Researchers of the University of Twente (UT; Enschede, Netherlands) have, for the first time, succeeded in connecting two parts of an electronic chip using an on-chip optical link, all fabricable in silicon with standard CMOS technology—a long-sought-after goal, as intrachip connection via light is almost instantaneous and, crucially, provides electrical isolation. Such a connection can, for example, be a safe way of connecting high-power electronics and digital control circuitry on a single chip without a direct electrical link. Vishal Agarwal, a UT PhD student, created a very small optocoupler circuit that delivers a data rate of megabits per second in an energy-efficient way.1
Such a device will allow the high-power portion of “smart power” chips to be isolated from the digital control circuits, guaranteeing safe operation in application areas like medical electronics and automotive. Conventionally, a bulky off-chip optocoupler is used for this. The new on-chip optocoupler can be integrated with the electronics, is about 0.008 mm2 in size, and consumes minimal energy.
An on-chip optocoupler requires both a light source and a photodetector. However, a light source is very difficult to create in silicon. Silicon LEDs created thus far on chips emit small amounts of infrared (IR) light with low efficiency—in addition, silicon detectors do not work well in the IR.
Avalanche-mode LED makes it possible
Agarwal took advantage of previous research done by UT PhD student Satadal Dutta, showing that better results can be obtained by connecting the silicon LED “the wrong way.”2 This leads to an avalanche effect, resulting in the emission of broadband visible light; the emitter is called an avalanche-mode LED (AMLED). Similarly, rather than an ordinary silicon photodetector, a single-photon avalanche diode (SPAD) can be created to efficiently measure the visible light. The result is an efficient optical connection. The entire optocoupler, along with its robust low-power LED driver circuit, was fabricated in a standard 140 nm silicon-on-insulator (SOI) CMOS technology (see figure).
The principle worked: next for Agarwal, the challenge was to design an electronic circuit to properly control the LED and the detector, optimizing for energy consumption, speed, and use of space on the chip. For example, what is the voltage needed for operating the AMLED and the SPAD in the most efficient way, resulting in a good connection without wasting light? And how to position the light source and light detector on the chip, for the highest efficiency?
The 1 to a few Mbit/s data rate of the resulting optocoupler, which has an energy consumption of a few nanojoules/bit, is already an acceptable rate for many applications. However, according to Agarwal, this rate can be increased at least tenfold in the future.
REFERENCES
1. V. Agarwal, “Optocoupling in CMOS,” dissertation, University of Twente (Jan. 16, 2019); doi:10.3990/1.9789036547079.
2. S. Dutta, “Avalanche-mode silicon LEDs for monolithic optical coupling in CMOS technology,” dissertation, University of Twente (Nov. 8, 2017); print ISBN 978-90-365-4413-9.
