AlGaN/GaN transistors and circuits have been fabricated on semi-insulating SiC substrates. The process has been expanded to provide all the required elements of a full MMIC fabrication, including thin film resistors, MIM capacitors, and two levels of gold interconnect including air-bridge. Spiral inductors have been included in some designs. Large signal models of devices with various gate widths have been obtained, and a several experimental circuits with both microwave and digital functions, have been designed and measured.
AlGaN/GaN HEMT devices on selectively grown mesa structures on Si(111) have been grown and fabricated. A typical strain relief structure was grown over the entire 2" wafer. Subsequently, the epilayer was removed from most of the wafer surface by dry etching, leaving only the patterned mesas intact. Then, the wafer was returned to the growth chamber and the growth of the AlGaN/GaN heterostructure was completed. GaN does not nucleate on the Si(111) surface, thus the growth was highly selective, and moreover, due to the small size of the mesas, was crack-free. A five level mask set, consisting of mesa, ohmic, gate, passivation, and interconnect metal levels was used to complete the working devices. Devices with 0.8 µm gate length showed peak currents of 0.45 A/mm, and transconductance of 130 mS/mm. The devices were essentially free of current collapse and had fT of 9 GHz and fMAX of 22 GHz.
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