Gallium nitride (GaN) is a promising material for next-generation power devices due to its wide band gap, which allows a large breakdown electric field and the possibility of operating under harsh environmental conditions [1
]. Such characteristics make these devices promising for space applications, where temperature and radiation are key factors. Particularly, the development of new GaN high-electron-mobility transistors (HEMTs) with great characteristics, such as low on-resistance and parasitic capacitances, allow them to switch at higher frequencies with high efficiency, making them attractive. The inherent radiation hardness, the capability to withstand higher breakdown voltages, and the higher operating temperatures will enable this technology’s use in future space applications, such as telecommunications, Earth observation, and science missions [4
These promising advantages have pushed the research focus on the reliability of GaN devices under radiation conditions. In space environments, energetic particles which impact semiconductor devices lose their energy to ionizing and nonionizing processes while they travel through the devices. The energy loss causes the production of electron–hole pairs (ionization) and displaced atoms (displacement damage). Gamma irradiation is one of the tests used to evaluate the hardness of devices to be used in aerospace applications. Our main objective here was to study the degradation induced by the total ionizing dose (TID) effects of 60
Co γ-ray radiation on GaN HEMTs. The response to gamma irradiation is complex. Compton electrons induced by γ-radiation create electron–hole pairs, thus changing the occupancy of traps. Regarding this topic, many research papers have been published in recent years showing different behaviors depending on the dose applied and the structure of the HEMT being irradiated [6
]. In general, HEMTs irradiated with gamma rays exhibit a negative threshold voltage and a transconductance decrease, which can be explained by the creation of trap states throughout the structure and, in some cases, an increase in the two-dimensional electron gas (2DEG) sheet concentration [7
]. Some authors have reported strain relaxation at low-dose gamma irradiation, which enhanced the channel mobility [9
]. In contrast, other authors [11
] have reported a reduction of 60% of the drain current at around 70 krad(Si). Thus, the defects generated by the γ-irradiation are very sensitive to the structure, having defect creation rates dependent on the quality of the sample and the doping level.
On the other hand, it is well known that another problem related to the GaN HEMT structure is the electron trapping effects which decrease device performance [12
], in particular, the dynamic on-resistance (RON_dyn
]. This trapping reduces the current that the devices can drive below the device’s rated current and could be attributed to a trapping effect in different regions inside the device. In [14
], it was reported that trapping could be attributed to the device surface and buffer layer and that it was possible to distinguish between both. These trapping effects have been shown to be induced by the electrical field applied between the drain and the gate. Additionally, it was confirmed that the HEMT structure design is a key factor regarding the RON_dyn
. Different strategies can be used to mitigate the increase of RON_dyn
, mainly, the use of a p-GaN region close to the drain that is electrically connected to the drain edge [15
] and the optimization of the device buffer layer design [16
Taking into account the problems regarding the effects of γ-radiation and RON_dyn
, it is necessary to evaluate the effect of γ-radiation on RON_dyn
if GaN HEMTs are to be seriously considered for future space applications. In fact, international space agencies, such as the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA), have shown increasing interest in evaluating the development of GaN devices. Some testing activities have been included as part of a future radiation qualification process for wide band gap devices, as presented in [17
Analyzing the relevant literature shows that some contradictory results have been reported and the reasons for these differences are not evident. Furthermore, a previous study reported changes in RON_dyn
] with low-dose gamma radiation. Therefore, in this study, we investigated not only the static characteristics but also the RON_dyn
behavior of two different HEMT structures subjected to gamma radiation until 3.7 Mrad(Si). New results on this subject are shown in this work related to the HEMT structure and the biasing conditions applied during irradiation. These results show that, depending on the structure and the bias applied, trapping can be increased (case of negative gate bias) or reduced (case of shorted devices), which provides more information than previous studies, where only static characterization had been done.
2. Materials and Methods
Normally-off commercial AlGaN/GaN HEMTs on Si with a voltage rating of 600 V were used for the radiation experiment. We chose two different HEMT structures: One was a 600 V p-doped GaN gate injection transistor (HD-GIT) PGA26E07BA, manufactured by Panasonic, Inc. (Kadoma-shi, Osaka, Japan). The HD-GIT has an additional p-GaN region between the gate and the drain, in which the holes are injected during the off-state, compensating the electron trapping and avoiding RON_dyn
. The other was the GS66516T, a 650 V p-GaN gate with a metal insulator layer (MISHEMT), manufactured by GaN Systems, Inc. (Ottawa, ON, Canada). Table 1
summarizes the key parameters of the investigated devices.
For the TID radiation experiment, 12 devices from each HEMT structure were used. As shown in Table 2
, different bias conditions were applied during irradiation to evaluate the bias dependence of the TID response of the HEMTs. In addition, one sample of each device type was selected as a reference (control device) to confirm the proper operation of the measurement system; that is, these unirradiated devices were subjected only to electrical measurements after each step, without any bias or radiation conditions applied.
The test campaign was carried out in the CNA-RadLab facility at the National Center for Accelerators in Seville, Spain. The gamma irradiation contained a 60
Co gamma source with associated photon energies of 1.17 and 1.33 MeV (mean value: 1.25 MeV). The selected dose rate was 23.742 krad(Si)/h, which is within the “standard rate” window (0.36–180 krad(Si)/h) of the ESA, according to the TID Test Method [19
]. The dose rate was obtained by measuring the charge with two TM30013 ionization chambers (PTW-FREIBURG, Germany) and one multichannel electrometer, MultiDOS (PTW-FREIBURG, Germany), and also considering the environmental correction factor. The dose rate uniformity in the filter box was 98.5%. The devices under test (DUTs) were mounted on a printed circuit board which was placed into a 12 × 17 cm filter box to be subjected to radiation, in compliance with the European Space Components Coordination Basic Specification No. 22900 [19
]. This container had 2 mm of aluminum and 1.5 mm of lead in the outer layer and a 5 mm front cover of polymethyl methacrylate (PMMA) to achieve the charged-particle equilibrium.
Six irradiation steps were carried out during the campaign. Post-irradiation electrical measurements were performed after each exposure step for all the devices, including the control devices. At the end of total irradiation, two annealing steps were implemented. The first step consisted of room-temperature annealing under bias for 24 h. Afterwards, accelerated aging was carried out, where the devices were baked at 100 ± 5 °C under bias for 168 h. In both annealing steps the bias voltage applied on each DUT was the same as that during the irradiation steps.
Concerning the measurements, two types were performed: I–V measurements were done with a Keysight Power Device Analyzer B1505A (Santa Rosa, CA, USA), and for the RON_dyn
measurements, a custom circuit (Figure 1
) was implemented [20
The implemented switching circuit had the benefit of fully controlling the time that the voltage stress was applied to the GaN HEMT. Basically, it consisted of two transistors connected in series between the drain and the source, with a resistive load between both of them. Transistor Q1 was used to control the stress/trapping time. A resistive load Rload was used to set the current level when DUT was in on-state. Due to the inherent parasitic inductance (Lp) of the power Rload, two SiC diodes (C4D05120)—D1 and D2—offered a freewheeling path for the current when either Q1 or DUT was switched from on to off.
For the transistor Q1, a SiC MOSFET (C3M0065090) (Wolfspeed, Durham, NC, USA) was used in order to have a low output capacitance (CDS) and low current peaks due the charge and discharge of this parasitic capacitance. The values of the drain-to-source on-resistance (RDSON) of the DUT were obtained by measuring the device on-state voltage (VDSON) across it and dividing by the current (IDS) through the DUT. The drain current was measured using a coaxial shunt resistor of 98 mΩ (SDN-414-10), and for the VDS, a 300 V and 500 MHz passive voltage probe was selected.
Due to the high voltage applied to the DUT, the voltage across it represents a large dynamic range input signal for the oscilloscope input amplifier acquisitions, which can be overloaded, and as a result, an accurate determination of the on-state voltage would not be achievable. To avoid that problem, a voltage clamp circuit together with the passive voltage probe was used. In particular, the voltage clamp used was the commercial clp1500V15A1 from Springburo GmbH Emmendingen, Baden-Wurtemberg, Germany). The low range (2 V) was selected in the voltage clipper in order to have a faster response—200 ns in this case—considering the passive voltage probe and the voltage clipper. Precise frequency response compensation was done in the passive voltage probe to make up for the whole chain of the clipper and the voltage probe.
To control the “on-time” of the DUT, a generic MOSFET isolated driver SI8271BB (Silicon Labs, Austin, TX, USA) was used. This driver was selected due to the minimum supply voltage needed of 3 V. This low gate voltage was required to drive GaN devices with VG = 4 V, which allowed us to see any change in the trapping charges when measuring the RON_dyn. This is because, at a higher gate voltage, the 2DEG density at the AlGaN/GaN interface is higher, so the device is able to drive low drain currents without being affected by the trapped charges in the surface or the buffer. Otherwise, if we had used a lower gate voltage value, the density of the 2DEG would decrease and we could see any changes in the RON_dyn due to the trapping, even at low currents. Thus, in both cases, the trapped charges are present; however, in the case of the higher gate voltage, it would require higher currents to see the effect by measuring the RON_dyn. Thus, instead of increasing the current, which could induce other problems such as self-heating, which would change the dynamic response, we chose to use a lower gate voltage to see any changes in the device trapping.
The results confirmed the relevance of the GaN HEMT structure used, since the HD-GIT structure was robust in all of the steps of irradiation, with practically no changes in any of the properties measured. However, the MISHEMT structure suffered many changes. These differences were mainly due to two factors. The first is the removal of traps inside the HD-GIT device due to the use of an additional p-GaN region near the drain, which was electrically connected to the drain that effectively released the trapped charges. The second factor is the use of an insulator in the GaN MISHEMT. The insulator improved the gate performance, allowing a higher threshold voltage and reducing the gate leakage current, but in radiation environment conditions, the metal insulator has to be highly optimized; otherwise, some reliability problems can appear.
In this study, the results show that due to these differences, the GaN MISHEMT has different behavior depending on the bias applied during irradiation. In the case of shorted devices during irradiation, hole trapping in the insulator takes place, which means a reduction of the effectiveness of channel depletion. This hole trapping is due to the increase of the energy of electron–hole pairs due to radiation, which allows them to gain enough energy to become trapped in the gate dielectric [8
]. This induces a negative threshold movement, an increase of the drain leakage current, an increase in gate current, and a reduction of the RON_dyn
favored by the increase in the drain leakage current, as reported in [14
The second bias condition studied was the devices subjected only to drain voltage. These devices experienced two phenomena: the damage on the insulator generating hole trapping, such as the shorted devices, which explains the same increase in the drain leakage current, gate current, and the decrease of RON_dyn. However, the positive threshold movement of these devices was due to electron trapping at the surface, which is a common effect for these devices when submitted to high drain voltages.
When a negative gate voltage was also applied to the devices, both electron trapping on the surface due to the high drain voltage and electron trapping under the gate took place [23
]. Therefore, on these devices, three effects took place together: the hole trapping in the damaged insulator, which induced an increase in gate current; the surface trapping due to the high drain voltage applied, which induced the positive threshold shift; and electron trapping under the gate due to the negative gate voltage applied, which induced a more negative shift of the gate current compared with the different biased devices. This trapping under the gate partially compensated the effects of the hole trapping in the insulator, resulting in these devices suffering less of an increase in drain leakage current. This also meant a reduction of the detrapping rate, which favored a greater increase in the RON_dyn
instead of the decrease suffered by the different biased devices.
In this work, different behaviors were observed for GaN HEMTs subjected to gamma radiation which were structure-dependent. While HD-GIT HEMT characteristics were mainly unchanged during irradiation, the GaN MISHEMT structure underwent some changes. The results demonstrated that a degradation of the insulator took place during irradiation, which allowed hole trapping to induce a negative threshold voltage shift, an increase in the forward gate current and drain leakage current, and a reduction of the RON_dyn. Additionally, the devices subjected to drain voltage during irradiation also suffered electron trapping on the surface due to the reduced barrier heights of the traps, which was the result of radiation inducing a positive threshold shift. In the case of devices biased with positive drain and negative gate voltages, they also suffered from trapping under the gate, which compensated the hole trapping in the insulator and forced an increase in the RON_dyn.
Therefore, the structure is one of the main factors that determines the reliability of GaN HEMTs under radiation, and here, the HD-GIT proved to be much more robust than the MISHEMT. This can be due to two factors. The first is the low reliability of the MISHEMT insulator during irradiation, as it is a weak region for the injection of traps. The existence of a p-doped region near the drain which removes the trapping is also crucial because the degradations reported were mainly due to trapping effects. In addition, different behaviors can take place in a MISHEMT when applying gamma radiation depending on bias condition. The results obtained here on the RON_dyn are necessary to consider when framing AlGaN/GaN radiation assurance tests, and they should be especially considered when using GaN HEMTs for power conversion units in future space missions.