In this work, we investigated Ti/Al and Ti/Al/Ni/Au ohmic contact metallizations to n-GaN which were formed by a vacuum annealing method. Our effort was focused on minimizing the oxidation of the Al, which limits the performance of the Al-based contacts. We have also investigated the effect of the Ni/Au overlayers on the performance and morphological characteristics of the contacts. Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. It was found that the Ni/Au overlayer on the Ti/Al reduces the specific contact resistivity values slightly and at the same time causes an upward shift, by ∼200 °C, in the contact formation temperature. For the Ti/Al and Ti/Al/Ni/Au on n-GaN doped at 1.1×1018 cm−3, a minimum specific contact resistivity of 1.2×10−5 Ω cm2 was achieved after 950 and 1100 °C anneals, respectively. Auger electron spectroscopic studies revealed that obtaining optimum ohmic contact resistance requires the penetration of Al through the Ti layer to reach the GaN surface in addition to N outdiffusing from the GaN surface and into the metal layers. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer on the Ti/Al had the effect of increasing the surface roughness after annealing.

Antimonide-based p-channel HFETs with a 0.25 mm gate length have been fabricated with an InAlSb=AlGaSb barrier and a strained In 0.41 Ga 0.59 Sb quantum well channel. The modulation-doped material exhibits a Hall mobility of 1020 cm 2 =V s and a sheet density of 1.6 Â 10 12 cm À2. The devices have a maximum DC transconductance of 133 mS=mm and an f T and f max of 15 and 27 GHz, respectively. These values are the highest reported to date for this material system.

Schottky barrier rectifying contacts using e-beam-deposited platinum have been demonstrated on n-type β-SiC. The electrical properties of these contacts were examined as a function of annealing temperature using I-V and C-V measurements. Auger analysis was used to study the metallurgical reactions at the Pt/SiC interface. Short annealing cycles in the 350–800 °C temperature range led to formation of a mixed structure of PtSix and PtC at the interface, evidenced by migration of platinum into the SiC above 350 °C. The barrier height was found to increase from 0.95 to 1.35 eV with increasing annealing temperature. The rectifying characteristics improved following an initial 350 °C anneal and remained relatively stable up to 800 °C.

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