Gauging the State of GaN Power Amplification

Improvements in GaN power semiconductor technology and modular design are making possible high power continuous wave (CW) and pulsed amplifiers at microwave frequencies.

Gallium nitride (GaN) power semiconductor technology has contributed  a great deal to improved performance levels in RF/microwave power amplification. By reducing devices' parasitic elements, using shorter gate lengths, and using higher operating voltages, GaN transistors have reached higher output power densities, wider bandwidths, and improved dc-to-RF efficiencies. For example, by 2014, GaN-based X-band amplifiers capable of 8 kW pulsed output power were demonstrated for radar systems applications as replacements for traveling wave tube (TWT) devices and TWT amplifiers. By 2016, 32 kW variants of these solid-state GaN power amplifiers are expected. In anticipation of the availability of those amplifiers, some of the key characteristics and features of these high power GaN amplifiers will be reviewed. 

In the recent past, GaN has been the technology of choice for counter radio frequency electronic warfare (CREW) applications with tens of thousands of amplifiers delivered for field use. The technology is now being deployed in the airborne EW arena as well, with amplifiers under development capable of delivering hundreds of watts of output power across multiple octaves in the RF/microwave range. Several variants of these wideband EW power amplifiers are due for release in the current calendar year.

Areas for further research include improvements in linearity for high peak to average power ratio (PAPR) waveforms employed in many military communications systems, including common data link (CDL), wideband networking waveform (WNW), soldier radio waveform (SRW), and wideband satellite communications (satcom) applications. The “Bits to RF”  initiative at Analog Devices will integrate the company’s strengths in baseband signal processing and GaN power amplifier (PA) technologies. This integration will enable advances in PA linearity and efficiency through the use of such techniques as predistortion and envelope modulation.

In the last few years, GaN-based devices, both discrete field effect transistors (FETs) and monolithic microwave integrated circuits (MMICs), have been released and widely used in high power microwave amplifier systems. These devices, available from several foundry sources and device manufacturers, are typically fabricated on 100 mm silicon carbide (SiC) wafers. GaN on Si processes are also under consideration; however, the relatively poor thermal and electrical conductivities of Si offset the cost advantages in high performance, high reliability applications. These devices feature gate lengths as small as 0.2 μm and support operation into millimeter wave frequency bands. GaN-based devices have largely replaced gallium arsenide (GaAs) and Si laterally diffused metal oxide semiconductor (LDMOS) devices in many high frequency applications and all but the most cost-sensitive, lower frequency applications.

GaN devices are of interest to the RF power amplifier designer because they support very high operating voltages (three to five times higher than those of GaAs), and roughly double the allowable current per unit FET gate width compared to GaAs devices. These characteristics have an important consequence to PA designers, specifically a higher load impedance for a given output power level. Previous GaAs- or LDMOS-based designs often had extremely low output impedances relative to typical system impedances of 50 Ω or 75 Ω. Low device impedances place limits on achievable bandwidth, that is, as the required impedance transformation ratio between the amplifying device and its load increases, the number of components and insertion loss increase as well. Due to these high impedances, early users of these devices were in some cases able to achieve partial results by merely installing one in an unmatched test fixture, applying dc bias, and driving the device with an RF/microwave test signal.

GaN devices are also finding their way into high reliability space  applications due to these operating characteristics and their exceptional reliability. Life test data from several sources of these devices predict mean time to failure (MTTF) durations (for single devices) in excess of one million hours at junction temperatures of 225°C or higher. This exceptional reliability is mainly due to the high band gap value for GaN (3.4 for GaN vs. 1.4 for GaAs). This makes them more than suitable for high reliability applications.

The chief impediment to wider use of GaN in high power applications has been its relatively high fabrication costs—typically two to three times that of GaAs and five to seven times that of Si LDMOS-based devices. This has usually been a barrier to use in cost-sensitive applications such as wireless infrastructure and consumer handsets. GaN on Si substrate processes are available, albeit with the performance issues noted earlier, and devices from these processes are probably best suited for these cost-sensitive applications. In the near future, cost reductions, perhaps on the order of 50%, are expected as GaN device fabrication moves to larger wafers, with diameters of 150 mm and larger, now underway at several leading foundry sources for GaN devices.

Currently deployed radar systems for weather prediction and target acquisition/identification rely on TWT-based power amplifiers operating at C-band and X-band frequencies. The amplifiers run at high supply voltages (10 kV to 100 kV) and temperatures and are susceptible to damage from excessive shock and vibration. The reliability in the field for these tube-based amplifiers is typically 1200 h to 1500 h, which leads to high costs for maintenance and for spare parts.

As an alternative to these high power TWT-based amplifiers, Analog Devices developed an 8 kW, solid-state, X-band power amplifier based on GaN technology. The design uses an innovative, layered combiner approach to sum the contributed RF/microwave output power of 256 MMICs, each developing approximately 35 W output power. The combining methodology results in graceful performance degradation in the event of individual MMIC failures. This is in contrast to TWT amplifiers, which tend to suffer failures that are catastrophic in nature due to low levels of redundancy. For these solid-state GaN power amplifiers, the RF/ microwave combining architecture must present a reasonable balance between the isolation required between MMICs and the RF/microwave insertion loss of the overall network.

The 8 kW amplifier topology is modular, comprised of four 2 kW  amplifier assemblies with their output power combined using wavguide  structures (Figure 1). The amplifier can be mounted in a standard  19" rack enclosure. The current design of the amplifier (Figure 2) is configured for use with water cooling, although variants of the amplifier using air cooling are under development. Table 1 provides a summary of the performance levels for the water cooled 8 kW GaN PA.

The 8 kW SSPAs as designed in such a way that these modular SSPAs can be combined to produce even higher power levels. Work is under way to develop an amplifier that will combine three of these 8 kW SSPA modules to achieve aunit with a peak output power level of 24 kW across the same frequency range. Other configurations achieving power levels to 32 kW are feasible and are under consideration for further evaluation.

Analog Devices is currently developing an advanced power module, also based on GaN technology, that will double the RF/microwave output power of current modules. The module is designed to be hermetically sealed to support operation in extreme environments. This, along with next-generation combining structures with reduced insertion loss  (compared to current approaches), will extend pulsed output power to levels approaching 75 kW to 100 kW at RF/microwave frequencies. These advanced, high power SSPAs will include control and processor functionality to enable fault monitoring, built in test (BIT) functionality, remote diagnostic testing, and control of fast, real-time bias control circuits for the MMIC devices that power the amplifiers.

These GaN-based solid-state power amplifiers address the industry’s need for amplifiers with wide instantaneous bandwidths and high output power levels. Some systems attempt to meet these requirements using channelized or multiple amplifiers, each covering a portion of the required spectrum and feeding a multiplexer. This leads to increased cost and complexity and results in coverage gaps at the frequency crossover points of the multiplexer. A more effective alternative solution is  continuous coverage of wide frequency ranges at elevated power levels, as has been accomplished with two different GaN-based amplifiers covering VHF through L-band frequencies, as well as 2 GHz to 18 GHz.

For use from VHF through S-band frequencies, Analog Devices has  developed a very small, feature rich, multiple octave amplifier capable  of 50 W output power from 115 MHz to 2000 MHz. The amplifier achieves output power levels of 46 dBm (typically 40 W) across that full frequency range when fed with a nominal input signal of 0 dBm.

Packed in a compact housing with dimensions of 7.3" × 3.6" × 1.4," the amplifier includes BIT functionality for thermal and current overload protection, telemetry reporting, and an integrated dc-to-dc converter for uncompromised RF performance with input supplies ranging from 26 V_DC to 30 V_DC. Figure 3 provides a photograph of the amplifier, with typical measured performance data for output power vs. frequency presented in Figure 4.

To address wideband applications above 2 GHz, Analog Devices has also developed a GaN amplifier that produces 50 W continuous wave (CW) output power across the entire 2 GHz to 18 GHz band. This amplifier uses commercially available 10 W GaN MMICs with output power contributions summed by means of a wideband, low loss combiner circuit. Multiple amplifiers may in turn be combined to develop output powers as high as 200 W across this same 2 GHz to 18 GHz bandwidth. The driver amplifier chain is also based on GaN active devices. The amplifier operates from 48 VDC and features an internal voltage regulator and high speed switching circuits to enable pulsed operation with good pulse fidelity and fast rise and fall times. Table 2 lists specifications for this amplifier. Figure 5 offers a photograph of the amplifier, while Figure 6 presents the amplifier’s output power as a function of frequency from 2 GHz to 18 GHz.

This 50 W amplifier is one of a family of amplifiers covering the 2 GHz to 18 GHz band. ADI has also developed a compact, benchtop amplifier capable of 12 W output power (Figure 7) and a rack-mount unit developing 100 W output power (Figure 8). Other amplifiers, with frequency coverage from 2 GHz to 6 GHz and 6 GHz to 18 GHz, are under development. ADI is also working to increase the output power of these broadband amplifiers from their present levels to power levels of 200 W and higher. To achieve these higher output power levels, the company is developing modules with increased output power, as well as broadband RF power combiners with greatly improved combining efficiency and less loss than current power combiners.

These are a few examples of the performance levels possible with GaN-based, solid-state amplifiers. The unit costs of these amplifiers is expected to decrease in the future as more GaN semiconductor vendors move to larger wafer sizes and continue to improve their yields of devices per wafer. Systems operating at millimeter wave frequencies will see more use of GaN devices as gate lengths are reduced, enabling higher frequency operation for GaN-based SSPAs. It is clear that current GaN trends of improving performance and reducing costs should continue for some time.

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