In-P HBTs fill high- speed, low power optical tranceiver requirements

Data transmission requirements in fiber optic systems are quickly catching up to state-of-the-art electronics. As a result, what were once considered "exotic" technologies are now being evaluated for communication system insertion. Of these, In-P-based HBT technology has consistently demonstrated the highest clock rate, lowest power-delay product and best compromise between integration level and performance for critical optical transceiver components. Recently these benchmark results have been turned into real circuits with adequate margin for 40 Gbit/second transmission. A few of these results have been published, others have simply been announced via press release because of the intense competition among technologies and companies that wish to be players in the 40 Gbit/second build out.

Demand for high data rate transmission is ultimately driven by the end user, the consumer of bandwidth. The early consumers of bandwidth demanded high quality voice transmission and were satisfied with the equivalent of a 64 kbits/sec data rate. Today's consumers want streaming video at about 32 Mbits/sec but are currently satisfied with Web browsing at about a one Mbit/second data rate. Most networks are tree structures with single trunk lines serving many users. A reasonable multiplier, the number of end users a trunk line must support, is 10,000 for the telephone network.

A basic receive/transmit system for optical fiber communication shows the critical locations where the highest data rate circuits and components are required. Circuits such as transimpedance amplifiers (TIA) and modulator drivers require high analog bandwidth. A decision circuit and clock recovery component require very high clock rate operation and good bandwidth. The complexity of the entire high -speed portion is on the order of a few thousand transistors. This is clearly an application where technologies not known for high levels of integration can compete.

Lattices matched to In-P are capable of supporting almost all of the functions in the receive transmit module including the photodiode, high speed digital electronics, high bandwidth analog components, laser and even the modulator. Some of the functions can even be integrated on the same substrate. However, an expert panel at the 2001 GaAs IC Symposium with representation from Lucent, Nortel, JDS and others, unanimously concurred that the solutions in the near term will require hybrid rather than integrated approaches. Each individual component will be chosen based on its own price/performance. The cost savings from integration cannot yet overcome the performance penalty inherent in mixing several functions on the same die. No single technology is likely to dominate the receive/transmit module.

Bipolar transistors consistently outperform field effect transistors in switching applications. Field effect devices in the form of High Electron Mobility Transistors (HEMT) in In-P hold most of the high profile transistor speed records such as unity gain cutoff frequency (ft), maximum frequency of oscillation (fmax) However, when FETs are used in real digital circuits with significant fanout and interconnect the circuit speed drops more rapidly than for bipolar transistors. Bipolar transistors have higher transconductance per unit area or unit device current and result in more compact logic gates. .

As molecular beam epitaxy (MBE) develops and the ability to control stoichiometry improves, a large number of quaternary semiconductors (semiconductors composed of four elements) are possible with a continuum of electrical properties. Bandgap engineering is the discipline that gives a device designer freedom to choose different semiconductors for different functions in a single transistor. A prime example is the choice of InGaAs for the base of a transistor to obtain a low base-emitter turn on and high hole mobility and the choice of In-P for the collector to obtain high breakdown and good saturated electron velocity.

Process flow in In-P is very similar to Ga-As and has built upon the two decades of Ga-As development. Three and four-inch In-P semi-insulating substrates are used; they are currently four times the cost of comparable Ga-As wafers. There is considerable headroom in the technology to scale both vertical and horizontal dimensions simultaneously. The exclusive use of dry etching will eventually allow the formation of much higher aspect ratio features than those typical today. Vertical scaling to reduce transit times must be accompanied by lateral scaling to maintain low access resistance.

In-P is particularly suited to the integration of a PIN photodiode directly on the same circuit as a TIA. The photodiode can be made using the same epitaxial material stack as the HBTs in the TIA. The scheme can work well for short-haul transceivers where the responsivity of the PIN is not as critical as for long-haul systems. In long haul systems the PIN and HBT have competitive needs and so the integrated solution may not be favored.

The vertical cavity surface emitting laser (VCSEL) offers another opportunity for optoelectronic integration using In-P. The most promising VCSELS at 1.3 to 1.5 microns — the range for long-haul fiber systems are formed with lattice matched materials on In-P substrates. The possibility of integration with electronics exists but there have been no demonstrations as of this writing. Still, the VCSEL is a vast improvement over edge emitting lasers allowing multiple sources to be integrated in a package-friendly format.

As performance approaches the limits of the technology, certain tradeoffs can be made that enhance device performance at the expense of circuit performance. Two of the better known trades are ft for fmax through base scaling and ft for breakdown through collector scaling. Optical transceiver modules require both digital and analog circuits — sometimes on the same chip. It is important to have devices with both high ft and fmax. In an HBT a direct tradeoff can be made between ft and fmax. Thinning the base increases the base resistance but reduces the base transit time; ft, which is independent of base resistance, thus increases.

Silicon-germanium (Si-Ge) is several generations ahead in fine-line lithography. Typical Si-Ge HBTs have 0.14 to 0.18 micron emitter stripe width. Lateral scaling is of limited importance to HBTs but it does allow for base thickness scaling while maintaining relatively low distributed base resistance. Despite Si-Ge's lead in scaling, In-P has been demonstrated at much lower power-delay products.

The final test of the technology, when it has sufficiently matured to produce stable models and repeatable and reliable circuit runs, is the ability to realize the actual circuit functions typical in an optical transceiver. The more integration available for the necessary functions the lower the overall system cost.

HRL Laboratories LLC and others have demonstrated a number of highly integrated In-P-based circuits containing many of the components required for optical communications as well as a number of specialized circuits with low levels of integration that can be used in hybrid form or integrated with more complex functions. In 1998, we demonstrated a fully integrated 7.5 Gbit/sec optical receiver ASIC that monolithically integrated all the critical components, including a photo diode, the amplifier stage, clock and data recovery circuit (CDR), demultiplexer, and word-synchronization logic. It consumed three watts of power and consisted of 2100 transistors. This demonstration was done in our mature first generation technology with 75 GHz ft and 130 GHz fmax.

We have also fabricated smaller footprint circuits with a wider design margin for 40 Gbit/sec applications. Lucent Technologies, Bell Labs engineers designed and demonstrated 40 Gbit/sec MUX and DMUX circuits as well as a 74 GHz distributed amplifier in our second generation process. The 4:1 MUX operated to 50 Gbit/sec.

Recently, we designed and demonstrated a photonic A/D circuit in the more advanced process. The circuit consists of an optical receiver, a comparator bank and a pipelined thermometer to binary decoder. Four optical receiver pin photodiodes are integrated with the HBT process. The bandwidth of the photodiodes is 13 GHz. And, Lucent Technologies has demonstrated an integrated 40 Gbit/sec fully integrated CDR. This was the first report of a mixed signal IC operating at a 40 GHz clock rate. Our researchers have also demonstrated a 38 GHz monolithic microwave integrated circuit (MMIC) voltage controlled oscillator in Al-In-As/In-Ga-As technology. The designs were optimized for best phase noise and spurious characteristics. The monolithic voltage controlled oscillator exhibited a tuning range of 850 MHz, about double the total variation of the center frequency across a wafer.

Narrower bandgap semiconductors such as In-Ga-As are direct bandgap absorbers of 1300 and 1550 nanometer light and make excellent photodiodes. Because of the much lower surface recombination velocity in In-P than GaAs, In-P HBTs can be scaled to submicron dimensions resulting in record power-delay performance.

In-P has a 4:1 edge over SiGe in power dissipation for equivalent clock rate. The advantage of higher integration levels of SiGe do not extend to high speed circuits because these circuits tend to be power dissipation limited. In-P has been demonstrated to be capable of providing critical components of optical fiber transceivers at up to 50 Gbit/sec data rates with substantial margin. In-P photodiodes are routinely used even in 10 Gbit/sec systems and may be integrated at some time in the future for short-haul fiber systems at 40 Gbit/sec. Broadband amplifiers in In-P with greater than 70 GHz bandwidth and gain-bandwidth products of 330 GHz have been demonstrated in the identical technology used for high speed logic functions. Full CDRs with integrated voltage controlled oscillators and phase detectors have also been demonstrated in this technology. With small enhancements in yield, full serializer-deserializer functions should be demonstrable on In-P.

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