Germanium Diodes Mean Progress Toward Silicon-Chip Lasers

David Orenstein writes "Teams at Stanford and MIT have each reported getting strong light signals from germanium-based diodes on silicon at room temperature. Engineers have long sought to do this because, with further refinement into lasers, such diodes would allow for optical interconnects on chips. Optical interconnects could operate much faster and with less power than electrical (metal) ones that are becoming bottlenecks on current chips."

I thought they already existed

[guruevi]

I know for sure that I used Germanium diodes before and I'm pretty sure Germanium-based LED's have been developed before. Dunno what the news is.

Re:I thought they already existed

[Anonymous Coward]

2-3GHz is around the frequency that FR4 (fiber glass) material for building PCB starting to become lossy. You know some of the cheap plastic gets hot in the microwave oven, that's because the material become lossy and change the energy into heat. Same principle here.
We played with 3GHz and was already pushing it back then.

So transmitting that type of signals outside a chip for a long distance ~ 30-40cm to a backplane onto another card for say a router core or a blade server is going to take a bit more work if you want frequencies well into the tens of GHz.

Re:I thought they already existed

[LoRdTAW]

The real benefit is you wont have to worry about cross talk or other electromagnetic interference. The short haul of the board level optical interconnects means we can have very high speed chip to chip interconnects without worrying too much about trace routing or length. And LED's are quite efficient when it comes to turning to electrical power into light. Metal wires at high frequencies develop a high resistance which has to be overcome by using more energy.

Re:I thought they already existed

[weirdo557]

how would the switching times of these lasers compare to that or a wire trace though?

Vague impression?

[Kupfernigk]

Er...you do know that all the first transistors were germanium based and that early transistor computers used germanium? Before Schottky diodes, computer power supplies used germanium rectifiers because they were twice as efficient (half the heat) as silicon ones. And early audio amplifiers used germanium power transistors in the output stages because at the time they offered lower distortion than silicon, as they had better transfer characteristics in the crossover region. You could easily hear the difference between class AB tube amps, class B germanium amps and class B silicon into the early 70s. Germanium was initially seen as a low frequency technology because thin junctions were hard to form, but this is not necessarily true (Esaki (tunnel) diodes.)

Having said that you are entirely right in your main observation. The main problem for germanium has always been fabrication; no germanium ICs. This is because there is no germanium equivalent of planar technology. It has been known for a long time that if this could be overcome there would be a role for germanium. It's just that, as with so many apparently breakthrough technologies, making it happen turns out to be very hard.

Re:I thought they already existed

[jd]

That's cool, but with modern chip designs using electron tunneling for some of the effects, it can't be used chip-wide. On the other hand, light can cross through light, so you would be able to avoid tediously long tracks currently required.

There may be some additional interest in the aerospace industry for this. Optical circuits on the chips aren't going to be so affected by radiation, and by having more real-estate available for redundant components and optimal placement, they can improve the resistance to radiation considerably.

Not sure how much heat this'll cut down on, as the transistors are the big heat-producers. On the other hand, better placement means more even heat production which means they should be able to push the designs a little bit further.

Re:I thought they already existed

[kinnell]

The real benefit is you wont have to worry about cross talk or other electromagnetic interference. The short haul of the board level optical interconnects means we can have very high speed chip to chip interconnects without worrying too much about trace routing or length. And LED's are quite efficient when it comes to turning to electrical power into light. Metal wires at high frequencies develop a high resistance which has to be overcome by using more energy.

I'm not convinced. You can still get electromagnetic interference with light - look at TV remotes. Of course, if you use fibre optic cable it's not a problem, but that's akin to using coaxial cable to route electrical signals. While it would be possible to embed coaxial structures in PCBs to eliminate the possibility of cross-talk and noise, in practice this would be prohibitively expensive and the same result can be achieved with stripline and careful routing. The question is, what does an optical PCB look like? You'll still need copper for power distribution so the optical PCB will need to tolerate soldering temperatures. Do you have a layer of interwoven fibre optic cables? How do these interface with the components such that there is tolerance in the size and position of the terminals? Do you use mirrors and optical waveguides embedded in the substrate? If so how do you make this cost effective to manufacture? If you use fibre optics, you have a minimum bend radius, so you open up a whole new set of routing problems. While there are obviously clear benefits in theory, when it comes to actually implementing this as a cost effective PCB interconnect you'll have a whole set of new problems to deal with, and it's unlikely to be anywhere close in cost to gluing layers of copper and plastic together.