**1. Introduction**

Semiconductor technology used in satellites or exploration robots in harsh environments is mainly based on silicon semiconductor technology, but uses modules for heat dissipation, the hermetic structure, and the shielding structure for extreme environments, such as high temperature and radiation; however, these modules are generally heavy and complicated parts. This burden can be relieved if robust semiconductor materials can be employed in the electronics used for harsh environments. Among attractive candidates, AlGaN/GaN heterostructure field e ffect transistors (HFETs) are attracting much intention as power switching devices for harsh environmental applications thanks to GaN's superior radiation resistance [1]. Recently, studies on the radiation characteristics of GaN-based transistors have been widely conducted [2–4]. Especially, studies on the irradiation e ffects of protons occupying the majority of low earth orbits (LEO) were carried out [5–7]. In general, AlGaN/GaN HFETs irradiated with protons exhibit a positive shift in the threshold voltage ( *V*th) and a reduction in the drain current (*I*DS), which can be attributed to the displacement damage near the two-dimensional electron gas (2-DEG) [8,9]. Exceptionally, the improvement of carrier concentration also has been reported at a relatively low dose [10]. In this paper, a su fficiently high dose (5 × 10<sup>14</sup> cm<sup>−</sup>2) was used to deteriorate the irradiated devices and the irradiated devices followed the generally reported results [11].

The main advantage of an AlGaN/GaN heterostructure is a natural formation of a 2-DEG channel without intentional doping [12], which leads to high mobility with a high sheet charge density [13]. Therefore, AlGaN/GaN HFETs inherently operate as normally-on devices. However, for the circuit configuration and stable operation, it is essential to implement a normally-o ff operation [14]. A gate-recessed metal–insulator–semiconductor (MIS) structure was employed to realize a normally-o ff operation and has exhibited a stable *V*th over 1 V and a low gate leakage [15,16]. Gate reliability has been one of the critical issues of AlGaN/GaN HFETs and can be aggravated in gate-recessed MIS structures due to the processes of AlGaN barrier etching and insulator deposition [17–19]. In this study, we fabricated normally-o ff AlGaN/GaN MISHFETs by using a gate-recessed MIS structure and investigated the e ffects of proton irradiation on the gate reliability of normally-o ff AlGaN/GaN gate-recessed MISHFETs.
