Impact of SiNx Passivation on IDS,max of AlGaN/GaN HEMTs
- Category: MEMS & BioMEMS
- Tags: mohamed azize, tomas palacios
Print This Page
Download as PDFDielectric films such as SiNx, SiO2, and Al2O3 play key roles in AlGaN/GaN heterostructure field-effect transistors (HFETs) both as surface passivation and as gate-insulating layers[1]. Several groups, including ours, have observed that thin SiNx (<50 nm) deposition on AlGaN/ GaN HFETs by plasma-enhanced chemical vapor deposition (PECVD) can significantly change the two-dimensional electron gas (2DEG) density (ns). The origin of this change in ns has not been carefully analyzed until now. The study of the effect of a SiNx layer on AlGaN/GaN high-electron-mobility transistors (HEMTs) has been challenging since it is difficult to decouple the effects of the SiNx-induced strain from changes in surface potential.
In this work, we have investigated the impact of SiNx passivation in the transport properties of AlGaN/GaN heterostructures grown on Si(111) substrates. After the fabrication of standard AlGaN/GaN membrane HEMTs, the Si substrate underneath several devices was selectively etched away using a deep reactive ion etching system with SF6 chemistry[2]. Then, the transistors were passivated with compressive (LFSiN) and tensile (HFSiN) stress SiNx layers deposited by PECVD.
A comparative study of compressive and tensile SiNx dielectrics on AlGaN/GaN grown on Si (111) shows a decrease by ~ 40 % and an increase by ~13% in current, respectively (Figure 1). At the same time, a threshold voltage (VT) shift towards 0V is observed after the LFSiN deposition, unlike with the HFSiN passivation cap layer (Figure 2). It was found that surface strain induced by the passivation layer is the main contributor to the change in ns and current density in the GaN-based devices when tensile stress SiNx is deposited, unlike the compressive stress SiNx cap layer. These results pave the way to a new degree of freedom in the design of GaN electronic devices and local strain engineering.
-
- Figure 1: Current-voltage characteristics of AlGaN/GaN membranes with HFSiN and LFSiN passivation layers on the top surface. The insets show pictures of the different processed chips.
-
- Figure 2: Transfer characteristics of SiNx/AlGaN/GaN membranes before and after HFSiN and LFSiN back passivation layers. To cancel the piezoelectric doping resulting from the bending stress of the AlGaN/GaN membrane caused by the deposition of SiNx, free-standing SiNx/AlGaN/GaN/SiNx heterostructures have been fabricated.