New approach to chip design could yield light speed computing

Kar and his col­league Yung Joon Jung, an asso­ciate pro­fessor in the Depart­ment of Mechan­ical and Indus­trial Engi­neering, have devel­oped a series of novel devices that do just that. Their work was pub­lished Sunday in the journal Nature Pho­tonics.

Last year, the inter­dis­ci­pli­nary duo com­bined their expertise -- Kar's in graphene, a carbon-​​based mate­rial known for its strength and con­duc­tivity, and Jung's in the mechanics of carbon nan­otubes, which are nanometer-​​sized rolled up sheets of graphene -- to unearth a phys­ical phe­nom­enon that could usher in a new wave of highly effi­cient electronics.

They dis­cov­ered that light-​​induced elec­trical cur­rents rise much more sharply at the inter­sec­tion of carbon nan­otubes and sil­icon, com­pared to the inter­sec­tion of sil­icon and a metal, as in tra­di­tional pho­to­diode devices. "That sharp rise helps us design devices that can be turned on and off using light," Kar said.

This finding has major impli­ca­tions for per­forming com­pu­ta­tions, which, in simple terms, also rely on a series of on-​​off switches. But in order to access the valu­able infor­ma­tion that can be stored on these switches, it must also be trans­ferred to and processed by other switches. "People believe that the best com­puter would be one in which the pro­cessing is done using elec­trical sig­nals and the signal transfer is done by optics," Kar said.

This isn't too sur­prising since light is extremely fast. Kar and Jung's devices -- which are the first to inte­grate elec­tronic and optical prop­er­ties on a single elec­tronic chip -- represent a crit­ical break­through in making this dream com­puter a reality.

The com­pu­ta­tional mod­eling of these junc­tions were per­formed in close col­lab­o­ra­tion with the group of Young-​​Kyun Kwon, a pro­fessor at Kyung Hee Uni­ver­sity, in Seoul, Korea.

In the new paper, the team presents three such new devices. The first is a so-​​called AND-​​gate, which requires both an elec­tronic and an optical input to gen­erate an output. This switch only trig­gers if both ele­ments are engaged.

The second device, an OR-​​gate, can gen­erate an output if either of two optical sen­sors is engaged. This same con­fig­u­ra­tion can also be used to con­vert dig­ital sig­nals into analog ones, an impor­tant capa­bility for actions such as turning the dig­ital con­tent of an MP3 file into actual music.

Finally, Kar and Jung also built a device that works like the front-​​end of a camera sensor. It con­sists of 250,000 minia­ture devices assem­bled over a centimeter-​​by-​​centimeter sur­face. While this device would require more inte­gra­tion to be fully viable, it allowed the team to test the repro­ducibility of their assembly process.

"Jung's method is a world-​​class tech­nique," Kar said. "It has really enabled us to design a lot of devices that are much more scalable."

While com­puters process bil­lions of com­pu­ta­tional steps each second, improving their capa­bility of per­forming those steps, Kar said, begins with the "demon­stra­tion of improving just one." Which is exactly what they've done.