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The modeling of nanoscale transistors at THz frequencies is discussed. The simulation of these dcvices at such very high frequencies can not rely on the assumption that the temporal variations inside the device are slower than any electron kinetic time (i.e. the quasi-static approximation).The study presented here is twofold. First, a novel formalism for quantum transport under oscillating conditions is applied to discuss high-frequency transconductance of nanoscale MOSFET. Second, the importance of the displacement current is analyzed via the solution of a 3D Poisson equation. Preliminary results seem to suggest important effects due to the small number of electrons inside the system. 1.-IntroductionTechnology demands lead to the fabrication of smaller and faster devices. In this sense, it is predicted that next future transistors will be designed for channel length of few nanometers, and they will work at frequencies close to the THz range [ I ] . In this manuscript we are addressing important issues not Considered yet in the modeling of these THz transistors [2,3] In particular, we are interested in studying the performance of nanoscale double gate MOSFET at frequencies high enough to be comparable with the inverse of the electron transit time [4]. The modeling of Transistors at such very-high frequencies cannot rely on the quasi-static approximation used in most simulation frameworks [2,3]. In particular, we will study two diffcrent (but related) modeling issues.On one hand, electron transport in these small ( d o nm) devices can be studied assuming phase-coherent transport (without considering the effect of the phonon or impurity scattering mechanisms). Therefore, the modeling of these devices under high frequency conditions (on the order of the inverse of the electron transit time) needs to take into account the spatial and also the temporal phase-coherence of electrons. In section 2, we use a novel formalism to study instantaneous current in arbitrarily time-dependent driven quantum systems [ 5 ] . As an example, we provide numerical results for the high-frequency transconductance ( Y 2 , ) of nanoscale MOSFETs [SI.On the other hand, for very high frequency conditions, the total current through the device is the sum of the conduction current (related with counting electron) plus the displacement current (related with temporal variations of the electric field). Overall current continuity is only achieved when both components are considered simultaneously [ 6 ] . We have developed a detailed three-dimensional Poisson solver to study conduction and displacement currents in nanoMOSFETs via the classical Monte Carlo technique 171. In section 3, we show preliminary results for the simulation of the DG-MOSFET mentioned above where both, the displacement and conduction, currents are taken into account.Finally, in section 4 we present the conclusions of these preliminary results and outline our future work. 2.-Quantum instantaneous currentAs we have mentioned, the evaluation of the particle current in phase-cohere...