Microwave transistor technology based on gallium nitride (GaN) for Radar Applications

The following article examines the characteristics of the main materials currently used for manufacturing microwave transistors such as silicon (Si), gallium arsenide (GaAs), Silicon Carbide (SiC) and gallium nitride (GaN) and describes and condition the transistor operation when required high output powers, the order of hundreds and thousands of watts, usually necessary in radar applications.
Is displayed as microwave transistors made of GaN are suitable for high power applications due to superior physical and chemical properties of these semiconductors. If one adds the modern techniques of high-efficiency polarization, the transistors fabricated with gallium nitride technology emerge as suitable candidates for use in systems transmitters Radar

The vast majority of radar transmitters require active devices that can generate an RF output power of the order of kilowatts and even in the megawatt. Typically these applications are used for devices based on wave tubes. However, these devices are bulky, expensive and can have reliability problems. Although semiconductor-based amplifiers have a priori more efficient, have so far been limited by the voltage could be applied to the device because of the critical breakdown field inherent to these materials, which makes a current that is required very high and also a larger. Working with a high operating current efficiency decreases because of losses and the fact that larger devices have a high capacitance and low impedance thereby limiting the operating frequency and bandwidth [1]. GaN technology is now able to offer a solution to this problem.
Solid state amplifiers are now replacing the traveling wave tube (TWTA, Traveling Wave Tube Amplifiers) in some applications of high power microwave. However, lower operating voltages make the associated circuitry is very large which implies a more complex device while reducing the production yield and reliability. The technologies of wide bandgap semiconductors (WBG, Wide Band Gap) as the GaN can achieve power densities five times greater than those of conventional transistors, GaAs field effect so as heterojunction bipolar. The final advantage is the reduction of circuit complexity, higher gain and efficiency, and greater reliability. In particular, radar systems will benefit from the development of this technology.

The GaN is the future
The development of wide bandgap semiconductors such as GaN or GaN-based alloys, offers the possibility of manufacturing of RF active devices, especially power transistors HEMT (High Electron Mobility Transistor) with a significantly higher output power. This improvement in RF output power is due to the special properties of this material, among other highlights: high breakdown field, high saturation value of the EDV (Drift velocity of electrons) and when using substrates SiC, higher thermal conductivity. The data shown in Table 1 [2] allow comparison of materials Si, GaAs, SiC and GaN. The higher thermal conductivity of SiC and GaN reduces the increase in junction temperature due to heating. The breakdown field of five to six times the SiC and GaN gives an advantage to these materials compared to Si and GaAs for RF power devices [2]. SiC is a wide bandgap material (3.2eV) but has a low electron mobility, which hinders their use in high frequency amplifiers. SiC is also limited because the material wafers are expensive, small and of poor quality.
Although the mobility of carriers is significantly greater in GaAs devices, high peak velocity and EDV saturation of GaN HEMT compensates for its lower relative mobility allowing its use at high frequencies. These advantages of GaN coupled with high linearity and low noise HEMT architectures open the doors to these devices for use in the manufacture of high power radar amplifiers.
An additional advantage of the GaN HEMT is in the large energy offset between the conduction band of GaN and AlGaN barrier layer. This allows a significant increase in the density of carriers in the channel in the GaN-based HEMT with respect to other materials (up to 1013cm-2 and more). If we add the possibility to use a higher voltage we get an increase in power density. Power density is an important parameter for high power devices because the greater the smaller the die size and simpler are adjustments of input and output. Figure 1 shows the rapid progress of RF power density versus time for a FET (Field-Effect Transistor) X-Band GaN
The high operating voltages and high power densities are achieved with RF devices from wide bandgap offer many advantages in the design, manufacture and assembly of power amplifiers compared with the technologies of LDMOS (Lateral Double-MOS difussa ) or that of Silicon
MESFET (Metal Epitaxial Semicon-ductor Field Effect Transistor) GaAs. GaN HEMT technology offers high power per unit channel width, which translates into cheaper devices and smaller for the same power output, this not only makes them easier to manufacture but will increase the impedance devices. The high voltage operation is achieved with GaN technology eliminates the need for voltage converters and therefore also reduces the final cost of the system.


The path is clear
Figure 2 [2] shows a graph of output power versus frequency for solid state devices and microwave tubes that constitute the current state of the art.
Historically, the tube amplifiers, such as those controlled by screens, magnetrons, kystrones, traveling wave tubes and crossed-field amplifier (CFA, Cross Field Amplifier) have been used as power amplifiers in radar transmitters. These amplifiers generate high power but usually work with duty cycles (duty cicle) low. Klystron amplifiers offer more power magnetrons at microwave frequencies and also allow the use of more complex waveforms. Traveling wave tubes are similar to the klystron, but with higher bandwidth. The CFA is characterized by high bandwidth, low profit and be compact.
The power amplifiers (solid state SSPA Solid State Power Amplifier) support long pulses and waveforms with high activity cycles. Although the elements used in the SSPA individually have little power amplification can be combined to achieve it. Silicon bipolar transistors, the Gallium Arsenide MESFET and PHEMT (Pseudomorphic HEMT) of gallium arsenide are some of the elements used in the SSPA. The GaN HEMT can be combined to create an SSPA with an average output power greater and therefore greater Radar detection range.
As seen in Figure 2, solid state transistors produce RF power levels below 200 watts S-band and its output decreases as we increase the frequency [1]. The output power of RF GaAs FETs are approaching 50 watt S-band and about 1 watt in Ka1 band. GaAs FETs have a power output limited mainly by the low breakdown voltage of drenador1. Semiconductor devices manufactured with higher band gap materials such as GaN, whose performance significantly better.
With the passage of time has been appearing different figures of merit to assess these different semiconductors with potential for use in applications requiring high power at high frequency. Using these figures of merit is to bring together the most relevant properties of materials in a qualitative value. Thus the figure of merit of Johnson (ECR JFOM = vsat / p) takes into account the field of ECR breakdown and saturation of the EDV Vsat. As shown in Figure 3 [3], the Johnson figure of merit for GaN is at least 15 times that of GaAs.
Aethercomm believes that if the trend of growth of GaN is maintained at current rates, the expected behavior for GAN HEMT in 2010 will be the one shown in Figure 4. The GaN will soon surpass all competitors.

Efficiency is the key
The most modern radar systems used in military applications demand new requirements for RF power amplifiers due to the need to reduce the size, weight and cost. The biggest changes in the specification are increasingly focused on improving the efficiency of the amplifier to reduce DC power requirements and improve system reliability through reduced component power dissipation. Microwave devices based on wide bandgap technologies and high efficiency will also increase system performance.
The parasitic capacitance and high breakdown voltage of GaN HEMT them ideal for operation in modes of high-efficiency amplification Class E and Class F Both modes have a theoretical efficiency of 100%. Recently, some manufacturers have implemented GaN transistors hybrid class E amplifiers Typical results obtained are 10 watts of output power L band with efficiencies between 80% and 90%.
Aethercomm has delivered an F-class amplifier module for L-Band The desired output should exceed 50 watts with an efficiency of 60% for the entire amplifier. Due to the tight timescale of the program was necessary to use standard transistors encapsulated instead of developing as a hybrid solution.
The final stage power amplifier was implemented using a balanced pair of GaN HEMT encapsulated working class F. The matching networks including harmonic terminations necessary for operation in class F were designed initially considered an ideal model of the transistor. The following were introduced parasitic inductances and capacitances of the transistor package and modified the matching networks to maintain the required harmonic terminations at the transistor die. Then the amplifier was simulated using a nonlinear transistor model and modified the matching networks to optimize efficiency and power.
Built a prototype configuration for single-ended output stage of class F. There was a drain efficiency of 75%, an output power of 40 watts and a gain of 16 dB is a minimum. The results were very similar to those obtained in the simulation. There was available GaN devices suitable for low-power driver stage was designed using a three-stage GaAs MESFET working in class A. Initially it was believed that the stages of the driver should have worked in a highly efficient manner and attain PAE (Power Added Efficiency) required, but the analysis indicated that with proper sizing of transistors in class A operation was permissible . The driver had a gain of 40 dB and a power consumption of 10 watts.
The final configuration of the power amplifier had a peak PAE of 63% and an output power of 75 watts. The amplifier had an output power of 65 watts and 61% of EAP P2dB. Table 2 shows the characteristics of the amplifier for different output power values. Because the final stage of class F is polarized on the threshold, no drain current, the amplifier offers a wide operating range for low powers. The amplifier gain reaches a peak and then begins to shrink when it reaches peak output. Table 2 shows the efficiency of this design for different output powers.
Aethercomm has also developed a GaN HEMT device 200 watts on SiC substrate designed to maximize the PAE and maintain high power output to an operating frequency of 1215 MHz to 1390 MHz were observed efficiencies greater than 56% while maintaining output power levels in excess of 205 watts P3dB.
Many SSPA for radar applications are designed with RF semiconductor devices configured to work in class C. This form of polarization provides a very efficient operation for a period of a single transistor, however, the class C transistor has a gain so low, typically 6 dB, the advantage gained in efficiency is lost when required many additional steps gain to achieve the desired power output.


Conclusion
Future radar systems such as those based radar active phased array SSPA increasingly require increasingly efficient and small. The desire to achieve extremely fast scans, greater detection ranges, the ability to locate and monitor a large number of targets, a low probability of being intercepted and the ability to act as an inhibitor transistor technology require innovative and cost effective. Recent developments in the field of GaN HEMT design have made possible highly efficient amplifiers at microwave frequencies. GaN HEMT devices provide high peak current with low output capacitance and a breakdown voltage and an extremely high power density. This unique combination of features allows designers to get amplifiers with a much higher overall benefits to those achieved with devices based on alternative technologies that exist today.

References

[1] R.J. Trew, "Wide Bandgap Transistor Amplifiers for Improved Performance Power Microwave and Radar Applications," 15th.
International Conference on Microwaves, Radar and Wireless Communications, 2004 MIKON 2004, Volume 1, 17-19 May 2004 Page (s): 18-23 Vol.1.
[2] C.E. Weitzel, "RF Power Devices for Wireless Communications," IEEE MTT-s Digest, Volume 1, 2-7 June 2002 Page (s): 285 - 288.
[3] T. Kikkawa, K. Imanishi, M. Kanamura, and K. Joshin, "Recent Progress of Highly Reliable GaN-HEMT for Mass Production," CS ManTech Conference, April 24-27, 2006 Page (s) :171-174.

Author:

Altaix Technical Department of Electronics, S.A.L.