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."

Comments

And it's organic

[cheebie]

Wow, plant-based electronics! This will surely usher in a new age of biological computers that will be able to . . .

What? It's not a geranium diode?

Uh, how 'bout that new version of Firefox? Pretty snazzy, eh.

Re:I thought they already existed

[MichaelSmith]

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.

They seem to have improved on Germanium LEDs by doping them differently to the point where the can look into using photons to transmit information around a silicon chip in place of electrons. I imagine they will look into building light pipes out of silicon, ie, little optical fibres.

OT: somebody should teach ascribe how to use the title tag.

Re:I thought they already existed

[fuzzyfuzzyfungus]

Germanium semiconductors are old news(in fact, I have this vague impression that they might have gotten germanium working in fairly common use earlier than silicon); but, according to TFA, germanium-based light emitters built into silicon structures under more or less reasonable production and operation conditions, is what is new. This isn't about discrete components; but about structures built into larger silicon ICs.

The 40-year old promise

[Laaserboy]

The promise of making a laser from indirect bandgap semiconductors, then gathering investors, then losing the investors' money goes back to the Sixties at least.

Some scientists showed off SiC blue LEDs in the '60s that shown brilliantly like laser light, but were not the read deal. The real blue room-temperature laser had to wait for Nakamura and a direct bandgap material.

Doping, adding nitrogen, and adding defects to the lattice to produce more light is nothing new. Look at your stop lights. It's working there, but don't count on these indirect materials suddenly turning into lasers. No need to hold your breath.

A quick scientific note. Photons have a lot of energy, but not much momentum. You get hot on a sunny day, but not blown over by the sun. Electrons fall almost directly down in the bandgap diagram to produce light. This makes direct-gap semiconductors useful for lasers. The trick one can use is to provide momentum-shifting impurities to the lattice of an indirect bandgap crystal. The electron creates a photon by dropping directly down, but some other mechanism shifts the electron momentum to create an overall diagonal transition. It's not efficient, but it works.

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

[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.