An embodiment of a packaged RF amplifier device includes a device substrate, a transistor die coupled to the device substrate, and an isolation structure coupled to the transistor die. The transistor die has a top die surface, a bottom die surface, a semiconductor substrate, first and second transistors formed in the semiconductor substrate, a conductive structure at the top die surface and positioned between the first and second transistors, and a low resistance path that extends vertically through the semiconductor substrate between the conductive structure and the bottom die surface. The isolation structure is coupled to the conductive structure and extends into an area above the top die surface between the first and second transistors. The isolation structure may be a wirebond fence, a conductive wall, conductive pillars or vias, or a plated trench that extends vertically upward from the conductive structure. The device may be encapsulated with molding compound.