Prof. Colombo Bolognesi | Millimeter-Wave Electronics Laboratory | www.mwe.ee.ethz.ch

Prof. Colombo Bolognesi Millimeter-Wave Electronics

MissionThe Millimeter-Wave Electronics (MWE) Group research activities focus on the AlGaInN/GaN, InP/GaAsSb and AlInAs/GaInAs/InP material systems and their alloys. We exploit “Bandgap Engineering” techniques to develop new millimeter- wave transistors such as Heterostructure Bipolar Transistors (HBTs) and High Electron Mobility Transistors (HEMTs) and their fabrication processes. Our projects carry a signifi-cant impact: over the years we have set multiple bandwidth records for InP bipolar transistors and AlGaN/GaN and AlInN/- GaN-based transistors built on either SiC or Silicon substrates.

“Bandgap Engineering” refers to the ability to tailor the electronic and chemical properties of semiconductor mate-rials by varying the composition and doping concentrations throughout the body of a semiconductor device. With the development of modern epitaxial growth techniques like mo- lecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD), the composition of semiconductor layers can be controlled to a scale of one monolayer (that is, an atomic bi-layer for compound semiconductors), or roughly 0.3 nm: it is literally possible to synthesize semiconductor devices by putting the crystal together one atomic layer at a time. Our ability to tailor semiconductor devices on the nanoscale for superior performance arises because most III-V compound semiconductors are soluble within each other and form homogeneous solid solutions or “alloys.”

At ETH Zurich this is done in the FIRST Laboratory, our sophisticated cleanroom facility for advanced Micro- and Nanotechnology (www.first.ethz.ch). In FIRST we exploit MOCVD and MBE and a full array of semiconductor fabrica-tion tools to realize nanometer scale devices.

In terms of teaching, our Group conveys the fundamentals of semiconductor devices to D-ITET students. The D-ITET curriculum is rather weak in the area, with a single fourth semester compulsory course. We are also training Master’s level students with the essentials of RF transmission lines as associated techniques (S-parameters, fundamentals of signal integrity concepts such as pulse dispersion and crosstalk) in fifth and sixth semester courses.

Curriculum VitaeProf. Colombo BolognesiProfessor of Millimeter-Wave Electronics

Degrees / Higher Education1994: PhD EE, UCSB, Santa Barbara, CA, USA1989: MSc EE, Carleton, Ottawa, ON, Canada1987: B.Eng. EE, McGill, Montréal, QC, Canada

Professional Career2008: Professor of Millimeter-Wave Electronics, D-ITET, ETH Zurich2006: Professor of THz Electronics, D-ITET, ETH Zurich2001: Full Professor of Engineering Science and Physics, Simon Fraser University, Burnaby BC, Canada1998: Associate Professor Engineering Science and Physics, Simon Fraser University1995: Assistant Professor Engineering Science and Physics, Simon Fraser University1994: BiCMOS Process Integration Engineer, Northern Telecom, Ottawa ON, Canada

Major Honors and Awards2008: IEEE Fellow (for contributions to Millimeter-Wave Antimonide-Based Heterojunction Bipolar Transistors)2003: IEEE Senior Member2001: “Frontiers in Research Award” of the Science Council of British Columbia1999: GaAs IC Symposium Best Paper Award1998: IEE Electronics Letters Premium Award

Professor Colombo Bolognesi

Prof. Colombo Bolognesi | Millimeter-Wave Electronics Laboratory | www.mwe.ee.ethz.chProf. Colombo Bolognesi | Millimeter-Wave Electronics Laboratory | www.mwe.ee.ethz.ch

Our research activities can be divided in two categories ac-cording to device type, specifically InP bipolar transistors and InP or GaN-based field-effect transistors. The topics are ad-dressed according to this logical scheme in what follows.

1. InP / GaAsSb Double Heterostructure Bipolar Transistors (DHBTs)Classical InP DHBTs incorporate a GaInAs base and an InP collector intended to cumulate the speed benefits of electron transport in GaInAs to the higher breakdown field of InP. The problem is that abrupt junctions between GaInAs and InP form a blocking barrier at the base / collector junction that traps electrons in the base layer, reducing gain and massive-ly increasing the base transit time. The classical solution to this problem has been to grade the composition of the collec-tor near the base with an AlGaInAs alloy (using digital alloy short-period superlattices or continuous compositional grad-ings). These structures are non-trivial to produce in a man-ufacturable fashion and they limit the high-current density operation of the devices, degrade the thermal resistance, and complicate the device fabrication process by making selective wet etching of the collector base junction impossible.

In 1996/97, Prof. Bolognesi (then at SFU in Canada) initiated work on the use of GaAsSb as a base layer for InP / GaAsSb / InP DHBTs. GaAs0.5Sb0.5 is nearly lattice-matched to InP and it was thought to lead to a type-II band alignment to InP which would allow the direct injection of electrons from a GaAsSb base into an InP collector across an abrupt heterojunction. The same idea was independently demonstrated by Bellcore and Rockwell workers, but both organizations abandoned the technology after disappointing initial results. This left the field wide open for Bolognesi: his group proceeded to experimen-tally confirm the band alignment between InP and GaAsSb and to develop an InP / GaAsSb DHBT transistor technology from the ground up. By 2001, Prof. Bolognesi and his group demon-strated 300 GHz transistors with a > 6V breakdown voltage: at the time, these were the fastest bipolar transistors in the world, regardless of material system. In collaboration with Agilent Technologies, a basic fabrication process was trans-ferred to the Palo Alto R&D labs which subsequently brought up a process in Santa Rosa. InP / GaAsSb DHBTs went into pro-duction in 2004 and they are now a key technology enabling all high-speed test equipment (such as PNAX network analyzers, Infiniium oscilloscopes, etc.) sold by Agilent. It is a particularly rewarding realization for our group that regardless of their stated technological preference for alternatives such as SiGe HBTs, InP / GaInAs DHBTs or HEMTs, a majority of scientists and device engineers characterize their devices with Agilent equipment incorporating InP / GaAsSb DHBTs –when all is said and done, that fact speaks to the manufacturability and value of the technology.

Since joining the ETH in 2006, Prof. Bolognesi’s group main-tained leadership with InP / GaAsSb DHBTs by developing mixed-group V graded base layers, electron beam lithogra-phy self-aligned processes as well as a unique Teflon based planarization process for the realization of integrated circuits. The Teflon planarization solved important shortcomings of the industry standard BCB (benzocyclobutene) process which de-grades Ohmic contacts due to its high curing temperatures, while offering far better dielectric properties. MWE recently published a DHBT with fT / fMAX = 429 / 715 GHz: this represents an absolute record for GaAsSb-based devices. Still unpub-lished results include transistors fT / fMAX = 500 / 800 GHz, us-ing conservatives interpretation of measurement data. A full MMIC design infrastructure is being developed for the re-alization of simple DHBT oscillator demonstration circuits above 200 GHz.

2. Field-Effect Transistors2a) GaN High-Electron Mobility Transistors (HEMTs)MWE has pioneered the development of GaN HEMTs for mm-wave applications since 2007. Whereas most groups were then focused on applications below 10 GHz, MWE forged ahead with high-speed deep submicrometer gate length de-vices to push cutoff frequencies well above 100 GHz. MWE was the first worldwide to demonstrate AlInN / GaN HEMTs on SiC substrates with fT / fMAX > 200 / 200 GHz by shrinking the source-drain distance in sub-100 nm gate devices. The work was highlighted by ScienceWatch as a Hot New Paper in En-gineering, amounting to about the top 0.1% most cited new of papers in the Engineering field (see http://archive.science-watch.com/dr/nhp/2012/12jannhp/12jannhpBolo/).

MWE pursued a similar course for GaN HEMTs built on high-resistivity substrates due to the realization that the ab-sence of natural lattice-matched low-cost substrates for GaN will limit the broad commercial applicability of GaN HEMTs. Whereas it was believed that GaN-on-Si HEMTs could not compete at high frequencies due to residual substrate con-ductivity, we quickly pushed cutoff frequencies near 150 GHz. To verify that proper RF interconnects could indeed be made on GaN-on-Si, MWE characterized coplanar waveguides be-tween 1 and 110 GHz and showed that line losses as low as 0.6 dB / mm could be achieved on GaN-on-Si. This proved that GaN-on-Si HEMT technology is indeed mm-wave compatible. For comparison, the best waveguide MWE made on semi-insu-lating Fe-doped InP showed a loss of 0.35 dB / mm at 110 GHz.In terms of mm-wave power, MWE demonstrated record pow-er densities of 5.85 W / mm at 40 GHz in AlInN / GaN HEMTs with

Research Activities and Achievements

FIB cross-section of 700 GHz InP / GaAsSb DHBT transistor (right) entirely fabricated at ETH by MWE in FIRST Laboratory, including crystal growth by metalorganic chemical vapor deposition (MOCVD). The emitter and base contacts are patterned by electron beam lithography: the emitter mesa width is 200 nm.

conventional annealed Ohmic contacts. More recently devel-oped regrown Ohmic contacts have allowed us to show output power densities of >1.6 W / mm at 94 GHz, which matches the best power output ever reported for GaN HEMTs at 94 GHz by workers at Hughes Research Laboratories in Malibu, CA, USA.

2b) InP / GaInAs Pseudomorphic High-Electron Mobility Transistors (pHEMTs)MWE develops InP / GaInAs pHEMTs for low-noise applica-tions, particularly as they pertain to deep space commu-nications and radio-astronomy (RA), two applications with the most stringent noise performance requirements. MWE pHEMTs have matched or bested the performance of the best low-noise transistor ever produced (the famed CRYO3 batch of TRW) in amplifiers produced by our Spanish collaborators in Centro Astronómico de Yebes.

MWE activities were constrained by the lack of responsivity from US custom epitaxial layer suppliers, leading us to the decision to develop our own InP pHEMTs in-house by molec-ular beam epitaxy (MBE) in FIRST Laboratory. Quick prog-ress was made since 2012, and we now produce world-class material using composite channels InAs sub-channels in a strained GaInAs main channel. The idea is to exploit the trans-port properties of InAs without using antimonide (AlGaSb) barriers which are chemically unstable to instead exploit the maturity and stability of InP HEMT technology. We view this as the most viable pathway to make useful and reliable ultra-low noise devices.

MWE members posing with a 40 keV electron beam lithography system used in the fabrication of our DHBT and HEMT mm-wave transistors in FIRST Laboratory. 30-50 nm footprint T-gates are routinely realized.

Six-finger ultralow-noise AlInAs / GaInAs pHEMT transistor developed by MWE for application in ESA’s deep space network at Ka-band. The gate finger footprint is 100 nm for K and Ka-band applications.