Integrated cascade of photovoltaic cells as a power supply for integrated circuits

ICs can be powered directly when a supply voltage source capable of generating a multiple of the open circuit voltage of one pn-junction is available on a chip. Two schemes have been investigated for cascading photovoltaic cells on the chip. The structures can be made compatible with standard bipolar processes. Deep ion implantations have been used here to realize the multiple-junction structure. Power losses due to photocurrents originating from insulation junctions in the cascade can be kept to a minimum. The surface areas of two adjacent cells have to satisfy a definite relation to ensure the build-up of output voltage. Operating under sunlight conditions, 0.6 mW/cm2 has been generated in a cascade of three cells. With appropriate optical coatings on the silicon surface and optimized cell dimensions, it should be possible to generate a few milliwatts per cm2. The structures may have applications in electronic circuits for glass fibres or can be used to power small CMOS circuits.