High-power RF and microwave signals are often associated with electron tubes, but solid-state devices have been gaining ground on their vacuum counterparts in recent years. With the maturation of gallium-arsenide (GaAs) power transistors, and as device designers continue to explore the capabilities of gallium nitride (GaN) and silicon-carbide (SiC) power-transistor technologies, solid-state devices offer noteworthy output-power levels.
The number of technologies supporting RF/microwave transistor developments has grown steadily over the years. Silicon bipolar transistors once handled the bulk of the solid-state amplification at radio frequencies. But over the last 30 years, GaAs field-effect transistors (FETs) have provided amplifier designers with higher-frequency performance while eventually matching the output-power levels of lower-frequency silicon bipolar transistors. In recent years, bipolar junction transistors based on SiC have established new marks for output power at lower microwave frequencies, while FETs and high-electron-mobility transistors (HEMTs) based on GaN have continued to boost solid-state power levels at higher microwave frequencies.
This transistor technological diversity is probably greatest for high-power applications around or below 1 GHz, such as in radio broadcast systems and in pulsed ultra-high-frequency (UHF) and very-high-frequency (VHF) radars. At these frequencies, silicon is still king, and a number of different transistor configurations have provided reliable results for many years. These include silicon bipolar transistors, silicon metal-oxide-semiconductor FETs (MOSFETs), and silicon lateral-diffused-MOS (LDMOS) FETs.
For example, model IB1011S1500 is a silicon bipolar power transistor from Integra Technologies designed for L-band radars at 1030 and 1090 MHz. The firm offers a range of high-power devices for air-traffic-control (ATC) and avionics applications, based on different device technologies that include silicon LDMOS and GaN HEMT technologies. The IB1011S1500 is designed for pulsed applications, and when fed with a 150-W, 10-µs, 1%-duty-cycle signal at 1030 MHz, delivers 1432 W peak output power with better than 48% drain efficiency. The company also offers a more broadband model IB0912M600 bipolar transistor, designed for L-band TACAN systems from 960 to 1215 MHz. When supplied with a 90-W pulsed input signal, it can generate 845 W peak output power and more than 56% efficiency at 960 MHz. Both transistors are housed in beryllium-oxide (BeO) packages to aid thermal management.
Freescale Semiconductor might be most closely identified with silicon LDMOS transistors, especially with that technology’s ubiquitous use in cellular communications infrastructure applications. In addition to a wide range of silicon LDMOS transistors for cellular, ISM band, and commercial L- and S-band pulsed applications, and its extensive lines of integrated circuits (ICs), Freescale also offers RF power GaAs FET transistors through about 6 GHz). As an example of its L-band LDMOS technology, model MRF6VP121KHR6 is an L-band power transistor for applications from 965 to 1215 MHz. It is designed for a supply of +50 VDC (150 mA quiescent current) and can deliver 1000 W peak output power (and 100 W average output power) with a 128-µs pulse at 10% duty cycle. The power transistor achieves 20-dB power gain with 56% drain efficiency, but is only rated to handle a 5.0:1 VSWR maximum load mismatch. It is internally impedance matched to 50O with integrated electrostatic-discharge (ESD) protection and supplied in a flange package.
The company also supplies unmatched lateral N-channel broadband RF power MOSFETs for applications requiring broadband amplification. The model MRFE6VP61K25HSR6 power transistor, for example, is +50-VDC device (100 mA quiescent current) that operates from 1.8 to 600 MHz with 1250 W CW output power at 230 MHz, 24-dB gain, and 74% drain efficiency. It can also deliver 1250 W peak output power at 230 MHz with a 100-µs pulse at 20% duty cycle, 22.9-dB gain and 74.6% drain efficiency. This device has been designed for applications in industrial plasma exciters, broadcast, aerospace, and land mobile communications and can tolerate a load mismatch as severe as 65.0:1 at +50 VDC.
In attempting to provide a more-robust LDMOS transistor, NXP Semiconductors last year announced its XR family of “eXtremely Rugged” LDMOS RF power transistors (see figure). Designed to withstand the severe conditions of applications such as industrial lasers, metal etching, and concrete drilling, these transistors are built to survive VSWR mismatches as severe as 125.0:1, which was the limit of the company’s mismatch test system. One of the targets in developing the devices was to exceed a VSWR of 100.0:1, ruggedness thought to be necessary for many Industrial-Scientific-Medical (ISM) band applications. The first device in the product line is model BLF578XR, with 1400 W pulsed output power from DC to 500 MHz. It delivers 24-dB small-signal gain at 225 MHz with 70% drain efficiency, and can handle a load mismatch of 125.0:1 through all phases at 1200 W output power.
Several years ago, devices based on SiC substrates showed great promise for use in high power continuous-wave (CW) and pulsed RF power applications. Device manufacturers including Cree and Microsemi Corp. developed RF power transistors based on the SiC material, including static induction transistors (SITs) for high-power pulsed radar applications working with UHF and VHF signals. Microsemi, for example, still offers the model 0405SC-2200M Class AB, common gate, depletion mode SIT for use at a drain voltage of +125 VDC. It provides 2200 W peak output power from 406 to 450 MHz with 7.5-dB typical gain when operating with 300-µs pulses at 6% duty cycle. It offers 55% drain efficiency and can handle load mismatches to 10.0:1. Suitable for UHF weather radar and long-range tracking radar, the power RF SiC transistor is supplied in a rugged flange-mount package.
SiC has excellent thermal properties, however, and is being used by many companies in conjunction with devices based on GaN epitaxial material, to form GaN-on-SiC power transistors. M/A-COM Technology Solutions is one of the growing list of suppliers for GaN-on-SiC power transistors. These devices offer unparalleled power densities at higher frequencies from smaller transistor cells, but require thoughtful thermal-management planning. For example, M/A-COM’s model MAGX-000035-150000 is a GaN-on-SiC power transistor that can provide 150 W CW output power from 30 to 3500 MHz with as much as 30 dB gain. The firm’s model MAGX-002735-180000 GaN-on-SiC transistor operates from 2700 to 3500 MHz with 180 W peak output power for a 500-µs pulse at 10% duty cycle. It can deliver 13-dB typical gain over that frequency range. In addition to these discrete transistors, the company also offers “pallet” circuits or amplifiers, such as the model MAPG-002731-330L0S which combines two discrete devices on a printed-circuit board (PCB) with coaxial input and output connectors matched to 50 Ω and bias connections for ease of use. The model MAPG-002731-330L0S pallet operates from 2700 to 3100 MHz with 330 W peak output power when running with a 300-µs pulse at 10% duty cycle; it yields 11-dB gain.
Rather than mounting GaN devices on SiC, Nitronex has developed its SIGANTIC® NRF1 GaN-on-silicon process to support high CW and peak output power levels from its devices. In addition to supporting high-performance levels, the process is economically attractive, since it is production qualified for fabrication of GaN-on-Si devices on standard 4-in. silicon semiconductor wafers. The firm’s model NPT1007 GaN-on-SiC power transistor, for example, is a +28-VDC device that can be used from DC to 1200 MHz. It is designed for push-pull applications and can withstand load mismatches as severe as a 10.0:1 VSWR without damage or degradation. The transistor is rated for CW power levels to 200 W at 900 MHz with better than 18-dB small-signal gain and 63% typical drain efficiency. The company offers an excellent 25-page white paper on thermal management of GaN-based power transistors (application note AN-012) as a free download from its website.
In addition, Freescale offers an excellent application note on evaluating the temperature of RF transistors, “Thermal Measurement Methodology of RF Power Amplifiers.” Copies are available for free download from www.freescale.com.
Late last year, Nitronex announced its move to +48-VDC GaN-on-silicon technology with a process designated NRF2. Intended to provide higher power densities and higher gain than the +28-VDC NRF1 process, the new process is also claimed to provide improved long-term reliability with a mean time to failure (MTTF) of more than one million hours (114 years) at an operating temperature of +230°C.
In addition to the firms mentioned, power transistors are available from a wide range of suppliers. These include Advanced Semiconductor, Inc. (ASI), which designs and fabricates a number of replacement devices for older broadcast and aerospace power transistors; Integra Technologies, Inc., which provides silicon bipolar and MOSFETs and GaN HEMTs; IXYS RF, a supplier of silicon MOSFETs; Spectrum Devices, a source for silicon bipolars and MOSFETs; ST Microelectronics, which supplies silicon MOSFETs; TriQuint Semiconductor, which provides GaAs and GaN power FETs; and Richardson Electronics, a distributor for numerous device manufacturers.