Experimental study of the spatially-modulated light detector

Usually, integrated detectors in CMOS exhibit long recovery times, limiting the detector bandwidth to only a few MHz. This is due to the long absorpti on length and the slow diffusion speed of photo-generated carriers. Differe nt approaches have been proposed to serve these problems hereby taxing the compatibility with standard CMOS fabrication processing. We present a novel detector for high-speed light detection in standard CMOS. To solve the pro blem of slow CMOS-detector recovery, the incident light is spatially modula ted and the spatially modulated component of the photo-generated carrier di stribution is measured. Though only a single light input signal is required , from the detector on, analog signal processing can be achieved fully diff erentially. Subsequently, expected good PSRR (Power supply rejection ratio) allows integration with digital circuits. Avoiding hybridization eliminate s the conventional problems caused by bonding-pad capacitance, bonding-wire inductance. This reduces the associated signal degradation. In addition, t he very low detector capacitance, due to the low effectively used detector area and the low area capacitance of the n-well junction, yields high volta ge readout of the detector. This facilitates further amplification and conv ersion to digital signal levers. The detector will be applicable in arrays due to expected low cross talk. The expected fields of operation involve: s erial and parallel optical communication receivers (e.g. for WDM), DVD-read ing heads with integrated amplifier, etc. First measurements show 200 Mbit/s operation with a detector-responsivity o f 0.05 A/W at lambda = 860 nm and 0.132 A/W at lambda = 635 nm. The detecto r has inherently a low capacitance, in this case only 50 fF (for an effecti ve detector area of 70 x 70 mu m(2)). (C) 1999 Elsevier Science Ltd. All ri ghts reserved.