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Gallium Nitride (Gan) Microwave Transistor Technology for Radar Applications

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Gallium Nitride (Gan) Microwave Transistor Technology for Radar Applications Gallium Nitride (GaN) Microwave Transistor Technology For Radar Applications Aethercomm® 2910 Norman Strasse Rd., Ste. 105 San Marcos CA 92069 [email protected] Abstract: This paper reviews the relative merits of Si, GaAs, SiC, and II. GaN IS THE FUTURE GaN materials and describes how the attributes of each impact the The development of the wide bandgap semiconductor operation of microwave transistors for the generation of high RF materials, such as GaN and GaN-based alloys, offers the output power, on the order of hundreds to thousands of watts, as ability to fabricate RF active devices, specifically necessary for radars systems. It is shown that the superior physical AlGaN/GaN power High Electron Mobility Transistors attributes of GaN lead to microwave transistors that are extremely well suited for high power applications. The superior properties of (HEMTs), with significantly improved output power GaN combined with modern high efficiency biasing techniques performance [1]. The improved RF output power is made make GaN technology a prime candidate for use in transmitters for possible due to the much improved material properties: high radar systems. electric breakdown field, high saturated electron drift velocity, and when epitaxially grown on semi-insulating SiC I. INTRODUCTION substrates, improved thermal conductivity. The data in Table Many microwave radar transmitters require active 1 [2] allows a comparison of the most important performance devices which can produce RF output power in the order of metrics of silicon (Si), gallium arsenide (GaAs), silicon kilowatts to even megawatts. Routinely microwave traveling- carbide (SiC), and gallium nitride (GaN). The larger thermal wave tube devices are utilized for this application. However, conductivity of SiC and GaN enables lower temperature rise the currently used traveling-wave tubes are inefficient, large, due to self heating. The five to six times’ higher breakdown expensive, and have suspect reliability. While field of SiC and GaN is what gives those materials the semiconductor-based amplifiers in principle can offer a more advantage over Si and GaAs for RF power devices[2]. SiC is effective solution, up to recently semiconductor transistors a wide bandgap material (3.26eV), but suffers from poor have been limited in the DC voltage that can be applied to electron transport properties, which hinders its use in very the device by the inherent critical breakdown field that the high frequency amplifiers. SiC has also been limited by material can sustain. Since limited DC voltage can be expensive, small and low-quality substrate wafers. applied, high RF power operation requires large DC and RF current, which in turn requires large area devices[1]. High Table 1 current operation is inefficient due to series losses and due to Si, GaAs, SiC, and GaN Material Properties the fact that large area devices have inherently high Properties Si GaAs 4H- GaN capacitance and very low impedance which limit operating SiC frequency and bandwidth[1]. GaN technology now offers a Bandgap (eV) 1.11 1.43 3.26 3.42 solution to this dilemma. Relative Dielectric 11.8 12.8 9.7 9.0 Solid-state amplifiers are already replacing traveling- Constant wave-tube amplifiers (TWTAs) for a variety of microwave Breakdown Field 2.5e5 3.5e5 35e5 35e5 power applications. However, the low operating voltages of (V/cm) Si and GaAs devices lead to a large device periphery, Saturated Velocity 1.0e7 1.0e7 2.0e7 1.5e7 resulting in high device and circuit complexity and reducing (cm/sec) production yield and reliability. Wide bandgap technologies Electron Mobility 1350 6000 800 1000 like GaN can achieve power density five times higher than (cm²/V-sec) that of conventional GaAs-based Metal-Semiconductor Field Hole Mobility 450 330 120 300 Effect Transistor (MESFETs) and Heterojunction Bipolar (cm²/V-sec) Transistors (HBTs). This will ultimately result in reduced Thermal 1.5 0.46 4.9 1.7 circuit complexity, improved gain, efficiency, and higher Conductivity reliability. In particular, radar systems will benefit from the (W/cm-°K) development of this technology. Even though the mobility of the carriers is significantly tubes is shown in Figure 2 [1]. Historically, tube amplifiers, better for GaAs devices, the higher peak and saturation such as grid controlled tubes, magnetrons, klystrons, velocity for GaN HEMTs compensates for the relatively traveling-wave tubes (TWTs), and crossed field amplifiers lower mobility, enabling high frequency performance. These (CFAs) have been used as power amplifiers for radar advantages for GaN, with the high linearity and low noise of transmitters. These amplifiers generate high power, but the HEMT architecture, open the application of these devices usually operate with low duty cycle. The klystron amplifier for high power Radar amplifiers. offers higher power than the magnetron at microwave An additional advantage of GaN HEMTs stems from frequencies, and also allows the use of more complex the large conduction band energy offset between the GaN waveforms. The TWT is similar to the klystron, but with channel and the AlGaN barrier layer. This allows a wider bandwidth. CFAs are characterized by wide significant increase in the channel carrier density in GaN- bandwidth, modest gain, and compactness. Solid State Power based HEMTs with respect to other materials (up to 1013cm-2 Amplifiers (SSPAs) support long pulses and high duty cycle and above). This, along with higher voltage handling, waveforms. Individual SSPA elements can be combined to increases power density. Power Density is a very important produce sufficient amplification, despite the fact that metric for high power devices because a higher power individual SSPA elements have low power amplification. density yields a smaller die size and more easily realized Silicon bipolar transistors, and gallium arsenide MESFETs, input and output matches. The rapid progress in saturated and GaAs PHEMTs are some of the solid state elements used CW RF power densities versus time for GaN FETs at X band in SSPAs. GaN HEMTs however, can be combined to create is shown in Figure 1. a SSPA with a significant improvement in mean output power resulting in an improvement of the radar detection X-band power density range. 30 Field Plates 25 State of the Art (Single Device) 10000000 Klystron 20 SiN Gridded Tube Passivation; 1000000 Helix TWT CFA 15 Advanced Epi 100000 CCTWT SiC GaAs substrate 10000 Si BJT 10 PHEMT 1000 GaN sapphire 5 SiC (W) Power Average 100 Power Density (W/mm)Power Density GaAs 10 0 1 1/96 1/98 1/00 1/02 1/04 0.1 1.0 10.0 100.0 1000.0 date Frequency (GHz) Figure 1. Power density for GaN FETs at X band as function of year. Best values for GaAs and SiC Figure 2. Present Day State of the Art Microwave Tubes FETs are also shown. and Solid State Devices The high operating voltages and high power densities As shown, solid state transistors produce RF power that are possible with the wide bandgap RF devices offer a levels less than about 100 watts at S-band frequencies and number of advantages for power amplifier design, below, and their output continuously decreases with manufacture and assembly in comparison to Si LDMOS and increasing frequency[1]. The RF output power capability of GaAs MESFET technologies. GaN HEMT technology offers GaAs FETs approaches 50 watts at S-band and at Ka band is high power per unit channel width that translates into smaller limited to about one watt[1]. The GaAs FETs are limited in less expensive devices for the same output power, which are RF output power capability primarily due to the low drain not only easier to fabricate but also increase the impedance bias breakdown voltage[1]. Semiconductor devices of the devices. This enables lower complexity and lower cost fabricated from wide bandgap material, such as GaN, offer impedance matching in amplifiers. The high voltage significantly improved RF performance. operation possible with GaN helps in eliminating the need for voltage converters, further reducing system cost. Over time, various Figures Of Merit (FOM), for evaluating semiconductors for high power and high III. THE PATH IS CLEAR frequency potential have emerged. These FOM for high The present day state-of-the-art output power vs performance RF semiconductors attempt to combine the frequency for microwave solid state devices and microwave most relevant material properties into a number that provides an estimation of the comparative strengths of the associated Furthermore, wide bandgap, broadband high efficiency materials. The Johnson FOM (JFOM=ECR vsat/π ) takes into microwave devices will enable enhanced system capability. account the breakdown electric field ECR and the saturated electron velocity vsat and corresponds approximately to the The low parasitic capacitance and high breakdown product of device cutoff frequency ft times the corresponding voltage of GaN HEMTs makes them ideal for realizing the breakdown voltage. As can be observed from Figure 3[3], the Class E and Class F high efficiency amplifier modes. Both Johnson FOM for GaN is larger by at least x15 than that for modes have theoretical efficiencies of 100%. Recently, GaAs. several GaN transistor vendors have implemented Class E amplifiers in hybrid form. Typical results are ten watts 10000 output power at L-band with efficiencies from 80% to 90%. GaAs MESFET Aethercomm recently delivered a Class F high (Johnson's FOM) 1000 efficiency amplifier module operating at L-band to a major GaN HEMT (Johnson's FOM) defense contractor. The desired output power was to exceed 100 50W with an efficiency of 60% for the entire amplifier. Due to the tight delivery schedule, it was necessary to use off the shelf packaged transistors rather than developing a custom Breakdown Voltage (V) Voltage Breakdown 10 hybrid solution. 1 The power amplifier final stage was implemented 1.0 10.0 100.0 1000.
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