Low-frequency noises (LFN) and noise-like oscillations (NLO) in GaAs metal semiconductor field effect transistor (MESFET) channel current were investigated under sidegating bias conditions. It was found that the fluctuations of the channel current were directly dependent upon the sidegating bias. As the sidegating bias decreased

the amplitudes of the oscillations would increase correspondingly. Furthermore

the LFN and NLO would attenuate sharply when the sidegating bias increased to more than a certain voltage. Two mechanisms are presented to demonstrate that the effective substrate resistivity or the channel-substrate junction modulated by sidegating bias and deep level traps would take responsibilities for the LFN and NLO.