In this paper, hetero-dielectric-based MoS2FET device is designed with different channel lengths and different high-K dielectric materials for the application of a dielectric-modulated biosensor. The device performance has also been compared with a conventional MoS2FET transistor having SiO2 as an insulation layer. From the analysis, it is observed that the MoS2FET-based hetero-dielectric is having better performance as compared to MoS2/SiO2FET transistor. Similarly, when the dielectric K-values are increased, the hetero-dielectric MoS2FET transistor performance is further increased due to high gate control and low leakage current. A dielectric-modulated MoS2FET biosensor for different values of dry protein dielectric constants (K = 1 to 6) is also designed, and their sensitivity parameters are analysed. It is observed that when the dielectric constant and the channel length of the proposed device increase, the sensitivity of the biosensor also increases due to enhanced drive current.