation of a parabolic potential profile along the channel depth. The analytical threshold voltage is then derived from the surface potential model to comprise the unique feature of the biotransistor, which acts as a biosensor. A shift of the threshold voltage was used as a metric to ascertain the sensitivity after the biomolecule interacts with the biotransistor. Various device parameters were investigated in the developed analytical model. The characteristic trend is supported and verified via a simulation. Hence, the proposed model can provide a useful guideline for the optimal design and fabrication of a biotransistor.