Richard Peter Smith

Wide bandgap semiconductor devices and MMICs for RF power applications

High power densities of 5.2 W/mm and 63% power added efficiency (PAE) have been demonstrated for SiC MESFETs at 3.5 GHz. Wide bandwidth MMICs have also been demonstrated with SiC MESFETs, yielding 37 W at 3.5 GHz. Even higher power densities have been obtained with GaN HEMTs, showing up to 12 W/mm under pulsed conditions. Hybrid amplifiers using GaN HEMTs on SiC substrates have demonstrated a pulsed output power level of 50.1 W, with 8 dB gain and PAE of 28% at 10 GHz, and CW power levels of 36 W have also been obtained. A wide bandwidth GaN MMIC amplifier had a peak pulsed power level of 24.2 watts, with a gain of 12.8 dB and PAE of 22% at 16 GHz.

Applications of SiC MESFETs and GaN HEMTs in power amplifier design

Very high power densities have been shown for both SiC MESFET and GaN HEMT devices. Both of these active devices benefit from the high breakdown voltages afforded by their wide-bandgap semiconductor properties. The GaN device also benefits from current densities as high as 1 A/mm. This high power density, along with good efficiency and linearity, provide an excellent base for future military and commercial power amplifier applications. High power densities are possible using narrow band power-matching networks. Although the gain-bandwidth limitation is exacerbated due to the high-impedance load lines required, high power design is possible even over multi-octave bandwidths.

Linearity performance of GaN HEMTs with field plates

Recently, electric field modification with GaN-based high-electron-mobility-transistors (HEMTs) using field plates (FP) has resulted in dramatically enhanced power performance. Power densities up to 32 W/mm at 4 GHz have been demonstrated with power-added-efficiency (PAE) of 55%. When scaled to a large periphery, a total output power of 149 W was obtained at 2 GHz. Modern communication applications also require high linearity for power devices. Here we present the linearity performance of GaN-channel HEMTs with various FP lengths at biases up to 108V.

High power demonstration at 10 GHz with GaN-AlGaN HEMT hybrid amplifiers

The GaN/AlGaN-on-SiC HEMT is being pursued as an RF power device on which to base next generation X-band and K-band power amplifiers. Previous demonstrations of extremely high power density and total RF power from GaN/AlGaN HEMTs on SI SiC substrates (Sheppard et al., 1999; Wu et al., 1999) adequately demonstrate their potential for improved performance over GaAs-based devices. Considering the recent advances in III-nitride growth and processing techniques, it is not surprising that GaN/AlGaN HEMTs on SiC substrates are beginning to the reach the extremely high power levels that have been predicted for this material system. To continue this trend, we are pursuing high quality HEMT structures with 15% AlGaN molar fraction (MF) in order to fabricate large area devices and demonstrate high power levels. We report the demonstration of over 40 W from a single GaN/AlGaN transistor at 10 GHz under pulsed mode conditions. This high power demonstration includes the first implementation of SiC substrate vias on a GaN-on-SiC microwave transistor for improved gain at X-band.

An Internally-matched GaN HEMT Amplifier with 550-watt Peak Power at 3.5 GHz

A high-power amplifier using two 28.8-mm-periphery GaN HEMTs was demonstrated with all matching components inside the package. When biased at 55 V, a power bandwidth of 3.3-3.6 GHz was obtained, with 550-Wpeak output, 12.5-dB associated gain and 66% drain efficiency at 3.45 GHz

High power hybrid and MMIC amplifiers using wide-bandgap semiconductor devices on semi-insulating SiC substrates

An overview of hybrid and monolithic high-power microwave amplifiers using SiC MESFET and GaN HEMT active devices is presented. High power densities of 5.2 W/mm and 63% power added efficiency (PAE) have been demonstrated for SiC MESFETs at 3.5 GHz. This performance has driven the development of wide-bandwidth MMIC amplifiers, which have yielded 37 W of pulsed power at 3.5 GHz. GaN HEMTs on SiC substrates can achieve these high performance levels at frequencies where SiC cannot operate. At 10 GHz, a 12-mm GaN HEMT hybrid amplifier achieved a CW output power level of 38 W with an associated gain of 8 dB and PAE of 29%, complementing a previous pulsed result of 50.1 W. MMIC amplifiers have also been demonstrated using GaN-on-SiC technology. At 16 GHz, a two-stage GaN HEMT MMIC wide-bandwidth amplifier was capable of a peak power level of 24.2 watts with an associated gain of 12.8 dB and PAE of 22%. Recently, a 6-mm single-stage narrow-band MMIC amplifier has produced 32 watts of pulsed power at 10 GHz with an associated gain of 8.3 dB and a PAE of 35.3%. Finally, to validate progress in scaling unit cell performance to large devices, we have demonstrated 103 W of CW power from a single GaN HEMT transistor at 2 GHz with an associated drain efficiency of 52%.

AlGaN/GaN-on-SiC HEMT Technology Status

GaN-based devices offer significant advantages for next generation military and commercial systems. Military systems benefit from high power densities of 4 to 7 W/mm depending on bias conditions along with efficiencies over 60% at frequencies through X-band, and commercial systems take advantage of excellent linearity as well. In this paper, we will review a number of commercial products that only GaN technology can achieve. In addition to narrow-band circuits for highly linear commercial applications, results will be shown for two commercial GaN MMIC products that have been developed for general-purpose applications in the 2.5-6.0 GHz and DC-6.0 GHz bands. Additionally, results are shown for a 2-stage high efficiency S-band switch mode amplifier operating from 3.1-3.5 GHz. Significant progress has also been made in the development of 100-mm SiC substrates. Micropipe densities as low as 2.5 cm-2 have been demonstrated for 100-mm HPSI substrates.

Technology development for GaN/AlGaN HEMT hybrid and MMIC amplifiers on semi-insulating SiC substrates

The GaN/AlGaN-on-SiC HEMT is being pursued as the active element on which to base next-generation high-frequency power amplifiers. Advances in III-nitride growth and processing techniques for commercial production of optoelectronic devices are being applied to advance the state-of-the-art for GaN/AlGaN HEMTs. Demonstrations of extremely high power density and total RF power from these unipolar, high-frequency transistors adequately demonstrate their potential for superior power performance over GaAs-based devices at 10 GHz. In order to satisfy the requirements for high-power, wide-bandwidth amplifiers, the next level of innovation for this technology is to develop monolithic microwave integrated circuits (MMICs). The latest developments have allowed us to demonstrate the first operational MMIC amplifier in the GaN-on-SiC HEMT platform.

DEVELOPMENT OF NITRIDE-BASED TECHNOLOGY FOR SPACE: ENABLING NEW FRONTIERS

Gallium nitride based microwave technology is being developed in an Air Force Dual-Use Science and Technology project. The technology has previously demonstrated significant characteristics for high power operation and robustness in extreme environments. Its desirable features (such as high breakdown voltage), overall development issues, and systems implications are reviewed. In this context, the programs recent advancements in state-of-the-art performance are presented, such as 14.8 W pulsed at 10 GHz with a 6mm HEMT with 27% PAE and 11.7 dB gain.