Researchers at MIT have developed the first CMOS LED that could be a breakthrough for high speed chip to chip interconnect.
LEDs are made with III-V materials on separate chips. An LED designed into the CMOS chip would provide an integrated interface on a chip.
“This is designed in a standard microelectronics process, it’s a really integrated solution,” said Prof Rajeev Ram who leads the Physical Optics and Electronics Group in the research laboratory for electronics (RLE).
The CMOS LED was built in Globalfoundries’ BCDlite 55nm process and currently handle signals at 1020nm up to 250MHz with a power of 50 mW/cm2, or 0.95nW for the 30μm LED shown above. This provides a link up to 5m over multimode fibre, says a paper at the IEDM conference this week.
“To the best of our knowledge, this is the first example of a forward biased CMOS LED that has been coupled into optical fibre,” said lead author Jin Xue, a PhD student in RLE who led the research. “We take advantage of the available light collection to demonstrate an all-silicon inter-chip optical interconnect. We employ a Geiger-mode photodetector realized on the same BCDLite platform.”
“The indirect bandgap of Si makes light emission a slow process. If there are faster, competing non-radiative processes then light emission will be unlikely,” said Prof Ram.
“The most important non-radiative processes for micro-LEDs are related to defects and surfaces. The Si in CMOS is high-quality and has a low bulk defect density [so] our focus was primarily in keeping the injected electrons and holes away from surfaces. This was achieved by using a relatively deep vertical pn junction, slightly more than 1um below the Si/SiO2 top surface.”
“Unlike previous work on forward-biased Si LEDs, this device had a deep, vertical junction instead of a lateral junction. This junction was separated from the lateral oxide (STI) with a lightly doped region. [This combination] ensured that electron-hole recombination took place far enough from surfaces to allow for the radiative recombination to take place.”
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