In order to overcome the loss of performances issue when scaling resonant sensors down to
NEMS, it proves extremely useful to study the behavior of resonators up to large
displacements and hence high nonlinearities. A comprehensive nonlinear multiphysics model
based on the Euler-Bernoulli equation which includes both mechanical and electrostatic
nonlinearities in the case of a capacitive doubly clamped beam is presented. This purely
analytical model captures all the nonlinear phenomena present in NEMS resonators
electrostatically actuated including bistability, multistability which can lead to several
physical limitations such as noise mixing, frequency stability deterioration as well as
dynamic pull-in. Moreover, close-form expressions of the critical amplitudes and pull-in
domain initiation amplitude are provided which can potentially serve for NEMS designers as
quick design rules.