Revisiting Power RF/MW Transistors

Revisiting Power RF/MW Transistors

Power transistors continue to gain in performance at RF and microwave frequencies, drawing upon numerous semiconductor materials for improved power and efficiency.

Power transistors serve many function in RF and microwave systems, largely in the transmit portion of those systems. An earlier report on large-signal transistors in the April 2011 issue of Microwaves & RF detailed some of the more recent advances in transistors, notably in the areas of gallium nitride (GaN) and silicon-carbide (SiC) technologies. But no single, short report could do justice to the number of new transistors on the market and the number of suppliers offering reliable devices for commercial, industrial, and military applications.

Unfortunately, the "Editor's Note" in that same issue referred to Polyfet RF Devices as no longer being in business. Representatives from the firm have since reported in that they are indeed a "going concern" and have quite the expansion lines of silicon field-effect transistors for continuous-wave (CW) and pulsed applications, as well as a growing line of high-power GaN transistors. As an example, the company has long offered its silicon VDMOS power transistors for applications such as AM/FM broadcast transmitters, cellular and paging amplifiers, military radios, and even in research and medical magnetic-resonance-imaging (MRI) systems. The model SR726 is a silicon VDMOS power push-pull packaged transistor rated for 100 W output power at 175 MHz. It can dissipate as much as 465 W power by merit of its low junction-to-case thermal resistance of 0.35C/W and maximum junction-temperature rating of +200C. It is also rated for maximum drain-to-gate voltage of 36 V and maximum drain-to-source voltage of 36 V, with maximum gate-to-source voltage of 20 V and maximum drain current of a robust 46 A. The device delivers at least 10 dB power gain at 175 MHz with minimum drain efficiency of 75% when tested with drain-source voltage of 12.5 V and quiescent drain current of 0.80 A.

Polyfet's model SR746 is a higher-power push-pull silicon VDMos device rated for 300 W output power at 175 MHz. It has maximum electrical ratings of 125 V for drain-to-gate and drain-to-source voltages and 18.5 a quiescent drain current. It provides at least 16 dB power gain at 175 MHz with drain efficiency of typically 65%, under conditions of 50 V drain-source voltage and 0.8 a quiescent drain current.

The company has also developed extensive lines of silicon laterally diffused metal-oxide-semiconductor (LDMOS) power transistors, such as the model LY942, a push-pull device (two drains and two gates, single source) rated for 600 W output power at 80 MHz. The rugged transistor is rated for maximum drain-to-gate voltage of 110 V and maximum drain current of 24 A. It achieves minimum power gain of 19 dB at 80 MHz with 70% typical drain efficiency at the same frequency, operating with 50 V drain-source voltage and quiescent current of 0.80 A. The device can withstand load mismatches as severe as a VSWR of 13.0:1.

In recent years, Polyfet RF Devices has added to its device lineups with high-power-density gallium nitride (GaN) transistors fabricated on silicon-carbide (SiC) substrates. these GaN-on-SiC transistors are housed in thermally enhanced packages for excellent heat dissipation. As an example, model GP041 operates with supplies to +50 VDC and provides 11-dB minimum gain at 1 GHz with 48 V drain-to-source voltage and quiescent current of 0.05 A. The RoHS-compliant device is fabricated without internal impedance matching, allowing it to be used for broadband applications. It features typical drain efficiency of 35% at 1 GHz and 10 W single-ended output power at that same frequency. It can handle load mismatches equivalent to VSWRs of 10.0:1.

Last year, in support of S-band radar applications, Integra Technologies announced the availability of a pair of 100-W silicon LDMOS transistors for pulsed systems. Model ILD2731M140 is engineered for applications from 2.7 to 3.1 GHz in commercial radar systems, while model ILD3135M180 is designed for use from 3.1 to 3.5 GHz in military S-band radars. Model ILD2731M140 can deliver at least 140 W peak output power from 2.7 to 3.1 GHz with operating with a 300-s pulse width and 10% pulse duty cycle. It is specified for use under Class AB bias conditions. The single-ended device features gold metallization for reliability. It is supplied in an industry-standard ceramic flanged package with excellent thermal characteristics. Model ILD3135M180 is suited for use from 3.1 to 3.5 GHz and can generate at least 180 W peak output power with 11 dB gain when operating with a 300-s pulse width and 10% duty cycle. As with the model ILD2731M140 transistor, it is designed for Class AB bias conditions and incorporates gold metallization for high reliability. It is also supplied in a ceramic, flange-mount housing.

Integra also offers devices based on GaN, using its GaN-on-silicon-substrate technology for high power at reduced materials and manufacturing costs. Housed in BeO-free packages and using gold metallization, the devices employ internal impedance matching for optimized performance over distinct frequency bands. For example, model IGN2731M50 is an input-matched, GaN high-electron-mobility-transistor (HEMT) device developed for S-band radar applications over an instantaneous bandwidth of 2.7 to 3.1 GHz. It supplies at least 50 W output power under Class AB bias conditions, with typical gain of 8 dB when running with a 300-s pulse width at a 10% duty cycle. It can be used across a variety of pulse widths and duty factors, and can operate with high spectral purity into an output load as severe as a 3.0:1 VSWR. The GaN-on-silicon HEMT is rated for maximum drain-source voltage of 65 V. When tested at individual frequencies of 2.7, 2.98, and 3.1 GHz, the transistor can deliver from 50 to 80 W pulse output power with power gains ranging from 8.0 to 10.5 dB. The conditions for testing include 32-V drain-source voltage and 25 mA quiescent drain current.

One of the more significant omissions from this past April's power transistor report was Freescale Semiconductor. Although mentioned in the report, little was written about some of the firm's more recent RF power transistors, including a pair of new power LDMOS devices for wireless base stations. The two devices are the model MRF8P20165WH/S developed nominally for the 1930-to-1995-MHz PCS band and model MRF8P20140WH/S for 1880-to- 2025-MHz TD-SCDMA applications.

The broad instantaneous bandwidths of these RoHS-compliant devices is well suited to the increasing requirements in wireless systems for data transmissions. They deliver an instantaneous bandwidth of 160 MHz, making them well suited for next-generation amplifier systems that must handle ever-increasing amounts of data. The MRF8P20165WH/S and MRF8P20140WH/S transistors meet linearity requirements for PCS and TD-SCDMA standards while delivering efficiency of at least 43.7 percent when amplifying multiple wireless carriers separated by as much as 65 MHz (PCS) and 140 MHz (TDSCDMA). Both devices are dual-path designs, and can implement the final stage of a Doherty amplifier with one path as the main amplifier and the other as the peaking amplifier.

Freescale is also a leading supplier of silicon MOSFET devices for pulsed and CW applications, including the rugged model MRFE6VP5600HR6/MRFE6VP5600HSR6 (see figure), with internally unmatched and usable at frequencies from 1.8 to 600 MHz. It is available in versions capable of not only 600 W peak pulsed output power at 230 MHz, but also providing 600 W average power at that frequency with slightly less gain (thus the two model numbers). The pulsed conditions assume a 100-s pulse width at 20% duty cycle, resulting in a power gain of 25 dB with 74.6% drain efficiency. The CW operating conditions result in 24.6 dB power gain at 230 MHz with drain efficiency of 75.2%.

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