AlGaN/GaN heterostructures can operate at very high power in dc and rf regimes due to the superior properties of III-nitride materials. At the same time, the higher level of dissipated power causes higher overheating in the structures. Considering the fact that the temperature is a driving force for aging processes in devices, it is clear that the investigations of thermal effects in 2DEG of GaN-based structures are of great importance. Additionally, such studies result in a better understanding of the transport features which affect device performance, and allow us to determine the conditions under which hot-electron effects play the dominant role in electron transport. In this report we focus on thermal effects in transmission line model (TLM) patterns. The experimental results and numerical simulations of self-heating effects in TLM patterns processed on heterostructures with different layer design (different Al mole fraction of barrier layer, different substrates) are analyzed. The GaN-based heterostructures were grown by MOCVD. One of the investigated samples consists of a nucleation layer of AlGaN (16% Al), an undoped GaN buffer layer and an AlGaN (33%) undoped barrier layer. This type of structure was formed on sapphire and SiC substrates. Additionally, another two types of structures were grown on sapphire substrate: with wide bandgap barrier layer (with Al mole fraction of 75%), and with a thin AlN spacer layer (5nm) in between the AlGaN barrier layer (23 nm) and the GaN buffer layer (1300 nm). All samples were processed with TLM patterns with standard Ti/Al/Ti/Au contact metallization and annealed for 40s at 800° C. The conducting channel of the devices has a width W = 100 μm with intercontact lengths L of 1, 5, 10, 15, 20, 25, 30, 35 μm. The average overheating temperature over the active channel area for each structure type is estimated. The strong influence of the Al mole fraction of the barrier layer and also the substrate type of the structure on noise and transport properties is revealed and analyzed. It was shown that the implementation of wide bandgap barriers and high thermal conductance substrates allows us to improve the overall performance of the structures significantly. Optimal conditions for the observation of hot-electron effects are determined.