Hot electron microwave noise is studied in 0.2 … 7.5 micron-long samples of GaAs at strong electric fields at 80 K and 293 K ambient temperatures. A new noise source is observed in doped (3·10 17 cm -3 samples. This source is related to the impurity levels lying 0.2 eV above the principle edge of the conduction band. At a given electric field the doped short samples show a higher noise temperature as compared to pure samples of the same length. Onset of the resonance impurity scattering gives rise to efficient transformation of the drift energy into the chaotic one and stimulates the intervalley noise sources. The threshold voltages of these sources are determined. The enhanced hot electron diffusion is observed in the vicinity of the threshold voltages. The critical length is less than one micron in doped GaAs.