Power spectral density due to deep traps has been calculated in a junction field effect transistor (JFET) by a numerical procedure. Distribution of potential, density of carriers, and occupation factors were evaluated for any point of the structure. Different effects were found depending on the depth of the trap considered, such as spectra different from the pure Lorentzian shape or anomalous behavior of the noise amplitude with reverse voltage applied. The explanation of these effects may be useful for the characterization of midgap levels produced in JFETs under irradiation stress. Good agreement is achieved with the experiment. ©1995 American Institute of Physics.