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We use the stochastic linearized Poisson-Boltzmann equation to model the fluctuations in nanowire field-effect biosensors due to changes in the orientation of the biomolecules. Different orientations of the biomolecules with respect to the sensor surface due to Brownian motion have different probabilities. The probabilities of the orientations are calculated from their electrostatic free energy. The structure considered here is a cross section through a rectangular silicon nanowire lying on a an oxide surface with a back-gate contact. The oxide surface of the nanowire is functionalized by biomolecules in an electrolyte with an electrode. Various combinations of PNA (peptide nucleic acid), single-stranded DNA, and double-stranded DNA are simulated to discuss the various states of a DNA sensor. A charge-transport models yields the current through the transducer that compares well with measurements.