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We tested various ohmic contact metallization schemes on AlGaN/GaN HEMTs to achieve low contact resistance, good surface morphology and proper line edge definition. Mo, Ni and Pt intermediate layers in these schemes replaced the intermediate Ti layer interposed between Al and Au in Ti/Al/Ti/Au contacts. We recorded almost similar values of the contact resistance lying in the range of 0.3-0.5 mm on all the metallization stacks, but a significant difference was observed in their surface morphology and line edge definition. The formation of particular intermetallic compounds due to different intermediate layers was found to be responsible for this difference. Ti/Al/Ti/Au/WSiN contacts possessed excellent surface morphology. Mo based contact not only delivered good surface morphology and low contact resistance but also proper line edge definition. We found that the formation of the Al-Mo phase and GaMo 3 compound was the key factor responsible for its remarkable performance.

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PACS 68.60. Dv, 73.30.+y The performance of Pt/(Ti)/Au Schottky contacts on AlGaN/GaN heterostructures up to 360 ºC has been investigated by means of I-V-T measurements. A simultaneous increase of the effective barrier height and a decrease of the ideality factor with increasing temperature is observed. Barrier values of 1.05-1.15 eV and a n-values of 1.35-1.55 are obtained above 300 ºC. A reversible behaviour during the cooling is observed for the Pt/Ti/Au contacts, whereas a slight degradation is observed in the Pt/Au case. The insertion of an intermediate Ti layer is revealed to highly improve the thermal stability of the devices. The diode parameters are significantly enhanced throughout the whole thermal cycle when this intermediate layer is 10 nm thick.1 Introduction Recent progress in AlGaN/GaN high electron mobility transistors (HEMTs) have revealed the potential of nitride heterostructures for high power applications at high frequencies. However, to take further advantage of the thermal stability and chemical inertness of III-nitrides, thermally stable contacts are needed to improve device reliability. Self-or external-heating in high power or harsh environment applications, respectively, require metal contacts which do not suffer from degradation at work temperatures.The effect of high temperature (HT) annealings on morphology and electrical properties of Schottky contacts on nitrides have been reported in literature, see for example [1][2][3][4][5]. However, most of these works are based on post-annealing characterizations at room temperature (RT). The electrical performance of a device after annealing does not need to be identical to that at the time of annealing. In particular, only irreversible changes can be assessed by means of post-annealing measurements, whereas any reversible or transient change in the electrical behaviour is undetectable. Therefore, HT in situ measurements are required to evaluate the device reliability. In this work, we present the in situ electrical characterization of Pt/(Ti)/Au Schottky diodes on AlGaN/GaN heterostructures operating up to 360 ºC. The efficiency of an intermediate Ti layer of different thickness as a diffusion barrier has been evaluated.