Technical Insight
Technology choices for high-speed ICs (Mantech Conference Report)
The speed of an electronic circuit is limited primarily by the cut-off frequency of the transistors used to build it. The common figures of merit are the current gain cut-off frequency, or fT, and the maximum oscillation frequency, fmax. Analog circuits are usually evaluated by fmax, while fT is a good indicator of the switching speed of logic circuits. There are many technologies that can be used to implement the high-speed ASICs (application specific integrated circuits) that are required for optical networks. As a rule of thumb, the transistor technology chosen must provide fT and fmax better than four times the bit rate of the network in order to achieve the required switched speeds and proper system margins. In other words, OC-192 networks, which operate at a bit rate of 10 Gbit/s, require ASICs that are fabricated from a material system that can provide an fT and fmax of at least 40 GHz. However, it is also possible to use novel circuit technologies, such as distributed design, and the use of multiple phase half-rate clocks to reduce the cut-off frequency requirement to twice the bit rate. The fT of field-effect transistors, such as Si MOSFETs, GaAs MESFETs and GaAs or InP HEMTs, is limited by the carrier transit delay in the gate region. This transit time is determined by the lateral dimension of the gate electrode and by the electron velocity in the host material giving compound semiconductors an intrinsic advantage. But silicon manufacturers can fight back by employing ever-more sophisticated fine-line lithography tools. For the other major category of transistors, bipolar devices, the cut-off frequency is determined mainly by the vertical layer thickness and less by the lateral dimensions. shows the trends in cut-off frequencies for IC technologies over several decades, as indicated by the best fT data reported by research laboratories as well as the typical fT data in production lines. The pattern that emerges shows that the more advanced technologies are significantly faster than CMOS at the outset, and these speeds improve over time. However, CMOS improves at a faster rate, due largely to the greater amount of resources brought to bear on the problem, and the aforementioned decreases in the lithography line widths. In the real world, this translates into silicon muscling into markets that were established using compound semiconductor technologies. For example, OC-48 networks, which operate at 2.5 Gbit/s, were launched in the late 1980s using GaAs MESFETs and silicon BJTs because at that time they were the only production-worthy technologies with fT values of 10 GHz or more (recall the four-times rule-of-thumb mentioned previously). But, as CMOS technology improved in the late 1990s, most of the OC-48 ASIC market was taken over by silicon circuits that could now offer not only the necessary fT, but also lower cost, lower power consumption and higher integration capabilities. Today s market is dominated by OC-192, which has a 10 Gbit/s data rate. Most of the ASICs for these systems are implemented in GaAs and SiGe technologies with 0.5 m line widths. But encroachment by advanced 0.16 m CMOS technology has already begun the narrower line width making up for silicon s lower intrinsic speed. What lies ahead? Currently 40 Gbit/s ASICs for the next-generation OC-768 networks are still in the early stages of development. As shown in , there are many candidate technologies for this regime and, as of now, silicon CMOS is not one of them. In fact, there is no single technology that is capable of performing all the circuit functions and meeting all the performance requirements of a 40 Gbit/s transceiver. Thus we will see several ASIC technologies combined into transceiver modules. For 40 Gbit/s ASICs it is preferable to use a bipolar technology, either InP or SiGe, to realize low-power, high-speed digital circuits with high transistor counts, while employing GaAs or InP HEMT technology for analog functions that require low noise amplification and high-voltage driving capability.
