  In this paper, we develop an explicit model to predict the DC electrical
behavior in ultra-thin surrounding gate junctionless nanowire FET. The proposed
model takes into account 2D electrical and geometrical confinements of carrier
charge density within few discrete sub-bands. Combining a parabolic
approximation of the Poisson equation, first order perturbation theory for the
Schrodinger subband energy eigenvalues, and Fermi-Dirac statistics for the
confined carrier density leads to an explicit solution of the DC characteristic
in ultra-thin junctionless devices. Validity of the model has been verified
with technology computer-aided design simulations. The results confirms its
validity for all regions of operation, i.e., from deep depletion to
accumulation and from linear to saturation. This represents an essential step
toward analysis of circuits based on junctionless nanowire devices.
