The epilayers in pseudomorphic high-electron mobility transistor ͑pHEMT͒ structures are grown by metallorganic chemical vapor deposition on GaAs substrates. A treatment with ammonium polysulfide ͑NH 4 ͒ 2 S X to passivate the surface of AlGaAs barrier layer is performed. Then, the surface morphology of the sulfur-treated AlGaAs layers was investigated by atomic force microscopy. The chemical compositions of AlGaAs surfaces before and after S treatment are studied using X-ray photoelectron spectroscopy. Two metals ͑Au and Pt/Au͒ are used as Schottky contacts on the gate. The passivated Pt/Au gate pHEMT outperforms the other three pHEMTs investigated in this work in both dc and high-frequency characteristics. The Schottky barrier height varies from 0.734 eV for Au on the unpassivated device to 0.904 eV for Pt/Au on the passivated device. Sulfur treatment and Pt/Au metallization yield the highest turn-on voltage and reverse breakdown voltage. An outstanding feature of this Pt/Au gate high-electron mobility transistor with passivation is its high f max . Furthermore, at 300 K, the passivated Pt/Au gate pHEMT has an exceptionally high f max /f T -ratio of 3.95. Following full characterization of these transistors at dc and radio frequencies, these devices undergo high-temperature tests. Experimental data reveal remarkably favorable characteristics of the sulfur-treated pHEMT with a Pt/Au gate. Surface and gate engineering are applied to pseudomorphic AlGaAs/InGaAs/GaAs heterostructures in the device, to achieve an unprecedented combination of high dc and high-frequency characteristics.Both commercial and military interests in high-power, highfrequency microwave transistors have fueled recent research into GaAs-based high electron mobility transistors ͑HEMTs͒. 1 The vast majority of reported HEMTs exploit AlGaAs/GaAs heterojunctions in which electrons are transferred from the heavily doped AlGaAs to the narrow bandgap GaAs. [2][3][4] No donor atoms is intentionally present in the undoped GaAs layer; thus, electrons in the twodimensional electron gas ͑2DEG͒ channel do not undergo impurity scattering; they consequently have high mobility. Initial ideas concerning the accumulation of charge at a heterojunction interface and its potential usefulness for devices was addressed in the late 1960s. Furthermore, the development of metallorganic chemical vapor deposition ͑MOCVD͒ 5 and molecular beam epitaxy ͑MBE͒ 6 technologies have made the production of heterostructures, quantum wells, and superlattices. MOCVD epitaxial growth technology was described in the scientific literature in 1968 by Manasevit, 7 even though related experimental results had been previously proposed in the patent literature by other workers, 8 prior to 1967. MOCVD technology for the growth of III-V compound semiconductors developed rapidly to dominate the production of epitaxial materials for both research and production. In 1980, researchers 9 at Fujitsu first proposed an experimental AlGaAs/GaAs HEMT and suggested the term HEMT. The selective doping of ...

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