Universal PIN photodiodes in a 0.35μm BiCMOS mixed-signal ASIC technology

Abstract

We present an improvement of monolithically integrated photodiodes in a p-type substrate of a commercial high-speed 0.35&mu;m SiGe heterojunction bipolar transistor (HBT) BiCMOS technology. These photodetectors (PDs) combine low capacitance with high bandwidth and responsivity. Slight process modifications of the standard HBT process have been introduced in order to decrease leakage currents and enhance reach-through stability of the PDs. These modifications have been chosen carefully in order not to alter any other transistor parameters as shown in [1]. To enable low capacitances of the PDs very lightly p-doped epitaxially grown layers of different thicknesses over highly p-doped substrates have been investigated. The improvement becomes manifest, e.g. in a bandwidth of 557MHz and a responsivity of 0.19A/W of a finger photodiode at blue light and a reverse bias voltage of 4V in a 10&mu;m cathode digit-spacing configuration. The capacitance of this finger photodiode is 150<i>f</i>F, overtopping the regular PIN photodiode published in [2] for the same light-sensitive area with a capacitance of 225<i>f</i>F. Results of detectors with interdigitated cathode distances of 5&mu;m, 10&mu;m, 15&mu;m and 30&mu;m are presented over the wide spectrum of technologically significant optical wavelengths from near-infrared to blue and ultraviolet. These detectors fulfil the requirements demanded by photodiode integrated circuits for universal backward compatible optical storage systems.