Getting to one Femtojoule per calculation

IEEE Spectrum – A new book called Chips 2020 was edited by this retired electrical engineer named Bernd Hoefflinge. Many experts came together to write a whole bunch of chapters on interesting new technologies that could emerge by 2020.

If we can get the energy of a very simple multiplier down from 1 picojoule—where it is right now—to about a femtojoule, we’ll be able to reach the energy efficiency we need to make the expectations for the coming decade. He likes to put that energy in terms of biology:

Bernd Hoefflinger: One femtojoule is 10 times lower than the firing energy of a synapse, be it cat or mouse or human.

Hoefflinger says we need more technology that exploits the third dimension on the chip. We’ve covered a lot of 3-D circuits activity at IEEE Spectrum lately: There’s 3-D packaging, where you have wires that connect one stack to another, which is increasingly being implemented. And then also three-dimensional transistors, which basically take a gate and wrap it around a channel. Intel introduced that technology last year, and it’s in production this year. Hoefflinger says we have to go even further:

Bernd Hoefflinger: We are going short on the third dimension…. There’s an incredible amount of worthwhile and very helpful activity on 3-D interconnects of chips. We have no activity that I can see on three-dimensional integration at the transistor level. And let’s be fair—the trigate, or the FinFET, is being called a three-dimensional device—that’s correct. But what we have to shoot for is one level further, and that is 3-D merged transistors.

3d merged transistors are two transistors combined together. So if you want to make a simple inverter, which is basically a p-doped transistor and an n-doped transistor, ordinarily both of those things have gates of their own. But in a merged transistor, they share a gate. So you have a PMOS transistor on one side, an NMOS transistor on the other side, and a gate in between them. Hoefflinger says that they’re occasionally referred to as “hamburger inverters.”

We basically need to change the way that circuits are architected to make them more like the circuitry that are used in communications. He suggests basically making it so that there is some sort of error-correction circuitry in chips, and the idea is that this will help us operate chips at a lower voltage:

Bernd Hoefflinger: We want to process it that we process both the signal and its complement—that is what differential signaling means. It is being used in the communication between chips, but it is not being used in logic any more. At some time, mankind produced the most effective computers, with differential emitter-coupled logic, in the days of bipolar technology. So it’s been around for decades, and it needs to be reconsidered.
