Advances in AlGaN/GaN HEMT Surface Passivation

Abstract

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not frequently provided in literature. In this work, we investigate several passivation techniques and study their performance under dynamic switching conditions, under off-state stress, and implement hyperspectral electroluminescence imaging to identify electric field nonunifornities. We investigate plasma enhanced chemical vapor deposition (PECVD) SiN, a common GaN HEMT passivation method, and demonstrate how to optimize the deposition conditions to effectively passivate the HEMT. Generally, a mixed frequency PECVD SiN process is used, where a combination of low frequency (LF) e.g. 100-360 kHz and high frequency (HF) 13.56 MHz plasmas are used to create the SiN film. However, the LF step could create undesirable damage to the surface of the AlGaN barrier from N+ ion bombardment, degrading device performance. We developed a bilayer PECVD process, where a 10 nm thick layer is deposited using HF plasma only in order to protect the AlGaN surface, followed by a HF/LF mixed frequency SiN deposition. Annealing the 10 nm thick HF layer densified the film and resulted in a HEMT with significantly improved current collapse. On the other hand, SiN layers grown in situ during MOCVD growth of the GaN/AlGaN layers have also been demonstrated to serve as an effective protective layer for subsequent HF/LF PECVD SiN deposition, yielding HEMTs with low current collapse. Atomic layer epitaxy (ALE) deposition of a thin (10 nm) AlN layer is evaluated, and is shown to serve well as a passivation layer. This is particularly advantageous, since the thin layer is beneficial for reducing gate capacitance for high frequency operation. Finally, a nanocrystalline diamond (NCD) layer is incorporated on the surface of the GaN HEMT, primarily to improve thermal management, but this also is effective as a surface passivation layer, mitigating surface charge trapping. Effective passivation of the AlGaN/GaN HEMT surface is essential for mitigating current collapse, and we demonstrate recent advancements in passivation technology.