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Copper-metallized gallium nitride (GaN) high-electron-mobility transistors (HEMTs) using a Ti/Pt/Ti diffusion barrier layer are fabricated and characterized for Ka-band applications. With a thick copper metallization layer of 6.8 μm adopted, the device exhibited a high output power density of 8.2 W/mm and a power-added efficiency (PAE) of 26% at 38 GHz. Such superior performance is mainly attributed to the substantial reduction of the source and drain resistance of the device. In addition to improvement in the Radio Frequency (RF) performance, the successful integration of the thick copper metallization in the device technology further reduces the manufacturing cost, making it extremely promising for future fifth-generation mobile communication system applications at millimeter-wave frequencies.

Schottky structures with copper and refractory metals as diffusion barrier for GaAs Schottky diodes were evaluated. These structures have lower series resistances than the conventionally used Ti/Pt/Au structure. Based on the electrical and material characteristics, the Ti/W/Cu and Ti/Mo/Cu Schottky structures are thermally stable up to 400°C; the Ti/Co/Cu Schottky structure is thermally stable up to 300°C. Overall, the copper-metallized Schottky structures have excellent electrical characteristics and thermal stability, and can be used as the Schottky metals for GaAs devices.

A highly selective wet chemical etch process for gate recess of the GaAs power metal-semiconductor field effect transistors (MES-FETs) was developed. The power MESFETs used in this study were epitaxially grown devices with a 20 Å AlAs etch-stop layer for gate recess. The selective etch process using citric acid/potassium citrate/hydrogen peroxide solution was studied. A selectivity better than 3800:1 was achieved for GaAs/AlAs layers. This selective etch was applied both to high-power, high-voltage power MESFETs and low-voltage large-periphery power MESFETs. For high-power applications, the 14.7 mm device had a breakdown voltage of 22 V. When tested at 1.88 GHz with a drain bias of 10 V, it provided a maximum output power of 38.8 dBm and a maximum power-added efficiency of 52.5%. For low-voltage applications, the 19.8 mm device was tested under IS-95 code-division multiple access (CDMA) modulation at 1.88 GHz with a drain bias of 3.5 V. Under CDMA modulation, the device showed an output power of 28.03 dBm with an adjacent channel power rejection of Ϫ29.6 dBc at 1.25 MHz offset frequency. Both devices also showed excellent uniformity in pinch-off voltages.

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