  The conductance change of nanowire field-effect transistors is considered a
highly sensitive probe for surface charge. However, Debye screening of relevant
physiological liquid environments challenge device performance due to competing
screening from the ionic liquid and nanowire charge carriers. We discuss this
effect within Thomas-Fermi and Debye-Huckel theory and derive analytical
results for cylindrical wires which can be used to estimate the sensitivity of
nanowire surface-charge sensors. We study the interplay between the nanowire
radius, the Thomas-Fermi and Debye screening lengths, and the length of the
functionalization molecules. The analytical results are compared to
finite-element calculations on a realistic geometry.
