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Technical Insight

Rockwell and UCSB develop Sb-based ICs (GaAs News)

Rockwell Scientific Company (formerly Rockwell Science Center) has received a contract worth $6.4 million from the US Space and Naval Warfare Systems Center, under the sponsorship of DARPA, to develop ultra-low power ICs for use in high-speed digital and millimeter wave applications. A major objective of the program, which is scheduled to last 3 to 4 years, will be the development of ICs using antimony as a primary constituent. Rockwell Scientific will combine the properties of Sb-based ICs with new techniques of circuit fabrication currently being developed at the University of California at Santa Barbara (UCSB). The aim is to produce a robust, manufacturable circuit process, initially for insertion into defense and space applications. This will include mixed signal ICs used in high-frequency circuits operating up to 160 Gbit/s and low noise amplifiers (< 1 dB) used to amplify weak signals in front-end receivers. Markets also exist for high-speed (low-resolution) analog-to-digital converters used in digital oscilloscopes, in addition to digital cameras and CD players. Superior properties The antimonide system being investigated employs AlGaAsSb confinement barriers and InAs active regions to make heterostructures that offer improved electronic properties compared to GaAs and InP. These properties include a higher electron velocity because the channel can be made with 100% indium, rather than 53% indium for lattice-matched InGaAs/ InP devices. The higher In concentration increases the carrier velocities and raises the operating frequencies. In addition, the mobility is also as much as 5 higher than GaAs, which leads to a much lower channel access resistance and therefore improved RF gain (see ). Turn-on voltages are also reduced due to the smaller bandgap of InAs, leading to lower power consumption. The latter favors the device for high-speed, low-power circuits, but makes them inappropriate for high breakdown voltage applications. Both HEMT and HBT structures will be grown by MBE using UCSB s reactor, in addition to a new MBE reactor obtained for the project at Rockwell Scientific s facility in Thousand Oaks, California. "Rockwell and UCSB will be working in parallel to develop device circuits and a manufacturing process for InAs/ AlGaAsSb heterostructures," says Bobby Brar, manager of Advanced III-V Devices at Rockwell Scientific. "UCSB has a framework for a process already, and has for example demonstrated AllnAs/ GaInAs HBTs with fmax values in excess of 800 GHz. Similar circuit designs for the new Sb-containing compounds will thus offer great benefits in terms of high-speed performance and low power consumption." Some challenges ahead "Sb-containing HEMTs and HBTs lead to a number of important gains, including the potential for millimeter wave devices with superior noise figures compared with InP HEMTs, but you also inherit a couple of disadvantages," says Mark Rodwell of UCSB s Department of Electrical & Computer Engineering. "Major advantages include an increased carrier velocity and 2D sheet carrier concentration, and reduced bulk and ohmic contact resistances." These properties together should provide an FET with a higher ft value and lower source resistance, which are excellent parameters for achieving devices with low noise figures. However, the bandgap of the InAs channel is small and this leads to challenges in managing avalanche breakdown. "This breakdown phenomenon adds noise and offsets gains in noise figure," explains Rodwell. "In addition, the potential of the material system is also contingent on circuit design and other device specifications being met." There is also the problem of suitable substrates. There is no insulating substrate lattice-matched to InAs, which is a requirement for HEMT devices used in RF applications. According to Brar, Rockwell Scientific will first employ lattice-matched conducting GaSb or InAs substrates to grow low-defect-density device layers. UCSB s wafer transfer process will then be used to remove the substrate before adding a suitable insulating substrate, such as GaAs. Rodwell notes that the transfer process is just one of the ways ahead, and device growth on GaAs substrates is also being investigated. However, work by Rodwell s group and others (i.e. HRL Labs and TRW) has resulted in high dislo-cation densities.
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