We investigate the potential distribution and breakdown of GaN-on-silicon HEMTs by using a test structure with a floating sense node located between gate and drain, in the access region. To demonstrate the effectiveness of the adopted method, we analyze two different wafers, having different 2DEG retraction in the OFF-state and different time to breakdown. We demonstrate that: 1) the floating node can effectively be used to evaluate the potential distribution in the access region; 2-D simulations are compared with the experimental data to demonstrate the effectiveness of the method and 2) the time to breakdown is strongly influenced by the distribution of the potential (and of the electric field) in the access region. A superior robustness is found on devices characterized by an improved distribution of the 2DEG potential between gate and drain. In contrast, devices with a lesser 2DEG retraction show early breakdown when subjected to OFF-state stress. For the first time, we demonstrate a direct correlation between the 2DEG retraction and the OFF-state long-term stability of GaN-on-Si HEMTs. The results reported within this paper provide further insight on the physical origin of time-dependent breakdown in GaN-based power transistors.