3Gb/s monolithically integrated photodiode and pre-amplifier in standard 0.18μm CMOS

Abstract

For cost, size, and assembly reasons monolithically integrated CMOS optical detectors are preferred in (very) short-range optical data communication [1,2] and in optical storage systems [3]. Another advantage of an integrated photodiode is that high interconnect capacitances and inductances are avoided. Furthermore many parallel optical receivers can be placed on a single chip at low cost, opening the door to optical interconnect. However the serious disadvantage of photodiodes integrated in standard CMOS is the low speed, reported up to 700Mb/s [2].In this work a fully integrated photodiode with pre-amplifier for bit-rates up to 3Gb/s in standard 0.18µm CMOS is presented. This represents more than half an order of magnitude speedincrease.For Gigabit Fiber Ethernet [5] 850nm light is used. For photodiodes in modern CMOS at this wavelength the majority of the generated carriers slowly diffuse towards junctions, resulting in a physical (intrinsic) bandwidth of the photodiodes in the low MHz range. This effect typically forms the speed bottleneck in integrated CMOS optical receivers. One solution [1,2] achieves 700Mb/s by cancelling the effect of the slowly diffusing carriers by subtracting two diode responses; this, however, results in lower responsivity and hence lower sensitivity. A solution is described for high-speed data communication with integrated photodiodes without reducing circuit responsivity, achieving 3Gb/s datarate by exploiting an analog equalizer.A minimal-distance finger n-well/p-substrate diode, (see Fig. 26.2.1), is used as the optical detector. Its overall response consists of three current contributions: two slow diffusion responses (in the n-well and in the p-substrate) and one fast drift current response. The latter is frequency-independent up to frequencies in the GHz range. The typically dominant substrate current has a bandwidth that is several orders of magnitude lower than the bandwidths of the other current components. This substrate current component limits the overall photodiode bandwidth.The overall intrinsic photodiode response shows a slow decay starting in the low MHz range, due to the combination of the three current components. It can be shown [6] that the roll-off in the overall photocurrent response is only about 5dB/decade for frequencies between roughly 10MHz and the lower GHz range. In the low-GHz range, the roll-off is even lower (<4dB/decade) because the fast depletion region response dominates the overall photocurrent. Signals from the photodiode are low bandwidth (MHz range), but still relatively strong at very high frequencies (GHz range). Therefore an analog equalizer is introduced that compensates (in gain and phase) for the diode photocurrent rolloff in the range from DC to 1GHz. As a result, a 3Gb/s data-rate with a BER<10 -11 is achieved.The presented analog equalizer is designed to compensate the frequency characteristic of the applied photodiode from DC to 1GHz, using four high-pass filters. Although a parallel configuration shown in Fig. 26.2.2 is optimum w....

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