X-band radar is set to reap benefits of GaN technology

The US military employs RF transmitters and receivers for a plethora of applications, which include all-weather radar, surveillance, reconnaissance, electronic attack and communications systems. However, power amplifiers (PAs) in many systems still use vacuum tubes. As recently as last year, the US Navy was still funding research to improve this technology, in particular for high-data-rate communications and high-power, high-frequency radar applications. An ideal PA is small, light, cheap, reliable and efficient, and it should provide high power densities, transmit across a wide range of bandwidths and operate in a broad range of temperatures. Wide-bandgap semiconductor electronics can provide power amplification (and low-noise amplification) with the potential advantages of being more compact, robust and longer-lived than vacuum tubes. Before solid-state electronics can replace vacuum tubes, however, the technology must be optimized and shown to be manufacturable. Material quality and process technology will dictate device performance.

GaAs and silicon PAs are already being used in some of these military systems, but GaN (along with SiC) can potentially operate from VHF through X-band frequencies while providing higher breakdown voltage, better thermal conductivity and wider transmission bandwidths than conventional devices are able to offer. GaN transistors that are the same size as GaAs devices can operate at higher powers with higher impedance.

Within the field of RF applications, MMW communications links and X-band radar are two major areas of interest. Strategic military communications systems range from 7 to 44 GHz and beyond. Some of these will be space-borne, in which case high efficiency, reliability and low weight are all crucial. Radar traditionally requires very high pulse powers in the microwave bands from UHF to X-band (8-12 GHz) and beyond, and it includes a variety of ground, air, ship and mobile platform installations.

Millimeter-wave AlGaN/GaN HEMTs have been developed with an emphasis on both output signal quality and linearity, and the US Navy is now in the process of testing the lifetimes of GaN HEMTs and MMICs. Military systems that use active aperture antenna arrays need a linear amplifier behind each antenna element, and GaN is well suited to provide that amplifier for many of these communications and radar systems. The US Navy has also funded the development of high-power broadband AlGaN HEMT amplifiers that emit tens of watts of power at many frequencies for use as electronic decoys.

Other materials, including GaAs and SiC, cannot measure up to GaN s ability to provide high power and high frequency at the same time. GaN s wide bandgap of 3.4 eV, high electron saturation velocity (2.7 x 107 cm/s), low onset resistance and ability to operate at high temperatures together result in potentially high-efficiency devices. GaN can also operate at higher voltages: it has a breakdown voltage of 70 V compared with GaAs s 5 V and InP s 3 V. GaN s large bandgap also makes it much less susceptible to radiation damage, which provides an additional benefit for satellite systems.

Satellite-based communications transceivers need efficient, robust and reliable transistors that can act as power amplifiers at MMW frequencies. Jeong-Sun Moon at HRL Laboratories in Malibu, CA, who presented a paper entitled "Deep-submicron gate-recessed and field-plated AlGaN/GaN HFETs for millimeter wave applications" at the MRS Fall 2004 meeting in Boston last December, believes that GaN devices compare well to GaAs for Ka-band and even higher-frequency communications equipment. "Current GaAs power HEMT technology is hitting a wall trying to deliver high power and high efficiency at the same time," he said. "GaN may overcome [GaAs] and move beyond it."

In his talk, Moon said that PAs based on GaAs PHEMTs could produce less than 6 W of output power in the Ka-band with a power-added efficiency (PAE) of about 23%. High-voltage GaAs FETs have shown better credentials with 1.5 W/mm power density, but so far the frequency performances have been limited. The MMW communications application could use more power but, equally important, it could also use higher efficiencies. GaN HEMTs have the potential to operate at power densities 10 times as high as those that GaAs PHEMT devices can cope with.

The efficiencies and power of GaN transistors have been increasing steadily. Also at the MRS meeting, Moon described GaN HFETs operating at 10 GHz with an output power density of 11 W/mm and a PAE of 50% at Vds = 30 V. At 30 GHz in the Ka band the same device produced a power density of 5.7 W/mm, with a PAE of 45% at Vds = 20 V - better than the best reported GaAs PHEMTs.

These GaN devices have other advantages too, including high-temperature operation, and they do not require as much off-chip circuit protection as GaAs transistors. Using GaN instead, those protective circuits could be eliminated for weight and cost savings. In addition, the ability of the GaN transistors to handle higher voltages may allow the systems to use fewer transistors in total.

Moon also described efforts to optimize AlGaN/GaN-based HFETs. Conventional T-gated HFETs operating at high frequencies showed a power output much lower than theoretically possible, which could be attributed to high field-induced trapping under high voltages and high RF power operation. The HRL team has also produced AlGaN/GaN HFETs optimized in a deep submicron field-plated and gate-recessed layout in order to operate the devices at higher frequencies than the X-band (i.e. in the K, Ka, Q and V bands) with high power and high efficiency.

There is some precedent for this design, with recessed gates and field plating employed to increase power performance at low GHz frequencies. The HRL group therefore made an AlGaN/GaN HFET on a SiC substrate with a recessed gate (figure 1). The gate foot dimension ranges from 0.23 to 0.14 μm, with a recessed depth of 10 nm. The gate-recessed and field-plated devices showed a source-drain saturation current density (Idss) of 0.7 A/mm with a pinch-off voltage of -2 V. The measured extrinsic transconductance was as high as 600 mS/mm - comparable to that of GaAs PHEMTs. The output power density and PAE of gate-recessed and field-plated AlGaN/GaN HFETs were almost twice those of baseline planar AlGaN/GaN HFETs.

In the meantime, GaN bipolar transistors are being developed through a collaboration between Photronix of Waltham, MA; Solid State Scientific in Hollis, NH; Boston University; and the Air Force Research Laboratory. The team is researching GaN n-p-n transistors for use in X-band radar transceivers (figure 2).

Although AlGaN/GaN FETs look promising, bipolar transistors offer several inherent advantages over FETs, explained William F Stacey from Photronix. Bipolar transistors are normally "off" devices that ought to provide more uniform threshold voltages, higher linearity and higher current densities than FETs. This is particularly attractive for applications that need ultrawide bandwidth, high linearity and high power.

At the Fall MRS Meeting, Stacey described the team s progress. It has received SBIR Phase 2 funding to develop the GaN bipolar transistor but is yet to report success. The group has, however, made two back-to-back p-n diodes on an AlN substrate, plus a GaN homojunction transistor on a SiC substrate.

These diodes were made by growing multilayer GaN films onto 1 cm2 AlN templates. The AlN layer sits atop a sapphire substrate, but Solid State Scientific, which supplied these materials, plans to provide AlN detached from the substrate. This unconventional AlN substrate is more compatible with GaN than either sapphire or SiC in a number of ways: the crystal structure, lattice size, thermal expansion, thermal conductivity and chemical properties are well matched. If large, uniform AlN substrates can be made cost-effectively, they could provide a substrate for high-quality GaN epitaxy (see Native substrates spar with the established technology).

At Boston University, researchers have optimized MBE growth to produce diodes that have the qualities that the transistor will need. Mesas were etched using inductively coupled plasma, and novel wafer processing techniques provided good uniformity as well as control of the etch depth.

The resulting diodes showed stable device characteristics over 20-475 °C, and two promising attributes in particular: a long-term stability at high current densities (in contrast with the behavior of SiC bipolar devices) and a p layer resistivity that decreased with increasing temperature (figure 3).

Good conductivity in the p layer is important, and difficult to achieve, says Phil Lamarre, president of Photronix. The resistivity characteristics mean that the diode (and eventually the transistor) becomes more efficient at elevated temperatures. "You want to run it hot," Stacey explained.

The latter may be important, because the majority of the waste heat in a transceiver module is generated from the power-output stage. As a result, many devices in use now require large cooling systems. If the n-p-n transistor retains this resistivity drop, then system designers may be able to eliminate the cooling systems, thus making the transceivers smaller and perhaps less expensive.

When it comes to military applications, the benefits of GaN transistors are clear, and this is driving their development. If manufacturing problems can be overcome, both X-band radar and MMW links featuring the technology look likely to emerge.