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This paper reports Al 0.27 Ga 0.73 N/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) with stacked Al 2 O 3 /HfO 2 gate dielectrics by using hydrogen peroxideoxidation/sputtering techniques. The Al 2 O 3 employed as a gate dielectric and surface passivation layer effectively suppresses the gate leakage current, improves RF drain current collapse and exhibits good thermal stability. Moreover, by stacking the good insulating high-k HfO 2 dielectric further suppresses the gate leakage, enhances the dielectric breakdown field and power-added efficiency, and decreases the equivalent oxide thickness. The present MOS-HEMT design has demonstrated superior improvements of 10.1% (16.4%) in the maximum drain-source current (I DS, max ), 11.4% (22.5%) in the gate voltage swing and 12.5%/14.4% (21.9%/22.3%) in the two-terminal gate-drain breakdown/turn-on voltages (BV GD /V ON ), and the present design also demonstrates the lowest gate leakage current and best thermal stability characteristics as compared to two reference MOS-HEMTs with a single Al 2 O 3 /(HfO 2 ) dielectric layer of the same physical thickness.

This study aims to explore phase transformation in plasmasprayed hydroxyapatite (HA) ؉ 10 wt% ZrO 2 -8-mol%-Y 2 O 3 composite coating, using separately prepared HA and ZrO 2 -8-mol%-Y 2 O 3 coatings as a control. Changes in the phase and chemistry of the coatings are characterized by X-ray diffractometry, with lattice-constant measurement (Cohen's method), and by transmission electron microscopy. Experimental results show evidence of diffusion, in the liquid state, of calcium ions from the HA matrix into the ZrO 2 . This behavior causes the formation of the following structural features in the composite coating: (i) a CaO-doped ZrO 2 solid solution (ZrO 2 -7.7 mol% Y 2 O 3 -4.4 mol% CaO); (ii) a mixture of ZrO 2 and CaZrO 3 having a crystal-orientation relationship; (iii) an amorphous phase containing elements of calcium, phosphorus, zirconium, and yttrium; and (iv) a remaining CaO-poor HA matrix (Ca 10؊x (HPO 4 ) x (PO 4 ) 6؊x (OH) 2؊x ; x ‫؍‬ 0.06). Rationales for the greatly decreased impurity phases of CaO and Ca 4 P 2 O 9 found in the composite coating are discussed.

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