Normally-off recessed MOS-gate AlGaN/GaN HEMTs with over +4V saturation drain current density and a 400V breakdown voltage

Abstract

Owing to outstanding properties of two-dimensional electron gas (2DEG) and GaN-base material, such as high electron mobility and high critical electric field, AlGaN/GaN high electron mobility transistors (HEMTs) have been considered as the next-generation power semiconductor devices with high power conversion efficiency, high switching frequency and high-temperature operation capability. For power switching applications, enhancement mode (e-mode, or say normally-off) transistors are desired for fail-safe operation and silicon-compatible gate drive circuit. However, e-mode operation is difficult for AlGaN/GaN-based HEMTs because of the natural existence of 2DEG. To achieve e-mode operation, several approaches have been reported, including gate recess structure, fluorine plasma ion implantation, p-type gate structure and selective channel regrowth, etc. This paper reports normally-off AlGaN/GaN HEMTs with a recessed MOS-gate. After mesa isolation and Source and drain metal, the gate recess process used inductively coupled plasma (ICP). In order to avoid severe etching damage and obtain high drain current density, etching power was carefully optimized. The etching rate is slow for accurate etching depth control to leverage the current density and threshold voltage. After that, an O2 plasma treatment was applied using a plasma asher to oxidize the damaged semiconductor surface. The oxide layer was then removed in HCl: DI-water (1: 3). The next step is an atomic layer deposition (ALD) of Al2O3 as the gate dielectric to increase the breakdown voltage. Follow after that, gate metal and pad metal. By the process, our team made three kinds of HEMTs with different recessed gate depths. The first one, which has been report before, exhibits a high threshold voltage of +4.6V, a specific on-resistance of 4mΩ-cm⁁2 and a drain current density of 108 mA/mm. This paper will show the others fabricated in the later researches. The device transfer curves show that this normally-off recessed MOS-gate AlGaN/GaN HEMTs exhibit threshold voltage of +0.9V and +2.1V, respectively. Additionally, the specific on-resistance and saturation drain current density of the device with 0.9V threshold voltage are 2.26mΩ-cm⁁2 and 326mA/mm, while 2.1V-Threshold Voltage-device are 3.03mΩcm⁁2 and 173mA/mm. Both of the two HEMTs have a breakdown voltage over 400V.