oh that's a very interesting question! -- HBM on the back side of chips has advantage and disadvantges. Logic atop HBM means power delivery becomes the bottleneck. This is equally problematic. My understanding is that the industry seems to prefer HBM on logic currently. Also for the bonding, I think we are going to see hybrid-bonding very soon! Instead of solder, we can directly connect the indivdual dies.

I think the switch to GaN is already happening for power, but Si is considered better for logic dies like the GPU. Also GaN is worse thermally than Si so the problems are actually amplified