Researchers of the University of Twente claim to have for the first time succeeded in connecting two parts of an electronics chip with an on-chip optical link. What makes the device extraordinary is that it uses only silicon-based technology for the digital control circuitry.
A light connection can, for example, safely connect high-power electronics and digital control circuitry on one chip, without a direct electrical link. Previously, such an optical link was impossible using standard silicon chip technology. However, Vishal Agarwal, a UT PhD student, managed to do so. He developed a very small optocoupler circuit that delivers data at a rate of Megabits per second in an energy-efficient way.
Using light can allow the isolation one part of a chip of another without any electrical connection. In so-called ‘smart power’ chips, the high-power portion can be isolated from the digital control circuits. This isolation ensures safe operation in application areas such as medical electronics and automotive. An optocoupler is used for this, but until now, this was a bulky device, separated from the actual chip.
Now, Vishal Agarwal has achieved design dream of having an on-chip optocoupler. His optocoupler can be integrated with silicon electronics using standard chip technology (CMOS). The optocoupler measures about 0.008 square millimeters in size and consumes minimal energy.
Integrating a light source and a light detector on a chip, is a difficult design challenge. Generally, special materials such as compound semiconductors will be needed that simply can’t be introduced in the CMOS process. Silicon is not a good light source by itself and a silicon LED on a chip would emit some infrared light with low efficiency.
Another issue is that a silicon-based detector doesn’t work well with infrared light. This is not a good starting point for a CMOS-process made optocoupler.
Previous research, however, by UT PhD student Satadal Dutta, demonstrated that better results could be achieved by connecting the silicon LED ‘the wrong way’. This connection causes an avalanche effect, resulting in the emission of visible light. In the same way, a light detector can be made in which a single photon can induce an avalanche. The result is an efficient optical connection.
The principle worked, now for Agarwal the challenge was to design an electronic circuit that controls the LED and the detector in the best way, that would optimize for energy consumption, speed, and use of space on the chip. Agarwal had to figure out all of the parameters to achieve an optimal connection. For example, what voltage is needed for operating the ‘Avalanche Mode LED’ (AMLED) and the ‘Single Photon Avalanche Diode’ (SPAD) in the most efficient way, resulting in a good connection without wasting light? Where to position the light source and light detector on the chip, to obtain the highest efficiency?
For his thesis, Agarwal came up with an optocoupler that can be fully integrated in CMOS, with a data rate of around 1 Megabit per second and minimum energy consumption. While for many applications, this is already an acceptable data rate, Agarwal believes it can be increased at least tenfold.
The research was done in two Electrical Engineering groups: Integrated Circuit Design (Digital Society Institute), led by Bram Nauta and Integrated Devices and Systems (MESA+ Institute) led by Prof Jurriaan Schmitz.
Vishal Agarwal from Chaibasa, India defended his PhD-thesis ‘Optocoupling in CMOS’ on 16 January.