1. Field of the Invention
This invention relates in general, to optical isolation circuits and, in particular, to optical isolation circuits which are fabricated using silicon integrated circuit techniques in conjunction with silicon-on-insulator substrates. The invention relates especially to monolithic silicon opto-couplers using silicon-based light emitting diodes.
2. Prior Art
Most optical isolator-couplers have used gallium arsenide (GaAs) based light emitting diodes (LEDs) and silicon based photo detectors to generate and receive an optical signal, respectively. Optical isolation circuits which use a GaAs LED and a silicon detector typically employ hybridization to make packaged opto-coupler units because of the very different processing conditions and materials required for the different devices. Silicon-on insulator (SOI) technology has emerged as a highly viable silicon technology for use in fabricating integrating circuits. A primary advantage of silicon technology is that, unlike hybrid processing, many devices can be included on a single die with the same amount of fabrication effort. SOI integrated circuits achieve component isolation via an intervening insulator rather than by back bias junctions as is the case with bulk silicon integrated circuits. However, the quantum efficiency of silicon diodes as a light source is very low when compared to that of forward biased GaAs diodes. Not only are GaAs LEDs more efficient at emitting light, but the wave length of the light emitted by a GaAs diode is readily absorbed by silicon which makes silicon detectors efficient at detecting light emitted by GaAs LEDs.
In the inventor's co-pending patent application, Ser. No. 08/139,818, entitled "OPTICAL ISOLATION CONNECTIONS USING INTEGRATED CIRCUIT TECHNIQUES", it was shown how a monolithic silicon opto-coupler can be made using a simple silicon diode as the LED in spite of its very low light output. A key in making a practical silicon monolithic opto-coupler using a simple silicon diode as an LED was to provide a powered low noise amplifier in conjunction with the silicon detector diode either separately or as an on chip device. This combination allows the use of the simple silicon LEDs with extremely low output levels including even the use of the simple silicon junction diode. It was shown that low speed optical couplers on SOI can be used as low cost digital isolation devices such as telephone and modem isolation devices. However, the band width of the monolithic silicon opto-coupler is limited unless increased levels of efficiency (greater than that of the simple junction diode) can be achieved from an LED whose fabrication is compatible with silicon processing.