The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input impedance GaN-based RF power devices have made substantial progresses in the last decade This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance The reliability and manufacturing challenges are also discussed

GaN-Based RF Power Devices and Amplifiers

Gallium-nitride power transistor (GaN HEMT) and integrated circuit technologies have matured dramatically over the last few years, and many hundreds of thousands of devices have been manufactured and fielded in applications ranging from pulsed radars and counter-IED jammers to CATV modules and fourth-generation infrastructure base-stations. GaN HEMT devices, exhibiting high power densities coupled with high breakdown voltages, have opened up the possibilities for highly efficient power amplifiers (PAs) exploiting the principles of waveform engineered designs. This paper summarizes the unique advantages of GaN HEMTs compared to other power transistor technologies, with examples of where such features have been exploited. Since RF power densities of GaN HEMTs are many times higher than other technologies, much attention has also been given to thermal management-examples of both commercial “off-the-shelf” packaging as well as custom heat-sinks are described. The very desirable feature of having accurate large-signal models for both discrete transistors and monolithic microwave integrated circuit foundry are emphasized with a number of circuit design examples. GaN HEMT technology has been a major enabler for both very broadband high-PAs and very high-efficiency designs. This paper describes examples of broadband amplifiers, as well as several of the main areas of high-efficiency amplifier design-notably Class-D, Class-E, Class-F, and Class-J approaches, Doherty PAs, envelope-tracking techniques, and Chireix outphasing.

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A Review of GaN on SiC High Electron-Mobility Power Transistors and MMICs

A 175-to-350 V hard-switched boost converter was constructed using a high-voltage GaN high-electron-mobility transistor grown on SiC substrate. The high speed and low on-resistance of the wide-band-gap device enabled extremely fast switching transients and low losses, resulting in a high conversion efficiency of 97.8% with 300-W output power at 1 MHz. The maximum efficiency was 98.0% at 214-W output power, well exceeding the state of the art of Si-based converters at similar frequencies.

A 97.8% Efficient GaN HEMT Boost Converter With 300-W Output Power at 1 MHz

A three-way Doherty 100-W GaN base-station power amplifier at 2.14 GHz is presented. Simple, but accurate design equations for the output power combiner of the amplifier are introduced. Mixed-signal techniques are utilized for uncompromised control of the amplifier stages to optimize efficiency, as well as linearity. The combination of the above techniques resulted in an unprecedented high efficiency over a 12-dB power backoff range, facilitating a record high power-added efficiency for a wideband code division multiple access test signal with high crest factor, while meeting all the spectral requirements for Universal Mobile Telecommunications System base stations.

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A High-Efficiency 100-W GaN Three-Way Doherty Amplifier for Base-Station Applications

A new Doherty amplifier configuration with an intrinsically broadband characteristic is presented based on the synthesis of key ideas derived from the analyses of the load modulation concept and the conventional Doherty amplifier. Important building blocks to implement the proposed Doherty amplifier structure are outlined, which include the quasi-lumped quarter-wave transmission line, as well as the Klopfenstein taper for broadband impedance matching. A 90-W GaN broadband Doherty amplifier was designed and fabricated and achieved an average peak output power of 49.9 dBm, an average gain of 15.3 dB, and average peak and 6-dB back-off efficiencies of 67.3% and 60.6%, respectively, from 700 to 1000 MHz (35.3% bandwidth). The amplifier is shown to be highly linearizable when driven with 20-MHz WCDMA and long-term evolution signals, achieving adjacent channel power ratio of better than -48 dBc after digital predistortion.

A Modified Doherty Configuration for Broadband Amplification Using Symmetrical Devices

We have successfully developed a 500W AlGaN/GaN HEMT power amplifier with a frequency of 1.5GHz in L-Band, operating at 65V drain bias voltage. This amplifier consists of 4-chips of HEMT die developed for L-band frequency operation with push-pull configuration. The developed amplifier has an output power of 500W and a high linear gain of 17.8dB at the frequency of 1.5GHz under pulsed conditions at a duty of 10% with a pulse width of 100musec. To the best of our knowledge, this is the highest power ever reported for L-band GaN-related amplifier

A 500W Push-Pull AlGaN/GaN HEMT Amplifier for L-Band High Power Application

A 2-stage 80 W amplifier, which consists of a 450 W saturated power GaN HEMT Doherty amplifier and a 30 W driver, was developed. At first we developed the 450 W GaN HEMT Doherty amplifier and obtained saturation power of 56.5 dBm(450 W) and drain efficiency of 55% at 6 dB back-off power showing typical Doherty amplifier behavior. Then we built the 2-stage amplifier up with the 30 W driver stage amplifier. With this amplifier we obtained 42% efficiency (including 30 W driver amplifier) and -50 dBc ACLR at the average power of 49 dBm(80 W) with saturation power of 56.5 dBm and Gain of 32 dB.

A 80W 2-stage GaN HEMT Doherty Amplifier with 50dBc ACLR, 42% Efficiency 32dB Gain with DPD for W-CDMA Base station

We developed a kW-class AlGaN/GaN HEMT pallet amplifier operating at S-band. The pallet amplifier consists of an internally partial-matched AlGaN/GaN HEMT optimized for S-band on a copper base with soft PC boards. The developed pallet amplifier showed excellent performance, which is output power of over 800 W, high linear gain of 13.6dB and high efficiency of 52% over the wide frequency range of 2.9-3.3 GHz, operating at 65 V drain voltage with the pulsed condition at a duty of 10% and a pulse width of 200 musec. With 80 V drain voltage operation the peak power reached to 1 kW with 49.5% drain efficiency and 14.1 dB linear gain at 3.2 GHz. To the best of our knowledge, this is the highest power pallet amplifier ever reported for S-band.

A kW-class AlGaN/GaN HEMT pallet amplifier for S-band high power application

A 40W GaN HEMT Doherty power amplifier (PA) for 2.5GHz band was developed. The Doherty PA was designed using large signal GaN HEMT models, and demonstrated a saturation output power of 54dBm (250W) and a drain efficiency of more than 60%. The measurement result shows good agreement with the large signal simulation result. We also investigated the Doherty PA linearity with digital pre-distortion (DPD) system, and obtained a drain efficiency of 48%, an ACLR of -53dBc and a power gain of 13.4dB at the average output power of 46dBm (40W) with 64QAM modulation signal. These superior performances show good suitability for 2.5GHz band WiMAX base stations.

A 40W GaN HEMT Doherty Power Amplifier with 48% Efficiency for WiMAX Applications

We investigated high temperature operation of AlGaN/GaN HEMTs. At channel temperature of 269 degC, a linear gain of 12.3 dB and a power added efficiency of 53.6% were achieved at 2.14 GHz, less than 50 V operations. These are sufficient performance to practical application. At channel temperature of 368 degC, the linear gain was 10.4 dB and a power added efficiency of 43.9% was achieved. We also investigated the temperature dependence of equivalent circuit values, and found that the temperature dependence of saturated output power and the linear gain is originated from the temperature dependence of electron velocity in the channel.

Seigo Sano

Papers

A 500W Push-Pull AlGaN/GaN HEMT Amplifier for L-Band High Power Application

A 80W 2-stage GaN HEMT Doherty Amplifier with 50dBc ACLR, 42% Efficiency 32dB Gain with DPD for W-CDMA Base station

A kW-class AlGaN/GaN HEMT pallet amplifier for S-band high power application

A 40W GaN HEMT Doherty Power Amplifier with 48% Efficiency for WiMAX Applications

High temperature operation of AlGaN/GaN HEMT