This paper presents a numerical simulation of a Si MITATT diode working in the submillimeter-wave and lower terahertz frequency range. A full band Monte Carlo simulation that included the microscopic details of carrier transport and a simpler drift-diffusion based model were used to investigate the diode DC, small-signal and large-signal properties of MITATT diodes operating between 200 and 300 GHz. Although the full band Monte Carlo simulation shows the effects of transient velocities and dead zones required by energy conservation during ionization, there is still a reasonable agreement between the two simulations. The results show that silicon based transit-time devices can produce significant power up to high submillimeter-wave frequencies. The paper will describe the simulation of transit-time devices at very high frequencies, compare results from a detailed physical Monte Carlo model with a numerically simpler drift-diffusion approach and describe device performance between 200 and 300 GHz. (c) 2006 Elsevier Ltd. All rights reserved.