Defects can greatly affect the electronic performance of semiconductor devices. As two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) are being considered for next generation nanoelectronics, it becomes vital to understand and control defects in these materials, especially under a tunable gating. The species and density of defects depend on the growth method as well as the postgrowth device fabrication processes. In this talk, I will present atomically resolved images of the defects and show how their local electronic properties are modulated by the application of a gate electric field. Chemical vapor deposition (CVD) is used in this study to reliably produce 2D TMD monolayers while allowing for incorporation of desired dopants. CVD-grown 2D TMD monolayers are implemented in heterostructures with highly doped silicon back gate and graphene nanoribbon top electrodes patterned by scanning probe lithography are attempted for improved gate tunability. We use a low-temperature scanning tunneling microscope (STM) to study the point defects in these devices. First, the identification of the defects is realized by combining scanning probe microscopy with density functional theory (DFT) calculations. Next, the effect of a gate electric field is explored and compared to DFT modeling. Our results will provide guidance on future defect engineering in gate-tunable 2D TMD based devices.