Packaged RF transistor amplifiers are provided that include a flat no-lead overmold package that includes a die pad, a plurality of terminal pads and an overmold encapsulation that at least partially covers the die pad and the terminal pads and an RF transistor amplifier die mounted on the die pad and at least partially covered by the overmold encapsulation. These packaged RF transistor amplifiers may have an output power density of at least 3.0 W/mm2.