Gallium Nitride-based (GaN) Heterostructure Field Effect Transistors (HFETs) allow for the realization of suitable amplifiers for high power applications. Due to their high band gap and excellent electrical properties, GaN-based HFETs are being investigated in a variety of applications. The focal point of the study at hand is the development of GaN-HFETs which will find their application in high voltage operation in the high frequency range. The processed devices exhibit breakdown voltages of more than 700~V and reduced leakage currents. A detailed analytical large-signal model including device parameters has been developed with respect to scaling mechanisms for high power operation. For an optimized assembly, the electrical and thermal impact of the layer stack has been analyzed in detail and optimized for high power operation. Furthermore, a thermal dissipation model has been established which allows for a loss-dependent determination of the device's junction temperature. Various GaN-based switch-mode amplifiers have been realized and characterized, which in conclusion provide reliable proof for the suitability of GaN HFETs for power applications in the HF and VHF range.