Researchers at STMicroelectronics have developed a silicon-based light-emitting device for optocouplers and similar applications. While the low light level that basic silicon can emit isn't very useful, silicon doped with erbium or one of the other 13 rare-earth elements can emit light at a specific wavelength.
The rare-earth metal is implanted in a special layer of silicon-rich oxide (silicon dioxide containing nanocrystals of silicon) that forms the gate dielectric of a MOS transistor (see the figure). Test devices implemented with erbium, terbium, or cerium emitted two orders of magnitude more light energy than the best previously achieved results.
For biasing currents of up to 50 µA, the silicon-based LED and an LED based on III-V materials (typically gallium arsenide) show a linear dependence of output power versus current and deliver comparable light outputs. Further, both have a quantum efficiency of about 10%, delivering about 5 µW at a bias of 100 µA. In the latest experiments, ST's devices delivered more than 1 mW of emitted light power for each square millimeter of silicon.
Silicon is useful as a photodetector and has long been exploited in many applications, including optocouplers. By using a silicon-based light emitter and detector, the costlier LED based on III-V compounds can be replaced, reducing the optocoupler's overall cost. Fabricated prototypes of a fully functional optocoupler prove that an all-silicon solution can deliver similar performance to that of the traditional silicon/III-V optocoupler.
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