III-V microelectronics technology
Fraunhofer IAF targets terahertz circuits
Gate scaling is the key to penetrating the depths of the sub-millimeter-wave frequency range. It improves RF performance, empowering active electronics at these ultra-high frequencies, say Axel Tessmann, Ingmar Kallfass and Arnulf Leuther from Fraunhofer IAF.
A
band of researchers around the world are united in their quest to build faster and faster circuits. If they can fulfill their dream, they will not only be able to investigate widely unchartered regions in the high millimeter, sub- millimeter and terahertz frequency range, but also start to develop novel systems operating in these spectral domains. Opportunities exist for spectroscopy in these spectral ranges along with trials of communication at staggering bit rates. What’s more, at terahertz frequencies in particular, there is the chance to fabricate incredibly compact imaging systems with tiny antennae operating at breathtaking bandwidths.
The most promising device for reaching these incredibly high frequencies is the MMIC. Unlike rival diode electronics and optical technologies, this miniature integrated circuit offers an incredibly attractive combination of value-for-money, mass manufacturability, small size and on-chip multi-functionality.
Many approaches can be taken to building a high speed transistor, and at Fraunhofer IAF we are pursuing an InAlAs/InGaAs metamorphic HEMT (mHEMT) architecture. This design has much to recommend it: a great deal of freedom, in terms of epitaxial design; outstanding electrical performance; and an easy-to- handle, underlying GaAs substrate.
We have been able to successfully scale this device down to gate-lengths of 20 nm, and can now offer circuit designers a tremendously fast transistor. Cutoff- frequencies, fT
, exceed 600 GHz and the fmax value is well
beyond 900 GHz. If these transistor speeds are to be really useful, they need to be exploited at the circuit level. To this end, we are continuously refining our passive components, the transmission line approach of the entire
Figure 1. Smaller gates can speed transistors. Between 2001 and 2010 Fraunhofer IAF made great strides in this direction, reducing the length of its gates employed in its InAlAs/InGaAs metamorphic HEMT technology
MMIC process, and the waveguide packaging technique for ultra high frequency operation. Our success has been built on long-standing expertise in the design, fabrication and packaging of MMICs based on metamorphic HEMT technology. The virtues of this class of transistor stem from its metamorphic buffer, which allows the use of substrates made from GaAs, rather than InP. These are larger, cheaper, and less brittle. In addition, mHEMTs grown on GaAs have a higher degree of flexibility for heterostructure growth, thanks to a lift in restrictions related to the lattice parameter.
The upshot of all of this is that we can perform successful epitaxial growth of InAlAs/InGaAs layers with very high indium concentration on 4-inch GaAs. The 1 µm-thick quaternary buffer that we employ begins with an Al0.52
Ga0.48 As layer, and the group III element gallium is linearly exchanged for indium. October 2010
www.compoundsemiconductor.net 21
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