Commercially available AlGaN/GaN high-electron-mobility transistors (HEMTs) are beginning to enter the public scene from a range of suppliers. Based on previous studies, commercial GaN-based electronics are expected to be tolerant to different types of irradiation in space. To test this assumption, we compared the characteristic electrical curves obtained at different X-ray irradiation doses for GaN HEMT devices manufactured by Infineon and Transphorm. The p-GaN-based device was found to be more robust with a stable threshold voltage, whereas the threshold voltage of the device with a metal-insulator-semiconductor gate was found to shift first in the negative and then the positive direction. This dynamic phenomenon is caused by the releasing and trapping effects of radiation-induced charges in the dielectric layer and at the interface of irradiated devices. As such, the p-GaN-gate-based GaN HEMT provides a promising solution for use as an electric source in space.

  • GaN MIS-HEMT together with p-GaN gate based GaN device are compared under different X-ray irradiation doses.

  • Dynamic phenomenon in threshold voltage of GaN MIS-HEMT after X-ray irradiation is deeply discussed.

  • Enhancement-mode GaN HEMT fabricated using p-GaN gate has a stable threshold voltage, possessing huge potential in space use of electric source.

When orbiting in space, satellites and their components must survive in its harsh irradiation environment. To be useful while in orbit, materials and devices must tolerate damage caused by numerous types of cosmic irradiation.1 The irradiation from solar flares primarily comprises X-rays, gamma-rays (γ-rays), protons, and electrons.2 A fundamental understanding of the reliability and failure mechanisms of these devices is critical to further technological development and commercialization.

In recent years, research groups from around the world have extensively studied the effects of 60Co γ-ray irradiation on the conventional depletion-mode (D-mode) AlGaN/GaN high-electron-mobility transistor (HEMT) (with a Schottky gate for contact between a metal and semiconductor).3 Significant degradation of AlGaN/GaN HEMTs was observed after γ-ray (60Co) doses of just tens or hundreds of Mrad(Si).4 The devices show a negative shift in their threshold voltage (VTH), which is predominantly due to an increase in trap density. Moreover, the results of other experiments suggest that damage due to particle irradiation is of greatest concern in GaN-based devices,5–7 which means that GaN-based HEMTs are more sensitive to displacement damage than ionization effects.

To date, enhancement-mode (E-mode) devices with a p-GaN gate have been considered to be the most commercially promising technological approach,8 and the irradiation reliability of p-GaN-gate-based GaN HEMTs requires in-depth study. In this paper, we compare the X-ray irradiation tolerance of devices with a p-GaN gate and a metal-insulator-semiconductor (MIS) structure, and discuss the mechanism of the VTH shift.

To gain insight into the irradiation tolerance of GaN-based HEMTs, we conducted irradiation-damage tests on two commercial GaN devices. The first device, the IGO60R070D1 from Infineon identified as device type “A”, has a p-GaN layer in the gate region to realize the E-mode. The second device, the TPH3205 from Transphorm Inc., is identified as device type “B”. This is a D-mode device in which the MIS-HEMT is separated from a cascade structure. No special structure was employed in either device to improve vulnerability to the radiation environment.

As shown in the cross-section image of device A in Fig. 1(a), the p-GaN thickness is approximately 240 nm. This device has a p-GaN ridge on each side of the drain electrodes to suppress current collapse,9 which is clearly observable in the magnified image outlined in red. Fig. 1(b) shows the corresponding cross-section structure of device B, the dielectric insulator thickness of which is a few tens of nanometers.

Fig. 1.

Cross-section images of (a) IGO60R070D1 and (b) TPH3205 devices.

Fig. 1.

Cross-section images of (a) IGO60R070D1 and (b) TPH3205 devices.

Close modal

In the experiment, the top surfaces of the GaN devices were subjected to X-ray irradiation at a normal dosage rate of 100 rad/s. The accumulated dose was varied from 100 krad to 200 krad to study the influence on the electrical parameters of devices A and B. After irradiation, the I-V curves were obtained instantly using an Agilent 1050B system.

Fig. 2 shows the output and transfer curves of device A with the p-GaN gate, in which no drain current (IDS) decay or VTH shift are evident. According to the inset of Fig. 2(b), the VTH shifts slightly in the positive direction, which agrees with previously reported results,10 and which can be ascribed to acceptor-like traps. As is well known, when exiting the epitaxial layer, high-density H+ contamination can lower the activation rate of the acceptor impurities during the epitaxial growth process.11 When irradiated by high-energy X-ray, the inactivated acceptor impurities (Mg ions) can be activated, which can lead to the VTH shift of the p-GaN gate device. This may be one of the irradiation-induced traps, with energy levels in the gap of 0.48 eV, 1.02 eV, 1.50 eV, 2.42 eV, and 3.28 eV.12 

Fig. 2.

(a) Output and (b) transfer curves of device IGO60R070D1 with p-GaN gate.

Fig. 2.

(a) Output and (b) transfer curves of device IGO60R070D1 with p-GaN gate.

Close modal

One proposed explanation for the high irradiation tolerance of GaN-based HEMTs is that the GaN material is so intrinsically defective that the creation of more defects by irradiation makes little difference.13 Another proposed explanation is that the threshold energy (Ed) value for the atomic displacement of GaN is higher than those of other III-V materials such as GaAs, so proportionately fewer atoms can be displaced.14 

Fig. 3 shows the output and transfer curves of device B with the MIS gate, in which we can see that the IDS value increases with increases in the incident dose from 100 krad to 200 krad. This can be attributed to increases in the density of the two-dimensional electron gas (2DEG) under X-ray irradiation. However, this value returns to a low level after one-hour annealing, as shown in the inset of Fig. 3(a). As shown in Fig. 3(b), the VTH first shifts dramatically in the negative direction with increases in the incident dose and then shifts in the positive direction to -14.5 V after one-hour annealing.

Fig. 3.

(a) Output and (b) transfer curves of device TPH3205 with MIS gate.

Fig. 3.

(a) Output and (b) transfer curves of device TPH3205 with MIS gate.

Close modal

With respect to the MIS-HEMT, the VTH can be calculated using the formula provided in:15 

V TH = 2 φ F + φ B + φ ms Q ot + Q it C ox Q m C ox
(1)

where φF is the potential difference between the middle of the band and the Fermi level, φms is the contact potential difference between the gate metal and semiconductor, and φB is the potential difference between the space charge in the semiconductor and insulator. Qm, Qot, and Qit are the mobile charges in the insulator, the fixed charges, and the interfacial charges, respectively. Cox is the insulator capacitance, which is expressed as:

C ox = ε ox ε o d ox
(2)

where εox and εo are the relative dielectric constant and permittivity of free space, respectively, dox is the thickness of the insulator, and the capacitance is assumed to be constant in the following analysis.

During the X-ray irradiation process, energy is transferred to an electron in the valence band from the incoming particle, which raises it to the conduction band and creates a corresponding hole in the valence band that causes the production of electron-hole pairs (ionization).16 The density of the electron-hole pairs is influenced by the sample quality and the doping level. Fig. 4 shows a schematic of the shift of the irradiation-induced excited carriers under electrical stress.17,18 A constant negative voltage is applied to the gate, which is used to accelerate the transfer of excited electrons to the potential well during the irradiation process. Meanwhile, parts of the excited holes are trapped at deep levels in the insulator to form positive fixed space charges, which cause the VTH to undergo a negative shift because of the increase in Qot, as shown in Eq. (1). This is a dynamic process influenced by both the irradiation and applied electrical stress on the gate.

Fig. 4.

Schematic of the shift in the irradiation-induced electron and hole pairs driven by the applied gate voltage.

Fig. 4.

Schematic of the shift in the irradiation-induced electron and hole pairs driven by the applied gate voltage.

Close modal

After a one-hour recovery period (device was placed in air at room temperature), the VTH exhibited a positive shift, as shown in Fig. 3(b), which is ascribed to the N vacancies and divacancies. As shown in the schematic in Fig. 5, these acceptor-like traps at the interface were negatively charged during the recovery procedure, which led to a decrease in the 2DEG density and a positive shift in the VTH. Overall, the increase in the interfacial negative charge Qit shifted the VTH in the positive direction, as can be inferred from Equation (1).

Fig. 5.

Schematic showing the trapped electrons reduce the density of 2DEG.

Fig. 5.

Schematic showing the trapped electrons reduce the density of 2DEG.

Close modal

In summary, the electrical characteristics of the p-GaN- and MIS gate GaN HEMTs (commercial devices) were studied under different doses of X-ray irradiation. The negative VTH shift in the MIS-HEMT might be caused by intrinsic free ions and irradiation-induced electron-hole pairs in the dielectric layer, and the positive VTH shift and current degradation can be modeled by the negatively trapped charge at the interface near the 2DEG. The experimental results suggest that GaN HEMTs with a p-GaN gate are a promising radiation-hardened solution for power-switching converter applications in space.

The authors declare no conflict of interest.

This work was supported by the National Key R&D Program of China (No. 2017YFB0402800, 2017YFB0402802).

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