GaN microelectronics  technology


of drain biases, remaining above 160 GHz up to VDS of 8 V. Increasing the drain bias to a VDS of 5 V resulted in a peak fMAX(U) of 200 GHz with an fT of 188 GHz at a VGS of -5.3 V. A cutoff frequency of 205 GHz is the highest-ever


achieved for any GaN-based transistors on any substrate. It is also the first time that AlInN/GaN-based devices have established the record for bandwidth in nitride HEMTs. Although one can expect records to continue to change hands between different types of GaN HEMTs, these results are extremely pleasing considering the rapidity of progress with the new AlInN/GaN heterostructures.


Analysis of the frequency performance, which will be published shortly, reveals that the effective velocity in AlInN/GaN channels is up to 60% higher than in the fastest AlGaN/GaN HEMT channels ever produced. Some theoretical and experimental work has linked higher channel velocities in AlInN/GaN channels to the higher 2DEG concentrations. These are believed to favor the faster dissipation of longitudinal-optical phonons in comparison to the situation in conventional AlGaN/GaN channels. Today, however, it is not possible to fully explain the improved performance observed with AlInN/GaN by this phenomenon. Further studies are necessary. For the sake of comparison, AlInN/AlN/GaN HEMTs were fabricated in a similar manner using epilayers grown on high-resistivity silicon (111) substrates. In this case, the epitaxial structure comprised a 60 nm AlN nucleation layer, 700 nm GaN insulating buffer and channel layer, a 1 nm AlN spacer layer, and an 8 nm-thick, nearly lattice-


matched, Al0.86In0.14N barrier. According to room- temperature Hall measurements, the channel electron sheet density was 1.95 x 1013 cm-2 and its mobility was 1,060 cm2/Vs. These inferior values compared to the above layers on SiC are a result of a reduction in crystal quality, which was highlighted by broader XRD linewidths. The 2DEG and mobility are 20-25 percent lower than that of the AlInN/GaN HEMT on SiC, and account for the increase in sheet resistance to 300 Ω/.


Devices with a 80 nm gate length were fabricated on these layers, which show simultaneous cutoff frequencies,


fT of 143 GHz and fMAX(U) of 176 GHz. Although these values are lower than those achieved with the 55 nm


Figure 7. The AlInN/GaN HEMTs


fabricated in the EPFL-ETHZ collaboration deliver a record breaking cut-off frequency for any form of GaN HEMT


devices on SiC, they are still the fastest nitride devices ever built on silicon. It is also interesting to note that the


fT’s obtained here for the 55 and 80 nm gate HEMTs on SiC and silicon substrates scale almost exactly with the inverse gate length. Clearly, the AlInN/GaN heterostructure has more surprises in store than one could ever anticipate!


AlInN/GaN HEMTs are fast emerging as an interesting alternative to conventional AlGaN/GaN HEMTs which offers much higher maximum current densities and transconductances in a (nearly) lattice-matched barrier system characterized by a superior thermal stability. Although the present article mainly focused on recent EPFL/ETHZ efforts toward extending the devices to higher frequencies, the fundamental properties of the AlInN/GaN system do lend themselves well to the realization of high- power, high-efficiency transistors, as recently reported by workers from Alcatel-Thales with the demonstration of 0.25 µm AlInN/GaN HEMTs with an output power of 10.3 W/mm and a power-added efficiency of 51% at 10 GHz [14]. The combined advantages of AlInN/GaN heterostructures for power HEMTs are extremely attractive and provide ample motivation to find solutions to remaining challenges associated with the material system: one can count on an increasingly important footprint for AlInN/GaN HEMTs in the field of GaN microwave and millimeter-wave electronics for the times to come.


References 1. S.Y. Park et al. Microelectronics Rel. 49 478-483 (2009) 2. A. Chini et al. Electron. Lett. 45 426-427 (2009) 3. M. Higashiwaki et al. Proc. SPIE 6894 68941L (2008) 4. Y. Cao et al. Appl. Phys. Lett. 90 182112 (2007) 5. M. Higashiwaki et al. Appl. Phys. Express 1 021103 (2008) 6. J. Kuzmík IEEE Electron Device Lett. 22 510-512 (2001) 7. F. Medjdoub et al. IEEE Electron Device Lett. 29 422-425 (2008)


8. F. Medjdoub et al. The Open Electrical and Electronic Engineering Journal 2 1-7 (2008) 9. L.D. Nguyen et al. IEEE Proc. 80 494-518 (1992) 10. L. Ardaravicius et al. J. Appl. Phys. 106 073708 (2009) 11. H.F. Sun et al. IEEE Electron Device Lett. 30 796-798 (2009) 12. H.F. Sun et al., IEEE Electron Device Lett. 31 293-295 (2010) 13. L. Zhou et al. Appl. Phys. Lett. 94 121909 (2009) 14. N. Sarazin et al. IEEE Electron Device Lett. 31 11-13 (2010)


August / September 2010 www.compoundsemiconductor.net 21


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