By means of the microscopic transport description supplied by a semiclassical two-dimensional Monte Carlo simulator, we provide an in depth explanation of the operation (based on electrostatic effects) of the nanoscale unipolar rectifying diode, so called self-switching diode, recently proposed in A. M. Song, M. Missous, P. Omling, A. R. Peaker, L. Samuelson, and W. Seifert, Appl. Phys. Lett. 83, 1881 (2003). The simple downscaling of this device and the intrinsically high electron velocity of InGaAs channels allows one to envisaging the fabrication of structures working in the THz range. We analyze the high-frequency performance of the diodes and provide design considerations for the optimization of the downscaling process. (c) 2005 American Institute of Physics.