I. INTRODUCTION

Harsh environment electronics play an important role in resource exploration and space mission. For example, electronic components equipped with radiation-tolerant technology are essential in space environment where strong cosmic-radiation is present. GaN-based electronics have been intensively developed by taking advantage of large breakdown voltage and high saturation velocity^[1,2]. In addition, GaN-based transistors have demonstrated robust operation in high temperature and radiation-abundant environment^[3-5].

In space, it is important to secure radiation-hardened characteristics of semiconductor components mounted on space electronic systems because there are abundant cosmic rays and solar particles including highly energetic protons. Protons with energies ranging from hundreds of keV to thousands of MeV exist along the low earth orbit, where most of satellites and spacecraft are allocated^[6]. The effects of proton irradiation on GaN-based transistors have been widely reported^[7-9]. The main degradation mechanism in GaN-based transistors induced by proton irradiation is strongly related to the defect centers. Protons knock out atoms thereby creating vacancies (Ga and N) and interstitials which act as defect centers. These defect centers cause the reduction of mobility in two-dimensional electron gas (2-DEG) channel by increasing scattering of carriers due to coulombic interaction. They also reduce the sheet carrier density in 2-DEG channel by trapping carriers^[10,11].

The reliability against the radiation effects should be addressed for GaN devices to be deployed in space. Among various issues in device reliability, GaN HEMTs have been reported to suffer from temporary instability, so called the current slump, when high electric field were applied between gate and drain^[12,13]. In this work, we carried out 5 MeV proton irradiation along with electrical stress tests to investigate the radiation effects on the electrically-induced degradation of normally-off AlGaN/GaN recessed MISHFETs.

II. DEVICE FABRICATION AND EXPERIMENT

A cross-sectional view of AlGaN/GaN recessed MISHFETs fabricated in this work is shown in Fig. 1. The epitaxial structure consists of a 4.6 μm GaN buffer layer on (111) Si substrate, a 517 nm i-GaN layer, a 24 nm AlGaN barrier, a 4 nm GaN capping layer, and a 10 nm in-situ $SiN_{X}$ layer. The gate-to-drain distance, gate length, and gate-to-source distance are 15, 2, and 3 μm, respectively, and the gate width is 100 μm. For the source and drain contacts formation, a recessed ohmic contact scheme was employed to reduce the contact resistance. A Ti/Al/Mo/Au (= 20/120/25/50 nm) metal stack was evaporated and alloyed at 830 ℃ for 30 s. Gate channel-recess process was performed following mesa isolation by a low-damage, Cl₂/BCl₃-based inductively coupled plasma (ICP) reactive ion etching. The AlGaN barrier was completely etched away for the normally-off operation. A 20 nm $SiN_{X}$ film was deposited by ICP chemical vapor deposition and annealed at 500 ℃ for 10 mins in $N_{2}$ ambient to stabilize the quality of an insulator. The following patterning process defined gate regions and a Ni/Au (=20/250 nm) metal stack was evaporated. Post-metallization annealing was also performed at 400 ℃ for 10 mins in $N_{2}$ ambient to improve the interface property.

Fig. 1. Cross-sectional view of normally-off AlGaN/GaN recessed MISHFETs fabricated on GaN-on-Si substrate.

Proton irradiation was performed with the energy of 5 MeV by MC-50 cyclotron at Korea Institute of Radiological and Medical Sciences. High total fluence of 5 10¹⁴ cm^-2 was chosen to ensure the device degradation. The samples under test were electrically floated. DC current-voltage characteristics were measured by using Agilent 4155A semiconductor parameter analyzer. High voltage stress was performed using Keithley 2410 and 2651A high voltage/current sources.

III. RESULT AND DISCUSSION

Fig. 2 shows the output and transfer characteristics of AlGaN/GaN recessed MISHFETs irradiated with 5-MeV protons. The $V_{th}$ was shifted by 1.5 V and the $I_{D}$ was reduced by 57.3 % after irradiation. The change of gate leakage currents was negligible. Displacement damage has been reported to be the primary mechanism of proton-induced degradation. As incident particles pass through the devices, they can displace the host atoms from the lattice and create defects or traps in the irradiated material. When the carriers in the channel become trapped in these defects of traps, they can’t contribute to the conduction of current flow. Therefore, the deterioration of device parameters is triggered^[14-17].

Fig. 2. (a) The output, (b) transfer characteristics of normally-off AlGaN/GaN recessed MISHFETs irradiated with 5-MeV protons. $V_{G}$ = 8 ~ 0 (-1 V step) in (a) and $V_{D}$ = 10 V in (b).

Short-term stress tests in which the devices were stressed at off-state were conducted before and after proton irradiation to investigate the radiation effects on the trapping effects. Stress conditions are the drain voltage ($V_{D}$) = 400 V and the gate voltage ($V_{G}$) = 0 V. The devices were stressed for 5 mins at room temperature.

Fig. 3 shows the output characteristics normalized by the maximum current before and after short-term stress tests with proton irradiation. The stressed devices exhibited more pronounced reduction of $I_{D}$ caused by charge trapping during short-term stress. This instability was reversible and fully recovered after 1-day ambient storage at room temperature.

Fig. 3. The normalized output characteristics before and after off-state stress tests. $V_{G}$ = 6 V.

When the devices are biased at high $V_{D}$, electrons in the 2-DEG channel become “hot” and can be trapped to the surface states in the access region (gate-to-drain) or buffer traps near the 2-DEG channel. These trapped electrons can reduce the $I_{D}$ by depleting the 2-DEG channel^[18-19]. After proton irradiation, the charge-trapping by short-term stress was aggravated as compared to non-irradiated devices. It suggests that the devices became more susceptible to hot electron-induced trapping during the stress tests after proton irradiation.

Proton irradiation can induce significant damage in the AlGaN/GaN heterostructure and exacerbate the trapping phenomenon. In order to investigate the change in the interface states before and after proton irradiation, CET map^[20] and conductance method^[21] were carried out. CET map, which is a useful technique to analyze overall trap behavior through the gate region, was extracted by the repetition of stress and recovery scheme on the gate. The 5 V of $V_{G}$ was used to induce bias stress instability and trap concentration ($N_{it}$) was extracted by the following equation.

where $\epsilon_{0}$ and $\epsilon_{d}$ are dielectric constants of air and insulator, $t_{d}$ is the thickness of insulator. The darker square block after proton irradiation in Fig. 4 indicates that the density of traps which have the same capture and emission time was increased by irradiation damage. The interface trap density ($D_{it}$) was extracted by conductance method with frequency dependent C-V characteristics measured at the frequency range from 1 kHz to 1 MHz. The $D_{it}$ was increased from 8 1012 to 1.9 10¹³ cm^-2 V^-1 at $E_{C}$ – $E_{t}$ = 0.44 eV before and after proton irradiation. Since the deterioration of the interface beneath the gate region was detected by CET map and conductance method, one can assume that the interface in the access region can be also degraded. $SiN_{X}$ gate insulator exists as a passivation layer in the access region. Therefore, the increase of interface states in the access region can adversely affect the trapping effects observed previously.

Fig. 4. CET maps in normally-off AlGaN/GaN recessed MISHFETs (a) before, (b) after 5-MeV proton irradiation.

Fig. 5. Interface trap density ($D_{it}$) extracted using conductance method in normally-off AlGaN/GaN recessed MISHFETs before and after 5-MeV proton irradiation.

Unlike aforementioned degradation characteristics, the off-state breakdown voltage of normally-off AlGaN/GaN recessed MISHFETs was improved after 5-MeV proton irradiation, as shown in Fig. 6(a). The irradiated devices exhibited the increase of the breakdown voltage from 530 to 670 V. The improvement of breakdown voltage in AlGaN/GaN high electron mobility transistors (HEMTs after proton irradiation was previously reported to be attributed to the reduction of peak electric field at gate edge^[22,23]. TCAD simulation using Silvaco Atlas was performed to investigate the electric field distribution in normally-off AlGaN/GaN recessed MISHFETs. Fig. 6(b) exhibited the simulation results employing negatively charged traps like Ref^[22] at $V_{D}$ = 600 V and $V_{G}$ = 0 V. The peak electric field of gate edge at drain side was reduced by 40 %, therefore the off-state breakdown voltage could be improved. The lateral field across the access region was increased as opposed to the field at a gate edge. In short-term stress tests, proton-irradiated devices exhibited more degradation by the same electrical stress than non-irradiated ones did. The increased instability caused by the trapping effects during the short-term stress can be attributed to the increase of trap density and lateral field between the access region after proton-irradiation.

Fig. 6. (a) The off-state breakdown characteristics measured from AlGaN/GaN MISHFET, (b) the electric field distribution obtained from TCAD simulation.

IV. CONCLUSIONS

The charge trapping-related instability of normally-off AlGaN/GaN recessed MISHFETs was investigated before and after proton irradiation. The short-term stressed devices showed the temporary reduction of $I_{D}$ due to charge trapping. The $I_{D}$ reduction induced by stress tests was increased after proton irradiation under the same stress condition. CET map and conductance method revealed that the interface states were increased by irradiation-induced damage. TCAD simulation showed the increase of electric field in the access region. Interface traps and electric field increased by the radiation effect will produce more charge trapping-related instability.