Self-heating in AlGaN/GaN high electron mobility transistors (HEMT) degrades device performance and reliability. Under nominal operating conditions, a so-called hot spot develops near the drain-side edge of the gate. The magnitude of the peak temperature at this local hot spot directly impacts device lifetime. Especially, such self-heating effects are aggravated in AlGaN/GaN HEMTs employing low cost silicon (Si) substrates that have relatively low thermal conductivity. In this work, GaN-on-Si HEMTs with symmetric geometries (identical gate-to-drain and gate-to-source lengths) were explored using thermoreflectance thermal imaging and infrared thermal microscopy to investigate and visualize the bias dependent self-heating phenomena. For the first time, a fully-coupled' electro-thermal modeling scheme was developed in order to validate experimental observations and study internal electro-thermal phenomena. It was found that under identical power dissipation conditions (P=VDS IDS=250 mW), a higher VDS bias (∼23V) results in 35% larger rise in peak junction temperature compared to that for a lower VDS (∼7.5 V) condition. The findings of this work confirm that bias conditions have a significant impact on device reliability and therefore must be considered when assessing device mean-time-to-failure (MTTF) through accelerated life time tests.