An In 0.45 Al 0.55 As/In 0.5 Ga 0.5 As metamorphic high-electron-mobility transistor ͑MHEMT͒ with an inverse-step InAlAs metamorphic buffer, grown by molecular beam epitaxy on GaAs substrate, is presented. A typical 1 m gate-length MHEMT has a drain current density of 471 mA/mm and a peak extrinsic transconductance of 258 mS/mm. High-frequency scattering parameter measurements yield a unity current gain cutoff frequency of f T = 35.2 GHz as well as a maximum frequency oscillation f max = 48 GHz. High two-terminal and three-terminal breakdown voltages are achieved mainly because of the high bandgap of the In 0.45 Al 0.55 As Schottky layer and the large conduction-band discontinuity ͑⌬E C ͒ at the InAlAs/InGaAs heterojunction. The large-signal power performance and noise characteristics are measured. Experimental results on the temperature-dependence study of the saturation drain current density, the extrinsic transconductance, the leakage current, the off-state breakdown voltage, and the threshold voltage ͑V th ͒ are presented. The V th shift of the device at elevated temperatures is as small as −0.294 mV/K. All of the experimental results confirm the exceptional potential of the InAlAs/InGaAs/GaAs MHEMT in microwave circuit and high-temperature applications.

As an alternative to AlGaAs/GaAs and InGaP/GaAs camel-gate heterostructure field-effect transistors ͑CAMFETs͒ for microwave applications, InAlGaP/GaAs/InGaAs pseudomorphic high-electron-mobility transistors ͑CAM-pHEMTs͒ are shown to have high breakdown voltage, high broad-plateau extrinsic transconductance ͑g m ͒, and small leakage current. Two-terminal gate-source breakdown voltage exceeding 20 V is achieved for CAM-pHEMT with Ni/Au gate metal. The transconductance curve is quite broad for a gate voltage range of approximately 3.6 V. Additionally, CAM-pHEMT exhibits relatively negligible temperaturedependent characteristics over the operating temperature range. Therefore, the studied device displays promise for hightemperature applications.

An AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor ͑pHEMT͒ with double ␦-doping carrier supply layers was fabricated and characterized. Estimates of typical device characteristics ͑extrinsic transconductance ͑g m ͒, drain current density ͑I D ͒, threshold voltage ͑V th ͒, current gain cut-off frequency ͑ f T ͒, maximum frequency of oscillation ͑ f max ͒, noise behavior, and large signal measurements͒ were made. A series of experiments was conducted to assess the effects of temperature on pHEMT. A peak g m of 161 mS/mm was obtained from the fabricated pHEMT with a gate length of 1 m for room-temperature operation. The transistor characteristics showed evidence of high breakdown voltages. With respect to the variation in two-terminal gatedrain breakdown voltage, this device had a positive temperature coefficient from 300 to 450 K, and a negative temperature coefficient at temperatures surpassing 450 K. Strikingly, high-frequency on-wafer measurements on the fabricated pHEMT were made up to 400 K, revealing nearly no change in f T . Not only was the device characterized and the complicated mechanisms at various temperatures elucidated, but also the favorable potential of the presented pHEMT for high-temperature and high-frequency applications was determined.

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This study proposes the investigation of using the (NH 4) 2 S x solution to form the AlGaN surface passivation on the AlGaN/GaN high electron mobility transistors (HEMTs). Both treatment schemes are implemented on separate pieces of the same HEMT wafer, including (NH 4) 2 S x pregate and post-gate metal treatments. Temperature-dependent characteristics of the HEMTs are also studied. Experimental results demonstrate that by the surface treatment prior to metal, the performance of the studied HEMTs can be improved, including thermal stability, dc and high-frequency characteristics. Furthermore, the interface state density (D it) of the studied HEMT is studied by the subthreshold slope method. To the best of our knowledge, this is the first report on comparison of AlGaN/AlN/GaN HEMTs with pre-gate and post-gate metal treatments.