A method for determining whether a sample suspected of containing an analyte does so, comprising the step of: providing a field effect transistor, the field effect transistor comprising: an electrically conductive elongate structure, said electrically conductive elongate structure having a width or diameter of less than about 1 micron, the electrically conductive elongate structure having a reaction entity immobilized relative thereto capable of interacting with the analyte, at least a portion of the electrically conductive elongate structure being addressable by the sample; and a gate electrode for gating the electrically conductive elongate structure characterised in that the method further comprises the step of adjusting the voltage of the gate electrode such that the electrically conductive elongate structure has a Debye screening length in the sample that is greater than the average cross-sectional dimension of the electrically conductive elongate structure, the Debye screening length being defined by λ = / p e 2 ε k B T wherein ε is the dielectric constant of the electrically conductive elongate structure material, p is the carrier concentration in the electrically conductive elongate structure, k B is the Boltzman constant, T is the temperature, and e is the elementary charge. providing a field effect transistor, the field effect transistor comprising: an electrically conductive elongate structure, said electrically conductive elongate structure having a width or diameter of less than about 1 micron, the electrically conductive elongate structure having a reaction entity immobilized relative thereto capable of interacting with the analyte, at least a portion of the electrically conductive elongate structure being addressable by the sample; and a gate electrode for gating the electrically conductive elongate structure an electrically conductive elongate structure, said electrically conductive elongate structure having a width or diameter of less than about 1 micron, the electrically conductive elongate structure having a reaction entity immobilized relative thereto capable of interacting with the analyte, at least a portion of the electrically conductive elongate structure being addressable by the sample; and a gate electrode for gating the electrically conductive elongate structure characterised in that the method further comprises the step of adjusting the voltage of the gate electrode such that the electrically conductive elongate structure has a Debye screening length in the sample that is greater than the average cross-sectional dimension of the electrically conductive elongate structure, adjusting the voltage of the gate electrode such that the electrically conductive elongate structure has a Debye screening length in the sample that is greater than the average cross-sectional dimension of the electrically conductive elongate structure, the Debye screening length being defined by λ = / p e 2 ε k B T wherein ε is the dielectric constant of the electrically conductive elongate structure material, p is the carrier concentration in the electrically conductive elongate structure, k B is the Boltzman constant, T is the temperature, and e is the elementary charge.