Theories on linear white noise sources such as thermal noise or shot noise are well established and massively used for low noise device modeling and circuit design. However, it has been experienced that diffusion noise in a large variety of pn diodes (transistors) can deviate from the expected value given by the Schottky theorem or by the Van der Ziel representation commonly used. In this work, more than ten types of pn junctions have been investigated, all featuring an increase of the diffusion noise floor in the low frequency band when operated under low d.c current conditions. These specific conditions certainly explain why such phenomenon has not been reported earlier; however, this noise degradation becomes a problem as many systems make use of pn diodes for low signal photodetection (PPD or CCD), operating at very low (dark) current. For the first time, we report current spectral densities deviations from the Shottky theorem at low frequency; a focus on the experimental workbench is given to remove any doubt regarding the opportunity to analyze data under concern. Then, low frequency noise spectra are presented for various diodes and pn junctions, and a model is proposed. Impedance spectroscopy is also used to support this study. Keywords-Low frequency noise, diffusion noise, noise frequency dispersion.