We present a microscopic analysis of the influence of the spatial correlations between local diffusion noise sources on the noise calculation in a submicron GaAsn+nn+diode under different applied voltages. The simulation is carried out using an ensemble Monte Carlo simulation. We demonstrate that in the case of submicron nonhomogeneous structures the use of the diffusion coefficient to characterize the local noise sources is not correct, specially under far‐from‐equilibrium conditions. The nonuniformity of the electric field and the nonstationary behavior of the electrons lead to significant changes in the spatial correlations with respect to the case of an homogeneous semiconductor. Therefore, the diffusion noise at the terminals must be calculated in terms of the correlations between the local noise sources. ©1995 American Institute of Physics.