Multiple layers of MoS₂ are used as channel materials in a type of field-effect transistor (FET). It was found that the hysteresis in transfer curves and low-frequency noise (LFN) characteristics are varied by the number of layers in MoS₂ due to the different influences of bulk traps. The LFN characteristics of a FET composed of a `bi-layer' MoS₂ channel, which was passivated with an atomic-layer-deposited (ALD) Al₂O₃ layer, follow the conventional carrier number fluctuation (CNF) model. However, FETs consisting of multi-layered MoS₂ channels (4, 7, 9, and 18 layers) show abnormal LFN characteristics, which substantially deviate from well-established 1/f noise models such as the CNF and Hooge's mobility fluctuation models. The bulk traps inside the MoS₂ layers are the origin of the abnormal LFN characteristics and the large hysteresis of FETs with multi-layered MoS₂ is due to its randomly embedded bulk traps. Secondary ion mass spectrometry (SIMS) confirms the existence of oxygen species that induce the electrical bulk trap in the MoS₂ layers.