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.

Aband 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, submillimeter 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-tohandle, 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. Cutofffrequencies, 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 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 Al0.52Ga0.48As layer, and the group III element gallium is linearly exchanged for indium.

Towards terahertz

Armed with an InGaAs channel, these HEMTs are the most advanced device technology for building the upcoming terahertz monolithic integrated circuits (TMICs). That’s because they deliver high transistor gain at very high frequencies and produce the lowest noise figure of any active device technology. The key to these high speeds has been a cut in gate length (see Figure 1). Advancing beyond 200 GHz while maintaining reasonable gain was not trivial - it demanded proper scaling of the entire process parameters.

 



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

 

So at every technology node adjustments were made to the scaling rules of the gate to-channel separation, source resistance, and electron density in the channel.

Increasing the indium content in the InGaAs channel has also helped to speed our mHEMTs (see Figure 2). Ramping the indium content to its upper limit to create a pure InAs film has increased electron mobilities and delivered superior charge confinement. In turn, the transit frequency, fT, of these mHEMTs has rocketed from 220 GHz for the 100 nm mHEMTs to 515 GHz for the 35 nm gate length devices. The faster variant can hit a drain current, Id, as high as 1600 mA/mm at a drain voltage of 1 V, thanks to a very low source resistance of only 0.1 Ω.mm. Both of the mHEMTs feature a 250 nm-thick MOCVD-deposited SiN layer, which is employed in all our devices.

 



Figure 2. Reduction in the gate size of Fraunhofer IAF’s transistors has been accompanied by adjustments to device design. This is evident in the layer composition of the 50 nm (a) and the 35 nm (b) mHEMT heterostructure. The 35 nm mHEMT layer sequence includes a double-side doped, single In0.80Ga0.20As channel to avoid short channel effects

 

There is more to realizing an IC with terahertz capability than producing super-fast transistors. Adapted passive circuit elements are essential for confining electromagnetic fields and suppressing unwanted substrate modes. To meet these needs we employ a grounded coplanar waveguide topology with coplanar transmission lines on the MMIC front side, connected to grounded backside metallization with miniaturized through-substrate vias.

This topology also provides a low source inductance of the active devices, along with compact transmission line dimensions. The crosstalk within the circuits is minimized by cutting the coplanar line ground-to-ground spacing to 14 μm. This, in turn, slashes chip size. To suppress substrate modes, a small spacing between the through substrate vias is necessary. By reducing the size of the vias from 35 to 20 μm, enough of them can be accommodated in the miniaturized MMIC topology. The final substrate thickness is 50 μm.

These processes have been used to build a mHEMT portfolio featuring 100, 50 and 35 nm gate length technologies, which can be used to fabricate circuits operating at up to 220, 340 and 500 GHz, respectively. A 20 nm gate length process is currently under development, which will replace the 35 nm technology and enable the design of novel terahertz ICs operating at 750 GHz and beyond.

How fast?

In response to the fabrication of faster transistors by our team and others around the world, commercial suppliers are starting to develop and fabricate frequency extension modules for accurate S-parameter measurements of ICs and modules up to approximately 1.1 THz. In addition, RF probes are now available for frequency bands between 0 and 500 GHz.

Although these efforts are welcomed, there are still major challenges associated with ultra-high-frequency measurements. For example, increases in operational frequency come at the penalty of a reduction in the dynamic range of these measurement systems. This increases the noise floor, complicating system calibration, which in turn affects the measurement of both active devices and passive circuit components. The consequence is that as we push further into unchartered frequency domains, we have to devote far more time to accurate calibration, testing and device model extraction in order to ensure successful circuit design and fabrication.

In addition to small-signal amplifiers, we are developing all of the required functional blocks for transmitters and receivers. This ever-expanding portfolio encompasses frequency generation and multiplication, power amplification and frequency conversion.

One example of our efforts is an all-MMIC-based broadband heterodyne receiver front-end spanning 268 - 306 GHz (see Figure 3). The wideband receiver is formed by a monolithic chip set that combines a cascading lownoise amplifier; resistive mixer with integrated frequencydoubler; LO power amplifier; and frequency-multiplier bysix. The result is a chip that delivers up to 8 dB of conversion gain and has a noise figure of 7.6 dB. These performance figures rival those of state-of-the-art Schottky receivers.

 

 



Figure 3. Fraunhofer IAF has fabricated a MMIC chipset for broadband performance at 300 GHz. This is based on active circuit concepts and employs 100 and 50 nm gate-length mHEMT technology. The chipset incorporates a low-noise amplifier, resistive mixer with integrated frequency-doubler, LO power amplifier and frequency-multiplier-by-six. The role of the balanced active frequency multiplier-by-six is to provide 0 dBm of output power in the 110 to 152 GHz range. When directly driven by the multiplier output power, the combination of frequency doubler and resistive mixer produces a conversion loss of 20 dB. With the intermediate LO power amplifier, conversion loss is reduced to only 12 dB across the 260- 308 GHz frequency range. The LNA provides pre-amplification by 20 dB at 290 GHz with an estimated noise figure of 7.5 dB. This takes the overall receiver performance to a maximum conversion gain of 8 dB.

 

Recently, we have focused our development on submillimeter- wave ICs and modules for operation above 300 GHz. This hinges on the realization of TMICs, which is the short-term target. Progress in this direction includes the fabrication of a 320 GHz mHEMT amplifier package with a waveguide module in split-block configuration (see figure 4). The MMIC is thinned down to a substrate thickness of 50 μm, allowing the use of very short bond wires. This ensures low parasitics and low loss. To increase operational reliability, the power supply was integrated into the module.

 



Figure 4. Fraunhofer IAF’s sub-millimeter-wave amplifier module in split-block technology. The dissection plane divides the input and output rectangular waveguides along the centerline of the longer side. The monolithic 50 nm gate length amplifier circuit is mounted between two microstrip lines realized on 50 μm thick quartz substrates. These two lines are serving as waveguide-to-microstrip transitions

 

Measurements of this amplifier reveal that it delivers a very flat gain characteristic. Linear gain exceeds 19 dB from 295 to 320 GHz, and it hits a maximum gain of 21 dB at 300 GHz (see Figure 5).

 



Figure 5. Fraunhofer IAFs mHEMT amplifier modules produce a maximum gain of 21 dB at 300 GHz. Small-signal gain exceeds 19 dB between 295 and 320 GHz

 

We have also produced a handful of amplifier MMICs with remarkable bandwidth and low noise figures for operation in frequency bands between 220 and 500 GHz. These amplifiers should help to spur development of high resolution imaging systems, wireless ultra-high-capacity communication links and sub-millimeter-wave spectroscopy.

One example of this type of amplifier is a four-stage, 460 GHz S-MMIC (see Figure 6). In this case, the amplifier was designed to deliver high small-signal gain and low noise. It fulfills these goals, delivering a peak gain of 16.1 dB at 460 GHz when driven at a drain voltage of Vd = 1 V, a gate voltage of Vg = 0.2 V, and a drain current of 23 mA (see Figure 7). Small-signal gain exceeded 13 dB across the range 433-465 GHz.



Figure 6. The four-stage, 460 GHz mHEMT amplifier S-MMIC employs transistors with a gate width of only 2 x 5 μm. Die size is only 0.37 x 0.63 mm2

 



Figure 7. On-wafer measured S-parameters of a four-stage mHEMT amplifier SMMIC. Small signal gain of more than 16 dB is realized at 460 GHz.

 

Amplifiers such as this, along with others that we have showcased in this article, are proof of our expertise in high-speed transistors and accompanying MMIC and packaging technology.

These recent scaling advances, which re-enforce our position as the leading pioneer of high-speed devices and circuits within Europe, have equipped our mHEMTs for terahertz operation and are paving the way for terahertz ICs.

Further reading

I Kallfass et al. 2009 Proc. of SPIE 7485

A Leuther et al. 2010 IPRM Technical Digest 425

A Tessmann et al. 2010 IMS Technical Digest 53