The epilayers in pseudomorphic high-electron mobility transistor (pHEMT) structures are grown by metallorganic chemical vapor deposition on GaAs substrates. A treatment with ammonium polysulfide (NH4)(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/Au) 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.