Reliability behavior of GaN HEMTs related to Au diffusion at the Schottky interface

Abstract

The Schottky contact on GaN HEMT power devices is a key element for high power performance and its long term reliability behavior. Up to now such GaN devices are still suffering very often by a leakage current increase during operation resulting in deteriorated transistor behavior. One reason for this problem is owing to insufficiently stable Schottky contacts based on NiAu metalisation. We analysed GaN HEMTs with different aging behaviors by means of HR‐TEM and EDX investigations. It could be shown that on transistors with increasing leakage current Au was found directly on the Schottky interface which may be responsible for the increase of the leakage current. Furthermore, there are strong indications that such Au is diffusing via the gate foot side wall Ni‐SiN interface towards the Schottky barrier. It is proposed that an additional metallic diffusion barrier may solve this aging problem. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Failure signatures on 0.25μm GaN HEMTs for high-power RF applications

“…For the most remarkable failure mode (2), many papers reports the strong effect in device performances given by the combined presence of high electric-field and high junction-temperature. Results in [6,8] point out the growth of pits and cracks extended all over the gate-drain edge region as the cause of the huge reduction of the channel performances without any leakage increase, while authors in [9,11] report that the Au interdiffusion in the Schottky contact produces the same effect in device performances even in pure thermal storage tests, together with the increase of the gate leakage. Even if all mentioned papers remark a significant degradation after long-term tests (more than 1000-h), the higher junction temperatures used in our tests can be the root cause of the pretty fast performance reduction obtained after only 24 h of stress.…”

Reliability studies on GaN HEMTs with sputtered Iridium gate module

“…Moreover for the Schottky gate contact diffusion of gold down to the AlGaN semiconductor has been observed in the case of Nickel contact [7]. The storage test is performed on wafer at 325°C for 1000 h. For the gate diode in forward direction a shift of typically 0.05 V after 1000 h testing is observed (Fig.…”

“…This risk is even more pronounced for classic gate technology without Nitride trench because gold may diffuse along the surface of the gate foot and over the edge of the Schottky metal to the semiconductor. Even slanted gate technology cannot guarantee a full coverage of the Schottky metal because adhesion of evaporated metal onto Nitride can be a problem [7].…”

“…Due to its high melting point (2450°C) intermixing with other metals is low. For Iridium as a Schottky gate contact on AlGaN/GaN-HEMT stable operation was found even at 400°C for 120 h [6].In addition to the gate sinking of the Nickel contact as described above [4], also diffusion of gold around Schottky metal edges into the AlGaN-layer has been detected [7]. Typically, the etched trench in the Silicon Nitride of the gate module cannot be fully covered by the Schottky metal on the sidewalls of the trench.…”

“…This risk is even more pronounced for classic gate technology without Nitride trench because gold may diffuse along the surface of the gate foot and over the edge of the Schottky metal to the semiconductor. Even slanted gate technology cannot guarantee a full coverage of the Schottky metal because adhesion of evaporated metal onto Nitride can be a problem [7].Here we propose a gate module that uses Iridium as a diffusion resistant Schottky contact and a fabrication method that independently from the shape of the gate foot allows for a full coverage of the gate contact and the adjacent area in the trench. Metal deposition methods that allow for a more conformal coverage of the surface are required to accomplish this goal.…”