Wide bandgap semiconductors are extremely attractive for the gamut of power electronics applications from power conditioning to microwave transmitters for communications and radar. Of the various materials and device technologies, the AlGaN/GaN high-electron mobility transistor seems the most promising. This paper attempts to present the status of the technology and the market with a view of highlighting both the progress and the remaining problems.

AlGaN/GaN HEMTs-an overview of device operation and applications

Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

The dc small-signal, and microwave power output characteristics of AlGaN/GaN HEMTs are presented. A maximum drain current greater than 1 A/mm and a gate-drain breakdown voltage over 80 V have been attained. For a 0.4 /spl mu/m gate length, an f/sub T/ of 30 GHz and an f/sub max/ of 70 GHz have been demonstrated. Trapping effects, attributed to surface and buffer layers, and their relationship to microwave power performance are discussed. It is demonstrated that gate lag is related to surface trapping and drain current collapse is associated with the properties of the GaN buffer layer. Through a reduction of these trapping effects, a CW power density of 3.3 W/mm and a pulsed power density of 6.7 W/mm have been achieved at 3.8 GHz.

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Trapping effects and microwave power performance in AlGaN/GaN HEMTs

Research work focusing on the enhancement of large-signal current-voltage (I-V) capabilities has resulted in significant performance improvement for AlGaN/GaN HEMT's. 100-150 /spl mu/m wide devices grown on SiC substrates demonstrated a record power density of 9.8 W/mm at 8 GHz, which is about ten times higher than GaAs-based FETs; similar devices grown on sapphire substrates showed 6.5 W/mm, which was thermally limited, 2-mm-wide devices flip-chip mounted on to AlN substrates produced 9.2-9.8 W output power at 8 GHz with 44-47% PAE. A flip-chip amplifier IC using a 4-mm device generated 14 W at 8 GHz, representing the highest CW power obtained from GaN-based integrated circuits to date.

Very-high power density AlGaN/GaN HEMTs

Record performance of high-power GaN/Al/sub 0.14/-Ga/sub 0.86/N high-electron mobility transistors (HEMTs) fabricated on semi-insulating (SI) 4H-SiC substrates is reported. Devices of 0.125-0.25 mm gate periphery show high CW power densities between 5.3 and 6.9 W/mm, with a typical power-added efficiency (PAE) of 35.4% and an associated gain of 9.2 dB at 10 GHz. High-electron mobility transistors with 1.5-mm gate widths (12/spl times/125 /spl mu/m), measured on-wafer, exhibit a total output power of 3.9 W CW (2.6 W/mm) at 10 GHz with a PAE of 29% and an associated gain of 10 dB at the -2 dB compression point. A 3-mm HEMT, packaged with a hybrid matching circuit, demonstrated 9.1 W CW at 7.4 GHz with a PAE of 29.6% and a gain of 7.1 dB. These data represent the highest power density, total power, and associated gain demonstrated for a III-nitride HEMT under RF drive.

High-power microwave GaN/AlGaN HEMTs on semi-insulating silicon carbide substrates

A SiC MESFET with 42 mm gate periphery on a single die had a maximum RF output power of 53 W CW with 37% power-added efficiency (PAE) at 3.0 GHz. This unprecedented power from a die with an area of only 3 mm/sup 2/ demonstrates the extremely high power handling capability of SiC microwave devices. Additionally, we report here SiC MESFETs showing 2.5 W/mm performance with 41% PAE at 8 GHz, demonstrating the utility of this technology at X-band.

L.T. Kehias

Papers

High-power microwave GaN/AlGaN HEMTs on semi-insulating silicon carbide substrates

SiC MESFET with output power of 50 watts CW at S-band