I. INTRODUCTION
The present article highlights some of the AlGaN/GaN high electron mobility transistor (HEMT) activities of the recently established Millimeter-wave Electronics Laboratory at ETH-Zürich. The Group was launched in 2006 with the mandate to develop better, higher performance heterostructure devices, principally for future telecommunication systems. Our three principal areas of interest are double heterojunction bipolar transistors (DHBTs) based on the InP/GaAsSb heterostructure system, InP HEMTs for low-noise applications, and GaN-based HEMTs, which are the focus of the present article. The fundamental philosophy underlying our AlGaN/GaN HEMT activities involves leveraging the ETH InP monolithically integrated circuit (MMIC) process capability to develop a GaN-based MMIC mm-wave platform.
Because of their now well-appreciated set of physical properties, GaN-based HEMTs remain the focus of intense interest for high-power, wideband, and/or high-temperature applications [Reference Palacios1, Reference Higashiwaki, Mimura and Matsui2]. The key benefits of the (Al,Ga)N/GaN material system for microwave and mm-wave HEMTs are of course associated with their wide energy gaps and the high two DEG-channel charge densities resulting from the spontaneous and piezoelectric polarizations associated with the lattice mismatch between (Al,Ga)N and GaN. In principle, this combination of characteristics makes possible rugged high-current drive wideband transistors capable of operating at high voltages and temperatures.
The highest speed GaN HEMTs have historically been implemented on sapphire or on SiC substrates. Because of their affordability, ample supply, and good thermal conductivity at operating junction temperatures, high-resistivity silicon (HR-Si) substrates provide a low-cost solution for the realization of GaN-based power transistors in the lower microwave frequency bands. Our group does not have a particular preference for any given substrate technology: historically, we have published results for devices grown on sapphire and SiC. More recently, since re-locating to ETH-Zürich, our work involved devices on HR-Si substrates because of a good relation with a supplier of epitaxial layers. As a result, our recent work established that GaN-based HEMTs grown on HR-Si also offer very good performances at mm-wave frequencies [Reference Sun, Alt, Benedickter and Bolognesi3, Reference Sun, Alt, Benedickter and Bolognesi4], culminating with the realization of 100 nm gate (Al,In)N/GaN HEMTs on HR-Si with cutoff frequencies as high as f T = 102 GHz for AlInN/GaN HEMTs grown at EPFL [Reference Sun and Alt5]. We will later show that these successes also carry over to devices on SiC substrates. All transistors discussed in this paper were fabricated on MOCVD-grown epitaxial layers.
In Section II, we compare the behavior of similar AlGaN/GaN HEMTs grown on HR-Si and sapphire substrates. We show that although the devices display similar DC and RF performances, they react differently to various factors such as the presence of passivation layers. As well, we demonstrate high-breakdown voltages with interesting cutoff frequencies in transistors with larger source-to-drain spacings. In Section III, we discuss some the work our group has carried out to extend the operating frequencies of AlGaN/GaN HEMTs culminating with cutoff frequencies exceeding 100 and 125 GHz for devices respectively grown on HR-Si and SiC substrates.
II. COMPARISON OF HEMTs ON HR-Si AND SAPPHIRE SUBSTRATES
AlGaN/GaN heterostructures used for this study were MOCVD-grown on either (111) HR-Si or sapphire substrates. On HR-Si a 2 µm GaN buffer was used, followed by 18.5 nm of Al0.27Ga0.73N and 2 nm GaN cap. A 2.9 µm GaN buffer was used on sapphire, followed by 25 nm of Al0.3Ga0.7N and a 2 nm GaN cap. The epitaxial layers are similar although not strictly identical, since they originate from different commercial vendors. Device processing details were given in [Reference Sun, Alt, Benedickter and Bolognesi3]. The as-grown electron sheet densities and mobilities on HR-Si (sapphire) were, respectively, 8.5 × 1012 (1.1 × 1013)/cm2 and 1200 (1000) cm2/Vs.
Circular transmission line measurement patterns reveal sheet and contact resistance values of 640 (550) Ω/sq. and 0.46 (0.55) Ω·mm on the HR-Si (sapphire) substrate. DC characteristics were measured using an Agilent 4156A semiconductor parameter analyzer. Figure 1 superposes representative DC characteristics for 2 × (0.1 × 75) µm2 devices before and after SiNx passivation for devices built on the HR-Si substrate. The static output characteristics were measured for the range V DS = [0 to 8] V with V GS = [0 to −4] V. The devices show similar output I–V curves for both substrates, and a detailed comparison of I–V characteristics was given in [Reference Sun, Alt, Marti, Vetter, Benedickter and Bolognesi6]. As expected, the drain saturation current increased after passivation: the drain current at V GS = 0 V increased from 425 mA/mm (500 mA/mm) to 545 mA/mm (590 mA/mm) on the HR-Si (sapphire) substrate. Figure 2 shows the transconductance characteristics before and after passivation for both substrates. We note that the threshold voltage shift toward more negative value following passivation. This may be due to an increase of sheet carrier concentration, but the phenomenon was not investigated in more depth. Peak DC extrinsic transconductances of 195 and 155 mS/mm were obtained for passivated devices on HR-Si and sapphire substrates, respectively. Devices on HR-Si show a higher transconductance due to the thinner (Al,Ga)N barrier layer (18.5 nm on HR-Si versus 25 nm on sapphire). The dc transconductance values indeed scale roughly with the nominal barrier thicknesses, and the same ratio roughly holds for the values extracted from RF measurements. Devices built on sapphire showed current collapse-related instabilities before they were passivated by a 100 nm thick SiN film deposition, as shown in the transconductance plot of Fig. 2(b). The gate diode leakage current on the passivated devices of Figs 1 and 2 was measured at the level of 7.6 and 36 µA/mm at V G = −7 V for the HEMTs on HR-Si and sapphire, respectively. The lower gate leakage on HR-Si is attributable to the low screw dislocation densities (~107/cm2) achieved by Nitronex on their GaN-on-Si layers (see www.nitronex.com more information). The lower gate diode leakage achieved in our GaN on HR-Si HEMTs translates into better I ON/I OFF ratios, as reported in [Reference Sun, Alt, Marti, Vetter, Benedickter and Bolognesi6].
Output characteristics of AlGaN/GaN HEMTs before and after passivation on HR-Si substrate. Devices built on sapphire show nearly identical characteristics and are therefore not shown here since detailed comparison of their I–V characteristics is given in [Reference Sun, Alt, Marti, Vetter, Benedickter and Bolognesi6].
Transconductance characteristics of AlGaN/GaN HEMTs before and after passivation: (a) on HR-Si substrate, and (b) Sapphire substrate. The unpassivated device on sapphire suffers current collapse which causes a drop of transconductance near V GS = 0 V. Passivation largely alleviates the current collapse on the devices built on sapphire.
RF measurements were performed from DC to 40 GHz with an HP8510 vector network analyzer using an off-wafer calibration standard and an LRRM calibration. The effects of probing pads were de-embedded using open and short patterns. Figure 3 characterizes the transistor microwave performance of devices on HR-Si and sapphire substrate for bias points corresponding to the maximum transconductance with V DS = 6 V. Although the devices on both substrates display quite similar DC characteristics, they react very differently in terms of microwave performance. A −20 dB/dec extrapolation of |h 21|2 and of Mason's maximum unilateral gain U yields f T = 49/56 GHz and f MAX = 90/108 GHz before and after passivation for the devices on HR-Si: the passivation therefore markedly enhances the RF performance on HR-Si. For devices on sapphire, the f T and f MAX decrease from 57 to 54 GHz, and from 102 to 77 GHz, following passivation.
Microwave performance of AlGaN/GaN HEMTs before and after passivation, with pad effects removed: (a) on the HR-Si substrate, and (b) on the Sapphire substrate.
It may also be of interest to consider at the raw (or extrinsic) device performance with pad effects included. For the devices on HR-Si the pre-de-embedding cutoff frequencies are f T,E = 38/46 GHz and f MAX,E = 73/88 GHz before and after passivation, indicating that the f MAX/f T ratio is essentially unaffected by the de-embedding process, thus contradicting the unfortunately too commonly held prejudice that GaN-on-Si devices necessarily suffer from conductive parasitic layers near the buffer/substrate interface. The parasitic buffer conduction can be satisfactorily addressed by optimizing growth conditions, as demonstrated here again on wafers from Nitronex. This view is strongly supported by our measurements of a CPW loss of only 0.82 dB/mm at 110 GHz for transmission lines built on HR-Si [Reference Sun, Alt, Marti, Vetter, Benedickter and Bolognesi6]. For devices built on sapphire, the pre-de-embedding cutoff frequencies are f T,E = 43/44 GHz and f MAX,E = 81/63 GHz before and after passivation. The role of passivation on the device RF performance is not straightforward when it comes to AlGaN/GaN HEMTs: an increase [Reference Harrison, Clayton and Jeffs7] as well as a decrease [Reference Green, Chu, Chumbes, Smart, Shealy and Eastman8] of f T and f MAX after passivation have been reported by different groups. The present results provide additional food for thought since devices undergoing nominally identical process steps in the same laboratory show dramatically different behaviors, despite the resemblance in the initial epitaxial layer stacks. Clearly then, the area of passivation effects in AlGaN/GaN HEMTs is far from exhausted and much remains to be clarified.
Table 1 shows the difference in the small-signal parameters extracted from S-parameter measurements before and after passivation on both substrates. The ratio of g M/2π(C GS + C GD) extracted from microwave measurements is in good agreement with the f T values obtained in Fig. 3. Our results show that the SiNx passivation has higher impact on the parasitic capacitance (such as C GS and C GD) for the GaN-on-sapphire devices than that on HR-Si. It should be mentioned that the passivated HEMTs built on HR-Si in fact feature a higher f MAX/f T ratio compared to the passivated devices on sapphire, and that the higher f MAX can be traced to the lower microwave output conductance g DS on the HR-Si substrate [Reference Sun, Alt, Marti, Vetter, Benedickter and Bolognesi6].
Device de-embedded parameters dependence on substrate type and passivation film.
In order to produce a high-gain millimeter-wave power amplifier featuring a high reliability, it is important to simultaneously achieve high transistor cutoff frequencies f T and f MAX and high breakdown voltages. GaN HEMTs have the potential to meet this requirement because the high electron velocities achieved under high electric fields can be combined with high breakdown voltage values achieved, thanks to the wide energy gaps of the materials involved. Although very high f T × BV products were reported on sapphire and SiC [Reference Kikkawa, Imanishi, Kanamura and Joshin9], there is a scarcity of reported results for GaN-on-silicon device. Figure 4 characterizes the transistor microwave performance of devices on HR-Si with source–drain distance of 4 µm: these devices were fabricated with a 0.1 µm gate centered in the source–drain space. A −20 dB/dec extrapolation of |h 21|2 and of Mason's maximum unilateral gain U yields f T = 43 GHz and f MAX = 77 GHz. Figure 5 shows the OFF-state breakdown voltage characterization of BV DS = 110 V and BV GD = 130 V, as determined by the drain current injection technique of [Reference del Alamo and Somerville10]. The f T × BV products reach values as high as 5.6 THz-V, and 10 THz-V when it comes to f MAX × BV. Still higher f T,MAX × BV products could be achieved by positioning the gate closer to the source contact.
Microwave performance measured at V DS = 6 V and V GS = −2 V for 2 × (0.1 × 75) µm2 HEMT. Extrapolation at −20 dB/dec yields f T = 43 GHz and f MAX = 77 GHz. Pads effects de-embedded.
Three terminal off-state breakdown characteristics of the conventional HEMT using the drain-current injection technique [10]. The drain current is fixed at 1 mA/mm and V GS is swept from 0 to −25 V (same device as Fig. 4).
Transistors featuring high-cutoff frequencies and large breakdown voltages are not only attractive for high-efficiency high-power amplifier applications (which demand minimal dispersion non-idealities), but also in wideband highly rugged low-noise amplifiers which can be built without input protection circuitry thanks to the low-noise figures achievable at X- and K-band frequencies [Reference Sun, Alt, Benedickter and Bolognesi3]. In this context, it becomes interesting to push AlGaN/GaN HEMTs to higher frequencies of operation. This is also of interest for us at ETHZ.
III. EXTENSION TO HIGHER FREQUENCIES
Perhaps because of the unavailability of truly semi-insulating silicon substrates, comparatively little work has been performed prior to 2008 to determine whether or not AlGaN/GaN HEMTs on silicon offer a viable technological solution for the realization of mm-wave transistors with interesting properties. We have first reported very good cutoff frequencies with f T of 75 GHz and noise figures interesting properties in 0.1 µm HEMTs grown on HR-Si [Reference Sun, Alt, Benedickter and Bolognesi3]. With increasing process maturity, we eventually achieved HEMTs with an f T of 90 GHz [Reference Sun, Alt, Benedickter and Bolognesi4]. On improved epitaxial layers featuring an electron mobility of ~1500 cm2/Vs from Nitronex Corporation, we now fabricate fully passivated (2 × 75) µm wide (Al,Ga)N/GaN HEMTs with 100 nm gate lengths and source–drain spacing L SD of 1 µm. These devices feature an f T = 104 GHz which is the highest ever achieved for GaN-based HEMTs on silicon substrates, shown in Fig. 6. Although the bulk of our activities have so far focused on AlGaN/GaN HEMTs on HR-Si because of an excellent supply relationship with Nitronex, our process also results in attractive performances when applied to GaN HEMTs on SiC: Fig. 7 shows that 100 nm gate transistors with f T of 126 GHz have been realized at ETH-Zürich. As far as we know, these are the fastest GaN-based HEMTs so far produced in Europe. More recently, we have also demonstrated the realization of 100 nm gate AlInN/GaN HEMTs with f T = 144 GHz and f MAX = 145 GHz using epitaxial layers grown on SiC at EPFL, thus setting a new record for AlInN/GaN and closing the cutoff frequency gap between AlInN/GaN and AlGaN/GaN HEMTs [Reference Sun11].
Microwave performance measured at V DS = 4 V and V GS = −3.25 V for 2 × (0.1 × 75) µm2 HEMT on silicon substrate. Extrapolation at −20 dB/dec yields f T = 104 GHz and f MAX(U) = 115 GHz and f MAX(MAG) = 125 GHz. Pad effects de-embedded.
Microwave performance measured at V DS = 4 V and V GS = −4.1 V for 2 × (0.1 × 75) µm2 HEMT on SiC substrate. Extrapolation at −20 dB/dec yields f T = 126 GHz and f MAX(U) = 119 GHz and f MAX(MAG) = 128 GHz. Pad effects de-embedded.
IV. SUMMARY
Since being established in 2006, the Millimeter-Wave Electronics Group at ETH-Zürich has developed high-performance AlGaN/GaN HEMT processes enabling the demonstration of surprisingly good RF and noise performances for material grown on HR-Si (111) substrates, therefore narrowing the performance gap between devices grown on silicon and on insulating substrates such as sapphire and SiC. The ETHZ AlGaN/GaN HEMT process architecture is not limited to HR-Si substrates, as it also provides excellent device performances for materials grown on SiC substrates, as exemplified by the realization of 100 nm gate HEMTs exhibiting cutoff frequencies of f T = 126 GHz on SiC semi-insulating substrates. Work is currently underway to exploit these device performances by establishing a power MMIC process at ETH-Zürich.
More recently, our Group set the record for the highest-speed AlInN/GaN HEMT ever reported with an f T = 102 GHz using EPFL material grown on HR-Si and demonstrated a reasonable output power of 2.5 W/mm at 10 GHz in 100 nm gate transistors built in a 1 µm source–drain space which only allowed low breakdown voltages which limited the output power level [Reference Sun and Alt5]. As of writing time, we have now established a new record for 100 nm gate AlInN/GaN HEMTs, again on EPFL material but this time grown on S.I. SiC to reach cutoff frequencies of f T = 144 GHz, which matches the fastest achieved for thin-barrier AlGaN/GaN devices of the same gatelength [Reference Sun11]. It is noteworthy that the fastest thin-barrier AlGaN/GaN HEMTs use thin ~2 nm SiN passivation/gate dielectric films and also feature relatively leaky gates.
Although an increasing number of research workgroups worldwide have undertaken the development of nitride transistors for mm-wave applications, few if any have shown activities in this direction in Europe. This is, however, a role that naturally fits the interests and expertise of our Group, regardless of substrate type.
ACKNOWLEDGEMENT
The support of the FIRST Laboratory personnel at ETH is gratefully acknowledged.
Haifeng Sun was born in Hunan, China. He received the B.S. degree in Applied Physics from Tsinghua University, Beijing, China, in 1997, and the M.S. degree in Electrical Engineering from the Institute of Microelectronics, Chinese Academy of Sciences, Beijing, China, in 2000. His Masters' thesis centered on the development of fully depleted SOI/CMOS devices and circuits. From 2001 to 2004, he joined Chinese Academy of Sciences, as an electrical engineer responsible for high-performance InGaP/GaAs HBTs. He began working toward the Ph.D. degree at Simon Fraser University, Burnaby, BC, Canada, in September 2004. In 2006, he transferred to the Swiss Federal Institute of Technology (ETH), Zürich, Switzerland, where he is currently completing the Ph.D. degree in the Millimeter-Wave Electronics Group. His current research interests are the optimization and characterization of high-performance GaN-based devices.
Diego Marti was born in Zürich, Switzerland. He received the M.Sc. in Electrical Engineering and Information Technology from the Federal Institute of Technology (ETH), Zürich, Switzerland, in 2009. His studies focused on circuit design and optoelectronics design and processing. For his Masters thesis, he designed and implemented wideband distributed amplifiers in a hybrid microstrip environment using 100 nm gate GaN HEMTs grown on high-resistivity silicon. Diego Marti started his Ph.D. work in 2009 at ETH and is working in the field of GaN HEMTs devices and circuit design and fabrication.
Stefano Tirelli was born in Reggio Emilia, Italy. He received the B.S. degree in Physics from University of Pisa, Italy in 2003, with a thesis carried out at California Institute of Technology, USA. He received his M.S. degree in Matter Physics from University of Pisa in 2008. His Masters' thesis, carried out at NEST Laboratory of Scuola Normale Superiore, Pisa, was focused on electronic transport in out-of-equilibrium superconductors. In January 2009 he moved to Switzerland to join the Millimeter-Wave Electronics Group at the Swiss Federal Institute of Technology (ETH), Zürich, where he is currently pursuing the Ph.D. His current research interests are related to the optimization of high-power GaN-based HEMTs and the investigation of electronic transport phenomena limiting the high-frequency performance in sub-100 nm gatelength devices.
Andreas R. Alt was born in Munich, Germany. He received his B.S. degree in Electrical Engineering in 2004 and the Diploma (Masters) degree in Electrical Engineering in 2006 from Technical University of Munich (TUM), Germany. In 2006 he began his Ph.D. degree in the Millimeter-Wave Electronics Group at the Swiss Federal Institute of Technology (ETH), Zürich, Switzerland. His current research interests are the optimization/characterization and modeling of high-performance low-noise InP-based high electron mobility transistors.
Hansruedi Benedickter was born in Zug, Switzerland. He received the Diploma degree in Electrical Engineering from the Swiss Federal Institute of Technology (ETH), Zürich, Switzerland, in 1976. He has been a Research Assistant and a Senior Research Associate at the Microwave Laboratory, Swiss Federal Institute of Technology, and, since 1987, at the Laboratory for Electromagnetic Fields and Microwave Electronics. His main research interests are microwave, millimeter-wave, and on-wafer measurement techniques.
Colombo R. Bolognesi was born in St-Lambert QC, Canada. He earned the B.Eng. from McGill University (Montréal QC) in 1987, the M.Eng. from Carleton University (Ottawa ON) in 1989, and the Ph.D. from the University of California, Santa Barbara (UCSB) in 1993, all in Electrical Engineering. His doctoral dissertation dealt with the growth, fabrication, and physics of InAs/AlSb HFETs and led to the first high-speed InAs/AlSb HFET high-yield process. In 1994, he joined Northern Telecom's Semiconductor Components Group in Ottawa, as a BiCMOS process integration engineer responsible for high-performance submicron polysilicon emitter BJTs with selectively implanted pedestal collectors. In 1995, he joined the Engineering Science and Physics Departments at Simon Fraser University to launch and direct SFU's Compound Semiconductor Device Fabrication Laboratory (CSDL). In 2001, his group demonstrated the first bipolar transistors in any material system with balanced f T and f MAX cutoff frequencies simultaneously exceeding 300 GHz with a 6 V breakdown voltage, by exploiting the new InP/GaAsSb type-II heterojunction system. In 2006, he took up the Chair of Terahertz Electronics at the Institut für Feldtheorie und Höchstfrequenztechnik (IfH) at the Swiss Federal Institute of Technology in Zürich (ETH-Zürich). In 2010, the THz Electronics Group was renamed Millimeter-Wave Electronics and split from the IfH to stand as an independent Chair at ETH. Professor Bolognesi's research continues to target the development of high-speed devices (HBTs, HEMTs) based on InP/GaAsSb/GaInAs and AlGaN/GaInN to extend their application toward higher frequencies. Among others, he and his team earned the IEEE 1999 GaAs IC Symposium Best Paper Award, and he was the co-recipient of the New Frontiers in Research Award from the Science Council of British Columbia in 2001. He serves/served on the Technical Program, Executive and/or International Steering Committees of several IEEE sponsored conferences such as the Device Research Conference (DRC'01 and '02), the Int. Electron Device Meeting (IEDM'01,'02, and '03 as the QEC Chair), and the Indium Phosphide and Related Materials Conference (IPRM'03, '07, '08, ‘09) as well as the Int. Symposium on Compound Semiconductors (ISCS'07, '08, ‘09).
