I don't really understand your explanation and I haven't really followed the discussion.
But if I understand correctly, apparently BDW has a 12% higher voltage for a given frequency, which flies in the face of
How's that possible? A smaller node should decrease voltage, right?
It could but higher voltage doesnt mean automaticaly higher power comsumption, i explain it to you before answering to Tuxdave since this will allow everyone to understand very easily what is discussed.
On the slide you posted the first parameter is called capacitance, to summarize these are parasistic capacitors that are inherently built in with the transistor.
Thoses parasistic capacitors act as some wheight attached to each transistor and they create an electrical inertia, that means that when you send a current to drive a transistor the capacitors will absorb this current and keep the transistor from being fed the command voltage until you have charged enough said capacitor, at this point the transistor input has reached a voltage high enough that the transistor start to conduct massively and drive the next transistors with their own attached parasistic capacitors.
Now if your process use higher voltage you can compensate by reducing the capacitance accordingly by the square or the voltage ratio, that means that if your voltage is 20% higher (a 1.2 ratio) the capacitances must be reduced by a ratio 1.2 x 1.2 = 1.44 to get the same efficency.
Now the parameter called leakage is extremely dependant of the supply voltage, if you want low leakage you have to design your transistor such that it start to conduct at a higher command voltage, the benefit is that the higher the threshold voltage at wich the transistor start to conduct the lower the leakage when the transistor doesnt conduct
You see, that s all trade offs here and there, often different process will be used depending of the usage, for high performance you want transistors that start to conduct with low command voltage as it allow high frequencies but this will increase leakage, for low power devices leakage is an important metric so it will be higher command (and hence supply) voltages but also less leakage and lower frequencies.
Edit : The slide doesnt contradict the 12% higher voltage to answer to your question.
Here's my thought process. At first glance, with a gate wrapping around a channel, it clearly looks like "more gate" and thus "more capacitance". But since the channel topology allows a much better current control, you can relax the gate capacitance (higher oxide thickness) and still end up with better current than planar. Who knows. It's not very obvious to me which way it's guaranteed to go. You may be right.
You may aslo be right, mind you, i did read a paper from Altera about specific process optimisations for their designs at TSMC, and they also have access to Intel process, they state that Finfets have much better transconductance but also substancialy bigger parasistic capacitances, i ll post the PDF if i can find it again.
As for the frequency-voltage curve, that's affected by process and design. Imagine a curve which represents all possibilities frequency and voltage design point that the CPU design team is targeting. If you pick one design (which represents the lowest voltage to hit that frequency) and start to adjust voltage/frequency from a fixed design (how much voltage do you need to hit a higher frequency, how much voltage can you drop for a lower frequency), that SECOND curve will always be suboptimal compared to the first curve. How's that for a plausible explanation.
You are right since starting from an optimum will forcibly yield a worse caracteristic when you move around this optimum, be it by excess or by default.
Finfets higher input capacitance mandate shooting for the lower possible voltage and to get this you have to :
Either decrease the transistor conduction threshold (Vth) but this will increase exponentialy leakage when Vth is decreased and this is not compatible with a design that wants to be low power like Core M.
Or you increase the transitor transconductance (gm) at a same convenient Vth but that s precisely what is difficult to achieve since moving this parameter require modding slightly the geometry or using different materials at some places, like germanium, and this will shift all other parameters including thoses that are already satisfactory, among other the sub threshold caracteristic wich define the leakage current.