devices with V DC g may be utilized for charge detection [24] as near the onset of non-linearity the change in amplitude as a function of V DC g is very large. In summary, we have studied the electromechanical properties of doubly clamped InAs NW resonators. Their size and tunable electron density allow us to map behavior that has not been manifested in a single device. We have observed and quantitatively explained the competition between the softening of stiffness of the restoring force of the resonator due to the variation of the electrostatic force under variable gate voltage. At larger voltages, the stretching of the nanowire leads to increased stiffness resulting in a non-monotonic dispersion of the fundamental mode with V DC g . The screening of electric fields due to the variation in the density of the electrons in our suspended FET devices modifies the amplitude because the variation of the screening length spans the cross sectional dimension of our nanowire. Further, the non-linear properties of our device can be understood qualitatively using the Duffing equation that explains the hysteretic response of the amplitude as a function of gate voltage. Thus in a single device, we demonstrate, separate and account for three diverse behaviors. Our measurements indicate that measuring electromechanical response influenced by charge screening could lead, in the future, to new ways to probe spin physics by exploiting spin-dependent charge screening [41]. Probing the physics by tuning electron density in NEMS devices may help probe the role of defects [42] and electron hopping as one moves from insulating to conducting regimes. Control over the non-linear dynamics may be achievable by controlling mode mixing and DC gate voltage. This work was supported by Government of India. J. Parpia was supported by DMR- 0457533. 9 [1] K. L. Ekinci and M. L. Roukes, Review of Scientific Instruments 76 , 061101 (2005). [2] R. G. Knobel and A. N. Cleland, Nature 424 , 291 (2003). [3] Y. T. Yang, C. Callegari, X. L. Feng, K. L. Ekinci, and M. L. Roukes, Nano Letters 6 , 583 (2006). [4] K. Jensen, K. Kim, and A. Zettl, Nat Nano 3 , 533 (2008). [5] B. Lassagne, D. Garcia-Sanchez, A. Aguasca, and A. Bachtold, Nano Letters 8 , 3735 (2008). ∗ deshmukh@tifr.res.in