Sketch summarizing the physical picture of disk-to-warm corona transition. A hydrogen ionization instability at ∼1 × 10⁴ K within the disk likely triggers the formation of the warm corona. For either a more massive BH or a lower Eddington ratio, the disk temperature becomes cooler, causing the transition radius (in terms of R_g) to move inward. Meanwhile, the hot corona expands (in units of R_g) as Eddington ratio decreases, to match the observed increase in Xray-to-bolometric luminosity fraction.