Abstract
1. Introduction
Silicon carbide (SiC) metal–oxide–semiconductor field-effect transistors (MOSFETs) have the advantages of high frequency, high efficiency, and high current density compared with Si devices. These advantages have attracted attention in the pursuit of lightweight structures, miniaturization, and low power consumption in the aerospace electronics industry; therefore, SiC power MOSFETs have very broad applications in the aerospace field[
In the space radiation environment, the total ionizing dose (TID) induced by gamma rays is one of the most important factors that cause the failure of electronic devices. Recently, the radiation response of SiC power MOSFETs to gamma ray irradiation has been studied by several authors[
Figure 1.Potential application examples using SiC devices at various voltage and frequency levels[
In this study, the relationship of the performance of SiC power MOSFETs and switching frequencies varying from 1 kHz to 10 MHz is first explored in the harsh TID environment. On the basis of the above work, the mechanism by which the operating frequency influences the radiation response characteristics of the device is further discussed, and we propose that it is feasible to improve the radiation resistance ability of SiC power MOSFETs by appropriately changing the switching conditions.
2. Experimental details
The experimental devices used were commercial 44 A 1200 V N channel SiC MOSFETs (CGE1M120060) fabricated at the Beijing Cengol Semiconductor Co., Ltd.. on 4H-SiC epitaxial layers. In fact, our experimental devices include SiC MOSFETs of Cree (C2M0080120D) and Rohm(SCT2450KE). Since the experimental results are basically similar, we only present the experimental results of this device (CGE1M120060) below. The samples were divided into five groups. The gate signal amplitude was 15 V, the high potential was 15 V, the low potential was 0 V, and the duty cycle was 50%. The switching frequencies of the five groups were set at 1 kHz, 10 kHz, 100 kHz, 1 MHz, and 10 MHz. The control group was divided into two groups: the first group was biased at 0 V gate bias, and the second group was exposed at a positive voltage of 15 V.
In the experiment, according to the test conditions given in the device manual, BC3193 was used to measure the breakdown voltage BVDSS and drain–source leakage current IDSS of the devices at room temperature (25 °C). A Keithley 4200CSC semiconductor device analyser was used to test the threshold voltage Vth before and after irradiation. These samples were irradiated at the Xinjiang Institute of Physics and Chemistry, Chinese Academy of Sciences by using a 60Co-γ source up to 300 krad(Si) at a dose rate of 200 rad (Si)/s.
3. Results and discussion
Fig. 2 shows the changes in the threshold voltage Vth in the total dose radiation environment of SiC power MOSFETs under the operating states of ON, OFF, and different frequencies. As shown in Fig. 2(a), The degradation in Vth was minimized at the off operating state with a gate bias of 0 V, while it was maximized when the gate bias was 15 V. The degradation varied at different frequencies between these two offsets. Note that the drift of Vth increased with the increase in the operating frequency at the same dose point (see the discussion and analysis below and Fig. 2(b)).
Figure 2.(Color online) SiC power MOSFET. (a) Variation in threshold voltage with the total ionizing dose at ON, OFF, and different frequencies. (b) Relation between the change in threshold voltage and applied switching frequency under the same total ionizing dose.
Fig. 3 shows the relationship between the drain–source leakage current IDSS (Fig. 3(a)) and the drain–source breakdown voltage BVDSS (Fig. 3(b)) of the SiC power MOSFET with various total doses under the ON, OFF, and switching conditions of different frequencies. As shown in Fig. 3, the trend of the two parameters with the change in the dose point of the irradiation was not the same exactly; the drain–source leakage current IDSS with the total ionizing dose gradually increases and the breakdown voltage significantly decreases when the total ionizing dose reaches a certain value. However, they all showed that the damage degradation strongly depended on switching frequency. For example, as shown in Fig. 3(b), under the same circumstances, SiC power MOSFETs switching on 10 MHz irradiation to 150 krad (Si) completely lost their blocking function, and 1 kHz samples in the 250 krad(Si) region lost the ability to block high voltage.
Figure 3.(Color online) SiC power MOSFETs at ON, OFF, and different frequencies. (a) Variation in drain–source leakage current with the total ionizing dose. (b) Change in breakdown voltage with the total ionizing dose.
The degradation of the properties and parameters of SiC power MOSFETs in the total dose radiation environment is mainly attributed to the oxide charge and interface states generated and accumulated by ionizing radiation near the SiC/SiO2 interface[
Under a switching frequency bias, although the bias voltage changes constantly, its range is between 0 and 15 V. Therefore, this mechanism also explains the parameters and performance degradation of SiC power MOSFETs at different frequencies. If we continue to infer according to this model, it is reasonable to conclude that the radiation degradation should be approximately the same regardless of the operating frequency, because under the 50% duty cycle condition, the total time of the 0 V bias and the total time of the 15 V bias during irradiation do not vary with frequency. Our test results show that the radiation damage, including the parameters and function of the SiC power MOSFETs, depends on the operating frequency to a certain extent. The static ionising radiation damage model of the SiC power MOSFETs cannot explain the actual radiation damage in the dynamic switching state.
We can try to understand the above phenomenon with the oxide hole trap model of the US Army Research Laboratory (ARL)[
Figure 4.A model hole trapping [(a) to (b)] and detrapping [(c) to (a)] processes are indicated, along with the intermediate compensation/reverse-annealing phenomenon [(b) to (c) and (c) to (b)][
To further verify our inference, we designed another irradiation experiment with an alternating gate bias at 15 and 0 V with different total doses under the same experimental conditions; the experimental results are shown in Fig. 5. No obvious recovery of the threshold voltage was found at the first low potential, because the positive charge accumulation process was stronger than the recovery process. At the second and third low potentials, a significant recovery of the threshold voltage was observed. At the dose point of 300 krad (Si), the negative drift of the threshold voltage was approximately –1.5 V, and this value is lower than the degradation seen in Fig. 2 under the minimum value of 1 kHz. These experimental results have confirmed the accuracy of our theoretical analysis to a certain extent.
Figure 5.(Color online) Comparison of threshold voltage of SiC Power MOSFETs with the total ionizing dose under 15 and 0 V segment gate bias, constant 0 V gate bias, and constant 15 V gate bias.
4. Conclusion
In summary, the results of this study show that TID radiation damage of the SiC power MOSFETs is related to the magnitude and direction of the electric field applied during irradiation, and it also strongly depends on the operating frequency. Under the same conditions, the TID radiation damage of SiC power MOSFETs will be aggravated as the frequency of the device increases. Further, a low frequency can be consciously selected or reduced to improve the radiation resistance ability of SiC power MOSFETs under permissible application conditions. The results suggest that we can develop a targeted TID test evaluation method according to the actual switching frequency of SiC power MOSFETs.
Acknowledgements
This work was supported by the National Natural Science Foundation of China under Grant No. 11975305 and the West Light Foundation of The Chinese Academy of Sciences, Grant No. 2017-XBQNXZ-B-008.
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