Based on Boltzmann transport equation, the drift-diffusion, hydrodynamic, and Monte-Carlo physical models are accurately developed. The model equations are self-consistently solved with Poisson equation, and with Schrodinger equation when quantization effects take place, in one and two-dimensions to characterize the operation and optimize the structure of mm-wave devices. The effects of the devices dimensions, biasing conditions and operating frequencies on the accuracy of the obtained model (simulator) results are thoroughly investigated. Based on physical understanding of the models, the simulation results are analyzed and conclusions are drawn to fully determine the limits at which a certain device simulator can be accurately and efficiently used to characterize the noise behaviour of mm-wave devices.