Our community is already exploiting the benefits of GaN in transistors and power diodes. However, further innovation in device design and processing is essential if these devices are to ever fulfill their true potential. In Japan, many companies are looking to do just this, and a scan through recent conference proceedings uncovers a wealth of claims for device records and novel processes.
One recent claim is for the highest frequency for a GaN transistor, which has been made by the National Institute of Information and Communications Technology (NICT), Tokyo. NICT reported an fT and fmax of 190 and 241 GHz at last year's International Conference on Nitride Semiconductors in Las Vegas, from HEMTs that have a maximum drain current density of 1.6 A/mm and a peak transconductance of 424 mS/mm.
NICT plans to demonstrate that its high-speed HEMTs can deliver power performance in the millimeter-wave range, particularly at 60, 76 and 94 GHz. "Generally there is no good solid-state amplifier for frequencies above 50 GHz in the world," said former NICT researcher Masataka Higashiwaki, who is now a project scientist at the University of California, Santa Barbara. Amplification at 60 and 76 GHz is needed for high-speed wireless LAN and automotive radar, and the 94 GHz band is just starting to be used for ultra-high-speed wireless communications that can complement optical fiber networks. These applications all demand high-power transmitters that are not available today. "In my opinion, GaN is the only material that has the possibility to realize it," said Higashiwaki.
The HEMTs' high speed is partly the result of an alternative SiN passivation step – catalytic CVD. This is a simple process that was invented in the 1980s by Hideki Matsumura's group at the Japan Advanced Institute of Science and Technology. "We put a silane and ammonia mixture by a heated tungsten filament, and the filament decomposes the gas by catalytic effects. SiN is formed on the substrate."
The catalytic CVD process is superior to the more common plasma processes because it doesn't damage the very sensitive GaN surface. Degradation of this material can produce defects and surface states that impair device performance and reliability. "Additionally, the quality of the SiN film deposited by Cat-CVD is much better than that by plasma," said Higashiwaki, "and film coverage is excellent from the early stages of deposition."
Reducing the length of the gate to 60 nm also helps to increase the HEMTs' speed. Higashiwaki says that getting to smaller dimensions wasn't that difficult: "There is no challenge on patterning. We used a normal electron-beam writer with an acceleration voltage of 50 kV and a usual triple-layer resist process." He says that 30 nm gates have been realized with a very similar process.
Completing the gate-fabrication processes was more challenging, however, because it was tough to develop a satisfactory lift-off process for the gate metal. "We used titanium instead of nickel – which is commonly used as a Schottky gate – because titanium has stronger adhesion." Titanium evaporation directly onto GaN leads to a high gate leakage, so the researchers have sandwiched a SiN layer in the middle. However, Higashiwaki is not sure whether the gate structure is optimal, so he is currently looking into other processes.
The third factor behind the record-breaking HEMTs is their thin, aluminum-rich barrier layers. These reduce unwanted "short-channel" effects that degrade device performance and prevent gate-length scaling. The MOCVD-grown high-quality epitaxial structure features a 6 nm thick Al0.4Ga0.6N barrier, a 1 nm thick AlN spacer and a 2 µm thick GaN buffer on 4H-SiC (figure 1). The high aluminum content in the barrier increases polarization and the density of the two-dimensional electron gas, which gets a further hike from the catalytic deposition of SiN.
The positive effect of catalytic CVD on the electron density is still to be understood fully, but Higashiwaki believes that the AlGaN surface barrier height is decreased after SiN deposition. Calculations can support this scenario, but Higashiwaki admits that they have no idea if the model provides an accurate account of what is taking place.
GaN overpowers silicon One company improving the performance of power devices is Panasonic, which has been developing a novel laser process for producing via holes through sapphire. AlGaN/GaN power HFETs with a blocking voltage of 10.4 kV were made by this method, with the details unveiled at December's International Electron Devices Meeting (IEDM) in Washington, DC.
This type of device can be used in various high-voltage applications, such as electric trains and electric power transmission. "10 kV blocking by GaN is the first demonstration that GaN can also be competitive with SiC, which has been the only choice for such high-voltage applications," said Panasonic's Tetsuzo Ueda, who is general manager of the company's Semiconductor Device Research Center.
Today's silicon devices can't compete at such high voltages, with insulated-gate bipolar transistors topping out at 6.5 kV, light-triggered thyristors doing no better than 8 kV and gate turn-off transistors restricted to 6 kV. The on-resistance of these devices is also inferior to wide-bandgap alternatives.
Panasonic's device has a major cost advantage over SiC competitors thanks to the cheaper prices of sapphire substrates. However, sapphire is an inferior thermal conductor. To address this weakness, via holes are formed and filled with metal, and the surface is passivated by polycrystalline AlN, which has a far lower thermal resistance than SiN (figure 2).
Ueda is rather coy about the details of the DC-sputtering step used to deposit the 1 µm thick layer of polycrystalline AlN. However, the research team has revealed that this film has a thermal conductivity 200 times as good as that of SiN. The breakdown electric field strength is also twice as high at 6 MV/cm, and this, alongside field plates and metal vias, gives the HFET its high performance.
Panasonic has improved and adapted its via hole technology for millimeter-wave devices, and it unveiled a 26 GHz amplifier delivering 22 dB of gain at the recent IEEE International Microwave Symposium in Atlanta. This record-breaking gain from a single-chip three-stage amplifier was attributed to integrated microstrip lines, which require the fabrication of via holes.
Higher breakdown voltages would also improve the performance of high-frequency HEMTs. The obvious way to do this is to add more aluminum to the channel layers. Conventional processes would then encounter the problem of a higher contact resistance, which limits output power. However, silicon ion implantation can be used instead, which produces a far lower contact resistance, according to a partnership between researchers at Mitsubishi Electric, RIKEN and Tokyo Institute of Technology, who also reported their findings at IEDM 2007.
The researchers assessed the impact of ion implantation by studying its effects on devices with three different channel and barrier structures: Al0.53Ga0.47N/Al0.38Ga0.62N; Al0.39Ga0.61N/Al0.16Ga0.84N; and Al0.18Ga0.82N/GaN. A silicon ion concentration of 1 × 1015 cm–2 was added by room-temperature implantation with a 50 keV tool. Five minutes of rapid thermal annealing under nitrogen gas at 1200 °C followed. During this the structures were capped with a 30 nm layer of SiN, which was deposited by plasma-enhanced CVD. The cap was subsequently etched and ohmic contacts added, before devices were isolated by zinc ion implantation.
Current–voltage curves were used to compare these devices and controls that had no silicon-ion implantation. The controls had a contact resistance in excess of 1 Ω cm2, while the ion-implanted Al0.53Ga0.47N/Al0.38Ga0.62N and Al0.39Ga0.61N/Al0.16Ga0.84N structures had resistances of just 5.3 × 10–3 Ω cm2 and 1.8 × 10–5 Ω cm2, respectively.
Ion implantation enabled the fabrication of HEMTs with high breakdown voltages. Conventional HEMTs with this design would typically break down at 200 V, but Al0.53Ga0.47N/Al0.38Ga0.62N transistors with gate-drain distances of 3 and 10 µm failed at 463 and 1650 V, respectively.
The researchers say that these results show that aluminum-rich AlGaN channel HEMTs are strong candidates for high-frequency devices, such as low-noise amplifiers, and they also make promising devices for high-power switching applications. Additional improvements are expected through the introduction of field plates that should deliver a further hike in breakdown voltage.
GaN power supplies One of the companies looking at HEMTs for switching applications is Toshiba, which is investigating this device for boost converters in PC power supplies. According to Toshiba's Wataru Saito, the GaN HEMT structure is attractive in this application, thanks to its combination of low channel resistance and high electron mobility. "This is an advantage of GaN HEMTs over conventional silicon and SiC power MOSFETs. The electron mobility in the SiC MOS-channel is 10 times lower than the AlGaN/GaN heterostructure." However, it is much harder to produce a normally off device, which means that the gate threshold voltage tends to be more than 0 V.
GaN HEMTs also suffer from a modulation of the on-resistance by current collapse, which leads to an increase in conduction loss. "As a result, power efficiency is degraded with the applied voltage," said Saito. Electron trapping is responsible, which is exacerbated by high electric fields that accelerate the electrons in the channel. However, this can be mitigated by the addition of field plates, which spread the electric field concentration over the gate and field plate edges.
Toshiba has evaluated the effects of single and dual field plate HEMTs (figure 3), and it presented the results at IEDM 2007. Dual field plates produce a far smaller on-resistance from 0 to 300 V, thanks to a reduction in the peak electric field strength.
The HEMTs are grown on n-type SiC, which can act as a back-side field plate that suppresses current collapse at voltages of 100 V or more. This type of substrate also produces a higher crystal quality than sapphire or silicon, and it cuts current collapse through a reduction of electron-trapping defects.
Measurements on a dual field plate 480 V 2 A HEMT revealed an on-resistance modulation of just 5% at 300 V. When the transistor was deployed in a boost converter circuit it delivered 92.7% efficiency at a 1 MHz switching frequency, which is comparable to a similar circuit based on a silicon MOSFET.
Saito says that more work is needed before these devices can be used in power electronic applications. "Power supply systems require a breakdown voltage of more than 600 V and a maximum current of more than 10 A. Our fabricated device characteristics were below those levels." Efforts are now being directed at improvements in breakdown voltage and the fabrication of large devices.
Further reading M Higashiwaki et al. 2008 Proc. of SPIE 6894. T Murata et al. 2008 MTT-S 1293. T Nanjo et al. 2007 IEDM 15.5.1. W Saito et al. 2007 IEDM 33.3.1. Y Uemoto et al. 2007 IEDM 33.1.1.
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