This brief proposes an analytical approach to model the DC electrical behavior of extremely narrow cylindrical junctionless nanowire field-effect transistors (JL-NW-FETs). The model includes explicit expressions, taking into account the first-order perturbation theory for calculating eigenstates and corresponding wave-functions obtained by the Schro center dot dinger equation in the cylindrical-coordinate. Assessment of the proposed model with technology computer-aided design (TCAD) simulations and measurement results confirms its validity for all regions of operation. This represents an essential step toward the analysis of circuits, mainly biosensors based on junctionless nanowire transistors.