The main goal of this work is to study the effect of the structure geometries on the time dependent current in the nano electronic devices. The Ramo-Shockley-Pellegrini theorems are used together with many-particle Monte Carlo simulator to study this problem. In particular, it is shown that when the lateral surfaces (Ly, Lz) where the total current is collected are decreased, while keeping the longitudinal dimension (Lx) fixed, the high frequency range where the AC spectrum is still meaningful increases.