Silicon RF and microwave power transistors have been supporting long-range surveillance radar systems since the early 1970s, starting at UHF with peak powers of 100 W at ~50 μs, 20 percent duty for the first UHF radar and L-band with peak powers of about 50 W at 100 μs, 10 percent duty and 20 W at 1 millisec, 20 percent duty cycle. The highest performance silicon products available today provide (see also Figure 1):
- UHF (406 to 450 MHz) 1 kW in a push pull package 300 μs, 10 percent duty cycle
- L-band (1200 to 1400 MHz) 370 W at 300 μs, 10 percent, 150 W at 2 ms, 10 percent duty cycle
- S-band (2700 to 3100 MHz) 120 W at 200 μs, 10 percent duty cycle
The historic silicon Class C amplifiers are limited by heat dissipation due to the potential for thermal runaway during long pulses (> 300 μs) and high peak powers. Silicon RF transistors—whether BJT, VDMOS or LDMOS—are not able to offer the combination of increased peak power performance and extended pulse (> 300 μs) characteristics for the next generation of radar systems.
Figure 1 Si Transistor peak power performance at various frequency bands (10% duty cycle).
Next Generation Systems Seek Operational Improvements
Long-range radar systems operating from VHF thru S-band are demanding increased performance from the RF power amplifier in order to support the improved ranging and sensitivity needed by today’s defense and air transport customers. A key lesson learned from the crisis on 9/11 was the need to extend the pulse width to at least 300 μs for long-range radar as well as the ability to increase the operating dynamic range of the power amplifier. The capability of model 0150SC-1250M, SiC SIT peak power performance operating at the pulse width of 300 μs, 10 percent duty cycle, is shown in Figure 2, direct from the Boonton Model 4500B peak power meter. The pulse droop is approximately 0.2 dB at power out of 1250 W peak.
Figure 2 Curve trace showing power droop of <0.2 dB (a) and output power of 1250 W (b) for SiC transistor.
The transistor design must meet the long-term reliability called for by the final customer. Since long-range radar systems have an expected life in excess of 30 years, equipment manufacturers need products designed and built with a high level of long-term reliability, as well as based on technologies that will be available to support the expected operating lifetime of the system. Microsemi Corp. offers a broad line of semiconductors used in legacy systems, including systems that have been in the field for more than 35 years. The use of all gold metallization and hermetic packages provides the transistor long-term reliability and the corporate commitment to the radar market ensures the source of supply for the long term. Figure 3 shows the packaged transistor and test fixture.
Figure 3 Packaged SiC transistor (a) and packaged SiC transistor in test fixture (b).
Microsemi Invests in Radar Power Amplifier Future
Microsemi’s Power Products Group Microwave RF Power Division is a supplier of RF and microwave power semiconductors for radar systems. It understands the operational requirements for solid state radar power amplifiers and has recognized the need for moving to wide band gap (WBG) semiconductor technology in support of the new radar systems. Microsemi initiated a program to bring SiC technology from R&D into the factory-establishing products that will support radar systems from VHF thru S-band. Higher frequency products will be designed and produced as WBG technology advances. At this time the WBG silicon carbide (SiC) material technology has progressed to the point were Microsemi can build high power devices with reasonable yields and consistent performance. The current focus is on developing an initial line of product to support new designs from VHF through S-band frequencies utilizing the proven SIT and MESFET technologies. Figure 4 shows the SiC transistor performance at various frequency bands compared to Si transistors from Figure 1.
Figure 4 SiC transistor peak power performance at various frequency bands (10% duty cycle).
WBG Material Features
WBG semiconductors offer three key operating characteristics for high pulsed power:
- Higher Band Gap Energy (~ 3x that of silicon): The ability to operate at higher junction temperatures, and therefore longer life than silicon for reduced field costs.
- Higher Critical Field (~ 10x that of silicon): Allows for high voltage operation to increase the power output from the same current.
- Increased Thermal Conductivity (~ 3x of silicon): Provides higher power density for higher peak power in the same-sized package as with silicon.
Figure 5 reviews the advantages of SiC over Si for transistor applications.
Figure 5 Advantages of SiC for pulsed RF power applications.
Extreme Electrical Ruggedness
As the SiC device functions much as a junction FET, the device has negative temperature characteristics—the device power output will decrease with increasing junction temperature. It will not go into thermal runaway, much like a silicon BJT or LDMOS device. Microsemi has tested and verified that these transistors will withstand a full 10:1 load mismatch under full specified operating conditions.
High Voltage Operation Simplifies Circuitry
The operation at a drain bias of 125 V reduces the complexity of the power supply and the peak current as well as offering higher terminal impedance, thereby reducing the complexity of the RF circuitry and making the device easier to match over extended bandwidths.
The first SiC products released include the VHF model 0150SC-1250M and UHF model 0405SC-1000M. These products are supplied in a commercially available hermetically sealed package. See Table 1 for key specifications and Figure 6 for power output and gain curves.
Table 1 Key Specifications
Figure 6 Gain and output power vs. input power (a) and efficiency vs. input power (b) for 0150SC-1250M (300 μs 10% duty, 125 V).
Microsemi has designed and characterized these SiC transistors using circuit materials and design criteria supporting next generation radar power amplifier equipment. The characterization of the product includes operational tests at various drive levels to verify the performance over the operational input power window and across the frequency band. Microsemi is developing SiC products to the full spectrum of next generation VHF thru S-band radar systems.
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