Abstract-This paper presents a monolithic optical detector, consisting of an integrated photodiode and a preamplifier in a standard 0.18-m CMOS technology. A data rate of 3 Gb/s at BER 10 11 was achieved for = 850 nm with 25-W peak-peak optical power. This data rate is more than four times than that of current state-of-the-art optical detectors in standard CMOS reported so far. High-speed operation is achieved without reducing circuit responsivity by using an inherently robust analog equalizer that compensates (in gain and phase) for the photodiode roll-off over more than three decades. The presented solution is applicable to various photodiode structures, wavelengths, and CMOS generations.

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The influence of different geometries (layouts) and structures of high-speed photodiodes in fully standard 0.18 pm CMOS technology on their intrinsic (physical) and eleetrienl bandwidth is analyzed. Three photodiode atrUCtures are studied: nwell/psubstrate. p+/nwell/paubstrate and p+/nwell. The photodiode bandwidths are compared for X=650 nm wavelength which is used in today's DVD OP tical pick-ups. Slow substrate current component limits the intrinsic bandwidth of nwell/psubstrate and p+/nwell/p substrate Dhotodiodes t o 70MHe and 100MHe. resmeetivelv.is used in today's DVD optical pick-ups. The physical and electrical bandwidth characteristics for nwell/psubs, p+/nwell/psubs and P+/nWell diode are discussed. The total bandwidth is hy approximation the lower of these two. 11, I~~~~~~~~ (PHYSICAL) PHOTODIODE BANDWIDTH A , ~i~~~~ nuell/psubstmteThe elefhienl bandwidth of these diodes in combinetion with typical transimpedance ampliflsrs, will be larger than the calculated intrinsic bandwidth. Thus, the parasitic diode capacitance has almost no influence on the total bandwidth of both photodiodes. By using only a p+/nwell photodiode (not connecting a substrate), the intrinsic diode bandwidth is 1 GHe. However, the eleetried bandwidth limitation of this diode due to its parasitic capacitance is important and can limit the total diode bandwidth which is by approximation the lower of the physical and the electrical bandwidth. The calculated responsivity of p+/nwell photodiode is 10 d B lower than in other two deflned diodes structures, requiring higher sensitivity of the subsequent electronic circuitry.

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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