The origin of low frequency (LF) noise of GaAs metal–semiconductor field-effect transistors (MESFETs) was examined. A few generation-recombination components dominate the flicker noise present in the measured spectral noise intensity. It follows that the noise scales proportionally to L−3 (L stands for channel length). The earlier is obeyed perfectly when the noise originated in the channel access areas is negligible compared to the LF noise of the region underneath the gate. Next, it was observed that the corner frequency of the Lorentzian noise components depends on the gate-to-source bias and does not depend on the device geometry. The third important observation is that the LF noise power does not depend on the source-to-gate bias at frequencies higher than 100 Hz. It was shown that both fluctuations of the mobility and fluctuations in the carrier density inside the channel could not explain the observed noise. We claim that charge fluctuations in the Shockley–Read–Hall centers found inside the depleted layer below the gate electrode are likely to be the origin of low frequency noise in GaAs MESFETs.