How about heat? Seems these days it's the heat above everything else that's the issue. And more density would only aggravate it.
Heat is mostly driven by leakage current and gate capacitance.
The big issue today is leakage currents. They typical account for around 30%-50% of total chip thermal budget, and they get increasingly difficult to control with smaller devices and lower voltages. They're also get worse with increased temperature(!).
The stacked devices here aren't the worst for leakage currents, but they're not fantastic either. Look at the 2nd graph in section 5: You'll see that the current never drops to zero over the range of gate-source voltages (for V_DS=0.7V). The minimum point is the best-case leakage current, and you can see it's well above zero! (The units on the vertical axis of the graph are unknown btw: The label reads as "current drain-source, arbitrary units")
What you loose in heat you gain in speed caused by proximity. Perhaps this will allow for lower voltage and thus less heat.
That's always an issue, but the industry seems to be moving away from 2D circuits.
Reducing trace length seems to be the way forward for faster/larger circuits. Signal propagation time on-die is becoming an issue.
Things like Huawei's Logic folding, or TSVs, and so on, attack the issue by reducing signal travel time.
This looks like another building block in that direction.
There's also some push at cooling chips from both sides.
I wonder if the proposed CPU/GPU laser cooling technique that was on here a few days ago would penetrate the Si layers?
Not an expert but I wondered this too and did some searching. My understanding is the laser cooling isn't expected to be applicable to silicon logic anytime soon. Its applications are more for specialized contexts like cooling quantum sensors, resonators, imagers, etc.
The big barrier remains heat and this 3D stacking (aka CFET) makes heat worse by increasing density. It's possible much of the density gains offered by CFETs will remain unutilized unless other approaches to solve the fundamental heat problem are found, possibly discovering new high-conductivity MDI materials.
The future is to replace the Si base with something else. Silicon Carbide has higher thermal conductivity. Bismuth-based composites provide much higher frequency.
https://semiengineering.com/the-race-to-replace-silicon/
https://www.plantengineering.com/semiconductor-material-that...
If heat is produced by conductors resistance then shorter paths would lead to less heat produced.
I'm not certain (never did hardware), but I thought the transistor switching cost was one of the bigger sources of energy loss, not internal conductor resistance between transistors?
Makes sense, I also doubt that they haven't minimized resistance to minimum already.
You're correct. Dynamic power consumption depends heavily on frequency, but it's definitely more of a limiting factor than static power consumption which as I understand it (I'm also not an expert) is mainly important for things like low power microcontrollers.
This seems like it could accelerate the transition to sub-1nm nodes (previously projected to mid 2030s), maybe by the end of this decade.