Historically, Vitesse Semiconductor of Camarillo, Calif., has used gallium-arsenide (GaAs) MESFET technology for its high-speed communication ICs. But increasing bandwidth and transmission data rates have forced the company to add a new weapon to its arsenal of compound semiconductor processes. It has developed indium-phosphide (InP) heterojunction bipolar transistors (HBTs) for a new line of chips capable of handling data rates in excess of 40 Gbits/s.
Initial development will be exploited to create physical-layer ICs for Sonet OC-768 applications. "Successive generations will deliver chips capable of handling 100-Gbit/s data rates," says Alan Huelsman, director of Vitesse's InP program. "In addition," he notes, "these HBTs will provide a path to the monolithic integration of long-wavelength optical sources and detectors and deliver true single-chip opto-electronic ICs (OEICs)."
To demonstrate the new technology, Vitesse has developed several voltage-controlled oscillators (VCOs), multiplexers, and buffer circuits. Internal tests indicate that these InP HBT-based devices perform close to their simulated frequencies. Using this technology, the supplier is now readying a five-chip solution for OC-768-compliant transceivers. This five-chip set is slated for release in the fall.
The ability to combine adequate gain, greater bandwidth, and higher breakdown voltage in a transistor makes InP HBTs attractive for such high-speed applications. Compared to conventional bipolars, the InP HBT offers seven times the improvement in current density, better reliability, and improved thermal management.
In fact, Vitesse has developed proprietary carbon-doping techniques and a base contact metal system for higher performance and reliability (see the figure). According to the company, carbon doping has lower diffusivity, while InP presents lower surface recombination velocity. The single-heterojunction InP HBT process also includes microstrip transmission lines, metal-insulator-metal (MIM) capacitors, and thin-film resistors.
Vitesse's performance roadmap shows that the first-generation HBTs, with an emitter width of 1.2 µm, achieve a transition frequency (fT) of 150 GHz and a maximum frequency of operation (fMAX) of 160 GHz. The next generation, expected to be ready for production by the second quarter of 2002, will offer an fT of 170 GHz and an fMAX of 200 MHz, with an emitter width shrunk to 0.5 µm.
Reducing the emitter width further to 0.35 µm, the third-generation InP HBTs will deliver an fT of 170 GHz and an fMAX of 250 GHz. Targeted for the second quarter of 2003, they also will include copper interconnects.
Approximately 50% of the supplier's 4-in. GaAs line in Camarillo has been converted to process InP wafers. By 2004, Vitesse hopes to migrate to 6-in. InP wafers.