The sustained development of digital electronics has driven for many years the advances of silicon device simulation. However, fundamental simulation issues that must be addressed in modern digital devices are also quite relevant for high frequency applications. Quantum effects must be considered in the simulation of aggressively scaled structures, while thermal effects are particular relevant for power applications. Conventional simulation approaches based on the standard semiconductor equations, although computationally efficient, do not always provide a sufficiently predictive model. More advanced physical approaches rely on an extended Boltzmann equation model that can include explicitly the detailed band structure, nonlinear transport effects, quantum corrections and a selfconsistent treatment for self-heating and phonon transport. Such complicated physical models are more easily implemented in particle Monte Carlo simulation.