Study on the Single-Event Burnout Effect Mechanism of SiC MOSFETs Induced by Heavy Ions

Abstract

As a prominent focus in high-voltage power devices, SiC MOSFETs have broad application prospects in the aerospace field. Due to the unique characteristics of the space radiation environment, the reliability of SiC MOSFETs concerning single-event effects (SEEs) has garnered widespread attention. In this study, we employed accelerator-heavy ion irradiation experiments to study the degradation characteristics for SEEs of 1.2 kV SiC MOSFETs under different bias voltages and temperature conditions. The experimental results indicate that when the drain-source voltage (V_DS) exceeds 300 V, the device leakage current increases sharply, and even single-event burnout (SEB) occurs. Furthermore, a negative gate bias (V_GS) can make SEB more likely via gate damage and Poole–Frenkel emission (PF), reducing the V_DS threshold of the device. The radiation degradation behavior of SiC MOSFETs at different temperatures was compared and analyzed, showing that although high temperatures can increase the safe operating voltage of V_DS, they can also cause more severe latent gate damage. Through an in-depth analysis of the experimental data, the physical mechanism by which heavy ion irradiation causes gate leakage in SiC MOSFETs was explored. These research findings provide an essential basis for the reliable design of SiC MOSFETs in aerospace applications.

1. Introduction

SiC MOSFETs exhibit outstanding advantages, such as high operating voltage, low power consumption, and high operating temperature, making them high-voltage power devices of significant interest in the aerospace field [1,2,3]. In response to the application requirements of the space radiation environment, both domestic and international researchers have investigated the effects of radiation on SiC MOSFETs, including total dose effects, SEEs, and displacement damage effects [4,5,6]. Studies have shown that SEEs, particularly SEBs, caused by cosmic rays and particles, seriously threaten the reliability of SiC MOSFETs. Robert et al. [7] found that when the incident particle linear energy transfer (LET) is greater than 10 MeV·cm²/mg and the bias voltage is 100 V, irreversible leakage current degradation can occur, and that at 400 V, the device will suffer SEB damage. They also noted [8] that the impact of the leakage current should be evaluated based on the application conditions of the device. Martinella et al. [4,5,9] found that the permanent damage caused by ions to the internal structures of SiC MOSFETs leads to a decrease in leakage current and breakdown voltage, which is directly related to the LET of the incident particles and the drain-source bias voltage. Witulski et al. [10,11] discovered that under the influence of an electric field, the electron–hole pairs generated by heavy ion ionization accumulate, causing the local temperature to reach 3600 K, surpassing the melting point of SiC and resulting in lattice damage, thereby causing SEB. In contrast, leakage current degradation is caused by the eutectic state of the SiC and the metal, without complete lattice structure damage. Combining domestic and international research progress, it is known that the radiation effect degradation performances and mechanisms of SiC MOSFETs differ from those of traditional Si-based devices. Research on the radiation effects of SiC MOSFETs is highly important for promoting their reliable space application.

This paper focuses on the 1.2-kilovolt SiC MOSFET by conducting accelerator heavy ion irradiation experiments to study the SEEs. The degradation performance of SiC MOSFETs under different bias voltages and temperature conditions was analyzed, and the physical mechanism of heavy ion irradiation leading to gate leakage was explored. This research provides a foundation for the reliable application of SiC MOSFETs.

2. Experimental Setup

The experimental samples used were fourth-generation symmetric trench gate SiC MOSFETs from Rohm (Kyoto, Japan), with an average operating voltage of 1.2 kV, packaged in TO-247-4 L. The heavy ion irradiation experiment was conducted using an HI-13 tandem accelerator in Beijing. The heavy ions chosen for irradiation were 138 MeV Cl particles, with LET of 14.63 MeV·cm²·mg⁻¹ and range of 26.11 μm in SiC. Before irradiation, all the samples were decapped and dies in them were exposed. Two samples were irradiated under each experimental condition, and a switch matrix was used to determine the power on and current monitoring of irradiated devices.

A block representation of the experimental setup (shown in Figure 1) was constructed to monitor the drain-source and gate-source currents of the irradiated device in real time. Two Keithley source measure units (SMUs), models 2470 and 2410, were used to bias gate (V_GS) and drain (V_DS), respectively. Furthermore, model 2410 was used to monitor the drain currents (I_D). The gate and drain were connected to the positive poles of the two source meters, and the source was connected to the negative poles of the two source meters simultaneously to monitor the two currents separately. The source meters were remotely controlled by a host computer for program control and data storage. Before the beam was turned on, the gate and source of the SiC MOSFET were short-circuited, i.e., V_GS = 0 V and V_DS = 100 V. After irradiation started, the process stopped when the total amount of incident ions reached 1 × 10⁶ cm⁻². Next, the irradiation continued after increasing the V_DS until the drain-source current exceeded 1 × 10⁻⁴ A, indicating the occurrence of SEB, and the irradiation experiment ended.

3. Results and Discussion

3.1. Degradation Characteristics of SiC MOSFET under Heavy Ion Irradiation

Figure 2 shows the changes in the gate current, drain current, and V_DS of the SiC MOSFETs with respect to the irradiation time during the heavy ion irradiation experiment. The heavy ion injection rate in this round of experiments was 1.18 × 10⁴ cm⁻²·s⁻¹. As shown in the figure, when V_DS < 300 V, the drain current of the device remains small and relatively stable, with minimal changes in V_DS. Above V_DS = 400 V, the drain current rapidly increases by three orders of magnitude, and when the voltage reaches V_DS = 450 V, the drain current instantly spikes, leading to SEB.

The effect of the gate-source bias voltage V_GS on device SEB degradation was studied. In this round of experiments, the devices were placed in an off state, V_GS = −10 V, and the heavy ion injection rate was 1.16 × 10⁴ cm⁻²·s⁻¹. The curves of the gate current, drain current, and V_DS of the SiC MOSFET with respect to the irradiation time during the heavy ion irradiation experiment are shown in Figure 3. When V_DS = 100 V, the drain current increases significantly compared with that at V_GS = −10 V, as shown in Figure 2, and the drain current shows great stability under different V_DS levels. Until V_DS = 400 V, the drain current gradually increases with the injection rate of the irradiated particles, and drain current degradation occurs, but SEB does not occur. Finally, when V_DS = 425 V, the current change slope increases. Under these conditions, the cumulative irradiation time is approximately 20 s, and SEB occurs when the current reaches the preset threshold. It can be concluded that when V_GS = −10 V and V_DS < 350 V, the SiC MOSFET has no sensitive response to radiation injection and can maintain a relatively stable working state.

The experiment was conducted again with the gate-source bias changed. Figure 4 shows the drain current and V_DS values of the SiC MOSFET with respect to the fluence with V_GS = −15 V. In this case, the heavy ion injection rate was 1.15 × 10⁴ cm⁻²·s⁻¹. As shown in the figure, when V_DS = 400 V, the drain current began to increase with increasing incident injection, and SEB occurred after approximately 20 s of cumulative irradiation under these conditions. Similarly, when the initial V_DS voltage was 100 V, the drain current was maintained at a high level, and the drain current was stable under different bias conditions. Figure 4 shows that the sample can maintain a relatively stable working state when V_GS = −15 V and V_DS < 375 V, and that it will not respond sensitively to irradiation.

Comparing the SEBs of the samples under three gate-source bias conditions, Figure 2, Figure 3 and Figure 4 show that the SEB threshold voltage decreases with increases in the absolute value of the negative bias of the gate-source voltage. This is because different gate-source biases cause different degrees of damage to the internal oxide layer of the SiC MOSFET [12]. A post-irradiation gate stress (PIGS) test was carried out to explore the relationship between the potential damage to the gate oxide and the gate-source bias voltage V_GS during the irradiation, and the results are shown in Figure 5. After irradiation with V_GS = 0 V, the gate leakage current I_GS had no obvious change; after irradiation with V_GS = −10 V, the device I_GS was one order of magnitude larger than that before irradiation; after irradiation with V_GS = −15 V, the I_GS changed significantly with the gate voltage, showing a trend of stabilizing at 1 µA, which indicates that particle irradiation under negative V_GS conditions causes the SiC MOSFET to have more serious oxide potential damage, verifying the conclusion that oxide potential damage becomes more serious with increasing gate-source bias stress.

The main leakage current transport mechanisms in the SiC MOSFET gate oxide layer include thermal electron emission, Fowler–Nordheim tunneling (FN), direct tunneling, and Poole–Frenkel emission (PF). For SiC MOSFETs, in general, when the electric field in the SiC barrier layer is sufficiently large, the reverse gate leakage current mainly originates from FN tunneling. PF emission is a trap-assisted transport mechanism that relies on defect traps in the gate dielectric layer. The more interface states and oxide defects there are, the greater the leakage current caused by PF emission, which means that the trap density in the oxide seriously affects the PF emission [13,14]. FN tunneling and PF emission can be described by Formulas (1) and (2), respectively [15]:

$${\mathrm{J}}_{\mathrm{F}\mathrm{N}}\propto {\mathrm{E}}^2\mathrm{e}\mathrm{x}\mathrm{p}\left(-{\displaystyle \frac{\mathrm{A}}{3\mathrm{h}\mathrm{E}}}\right),$$

(1)

$${\mathrm{J}}_{\mathrm{P}\mathrm{F}}\propto \mathrm{E}\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{\mathrm{B}\sqrt{\mathrm{E}}}{\mathrm{T}}}-\mathrm{C}\right)$$

(2)

where J_FN and J_PF are the FN tunneling and PF emission current densities, respectively. E is the electric field of the gate dielectric layer, which can be obtained by dividing the difference between the applied gate voltage and the flat band voltage by the thickness of the oxide layer. The h is the Planck constant. A, B, and C are all constants. Equations (1) and (2) show that in the FN tunneling model, ln(J/E²) is proportional to −E⁻¹, and in the PF emission model, ln(J/E) is proportional to E^1/2.

The I_GS~V data under the condition of V_GS = −15 V in Figure 5 were processed to obtain the characteristic curve relationships of ln(J/E²)~−E⁻¹ and ln(J/E)~E^1/2, as shown in Figure 6. In this calculation, all the constants are simplified to 1 for calculation. Under the condition of V_GS = −15 V, I_GS fits well with the PF emission model, indicating that the gate oxide is degraded after irradiation, and that the leakage current mechanism of the SiC MOSFET changes to PF emission. Therefore, the main reason for the degradation of the SiC MOSFET gate oxide is that the radiation causes defects in the gate oxide layer, and the carriers tunnel through the oxide layer under the action of the electric field, causing the oxide layer to fail. For this SiC MOSFET, there are two main mechanisms for inducing defects in its gate oxide:

• Atomic displacement: defects introduced by atomic displacement in SiO₂ caused by irradiation.
• Chemical bond breakage: defects introduced by chemical bond breakage in SiO₂ caused by electrical stress.

The PIGS results show that the gate leakage current I_GS of the device did not significantly increase when V_GS = 0 V or −10 V, indicating that the gate oxide of the devices did not suffer catastrophic damage. Therefore, it is believed that the gate oxide of the devices did not break down due to particle irradiation. Consequently, it is believed that the atomic displacement caused by this injection of heavy ions will not degrade the reliability of the gate oxide of the SiC MOSFET [16]. At this time, gate oxide defects are mainly formed by the breaking of O-Si-O chemical bonds when SiO₂ approaches its critical breakdown electric field under high voltage, which causes carriers to pass through the oxide layer via defect-assisted tunneling and to be collected by the gate, thereby causing the breakdown of the gate oxide layer.

3.2. Temperature Synergy Effect

Different temperature points were selected for the irradiation experiments to explore the relationship between the SEB sensitivity and temperature of the SiC MOSFET. Figure 7 and Figure 8 show that the SEB voltage of the device at 40 °C is 450 V, and the SEB voltage of the device at 70 °C increased by 50 V, which shows that different temperature environments cause the SEB threshold voltage of the sample to change. Furthermore, in the experiment at 70 °C, the experimental SiC MOSFET showed a significant decrease in leakage I_D at an early stage. When V_DS = 400 V, the I_D increased rapidly, by two orders of magnitude, and SEB occurred when V_DS was 500 V. However, in the 40 °C environment, the leakage current of the sample increased rapidly when the V_DS was 450 V, and it was determined that SEB occurred. This phenomenon shows that with increases in the working environment temperature of the SiC MOSFET, the SEB threshold voltage increases. Additionally, the SiC MOSFET experiences leakage degradation before the SEB occurs at any temperature when V_GS = 0 V.

Based on the PISG test, the relationship between the potential damage degree of the oxide and the environmental temperature was explored. Figure 9 shows the PIGS of the SiC MOSFET with irradiation at different temperatures. The results revealed that the gate leakage current of the SiC MOSFET increased to varying degrees after it was irradiated by heavy ions. However, a high temperature of 70 °C caused the gate leakage current of the device to increase significantly, and it fluctuated violently with increasing gate voltage stress. The leakage current of the SiC MOSFET irradiated at 40 °C was significantly greater than that of the device irradiated at 20 °C. A higher V_DS is required for catastrophic SEB damage to the devices at 70 °C, which leads to the generation of a greater instantaneous electric field during its SEB, as well as to more severe gate defects. Combined with Formula (1), it can also be proven that due to the defects introduced by irradiation, the gate leakage current mechanism of the SiC MOSFET changes to a PF emission model. At higher temperatures, the gate oxide layer of the device is more damaged, and the PF emission current increases, resulting in a larger gate leakage current.

Moreover, the threshold voltage of the device was analyzed. For the MOSFET with N-channel enhancement, the source and drain electrodes were connected, with the source voltage being V_S and the substrate voltage being V_B. When V_B = V_S = 0 V, Formula (3) is obtained [17]:

$${\mathrm{V}}_{\mathrm{T}\mathrm{N}}=\mathrm{K}\sqrt{2{\mathsf{\varphi }}_{\mathrm{f}\mathrm{p}}}+{\mathsf{\varphi }}_{\mathrm{m}\mathrm{s}}-{\displaystyle \frac{{\mathrm{Q}}_{\mathrm{o}\mathrm{x}}}{{\mathrm{C}}_{\mathrm{o}\mathrm{x}}}}+2{\mathsf{\varphi }}_{\mathrm{f}\mathrm{p}}$$

(3)

where K is the body factor, $\mathrm{K}={\displaystyle \frac{{\left(2{\mathrm{q}\mathsf{\epsilon }}_{\mathrm{s}}{\mathrm{N}}_{\mathrm{A}}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}{{\mathrm{C}}_{\mathrm{o}\mathrm{x}}}}$, ϕ_fp is the substrate Fermi potential, ϕ_ms is the metal semiconductor work function difference, Q_ox is the effective charge surface density of the gate oxide layer, C_ox is the gate oxide layer capacitance, q is the absolute value of the electron charge, q = 1.602 × 10⁻¹⁹ C, ε_s is the dielectric constant, and NA is the doping concentration.

According to Formula (3), the threshold voltage is divided into three main parts:

• Voltage drop on the gate oxide layer, caused by the charge that generates the strong inversion layer.
• Flat band voltages. The second and third terms are the flat band voltages of the MOSFET device.
• The surface potential is ϕ_s = 2ϕ_fp, where a strong inversion layer begins to form at the interface. The ϕ_fp formula can be expressed as follows:

$${\mathsf{\varphi }}_{\mathrm{f}\mathrm{p}}={\displaystyle \frac{\mathrm{k}\mathrm{T}}{\mathrm{q}}}\mathrm{l}\mathrm{n}{\displaystyle \frac{{\mathrm{N}}_{\mathrm{A}}}{{\mathrm{n}}_{\mathrm{i}}}}$$

(4)

For the SiC, the semiempirical formula for the variation in the intrinsic carrier concentration with temperature is as follows:

$${\mathrm{n}}_{\mathrm{i}}=1.7\times {10}^{16}\times {\mathrm{T}}^{{\displaystyle \frac{2}{3}}}{\times \mathrm{e}}^{-{\displaystyle \frac{2.08\times {10}^3}{\mathrm{T}}}}$$

(5)

Equation (5) shows that the intrinsic carrier concentration increases with increases in temperature, as shown in Figure 10. Therefore, increasing the temperature will reduce the substrate Fermi potential, which will lead to a negative drift in the threshold voltage. Therefore, under the same gate-source bias conditions, a high-temperature environment will lead to an increase in the effective electric field of the device channel and an increase in the channel carrier density, which will lead to an increase in the leakage current and an early onset of leakage degradation in a high-temperature environment.

4. Conclusions

Heavy ion irradiation experiments on SiC MOSFETs were carried out, and the performance degradation of SiC MOSFETs under Cl ions irradiation was experimentally studied. The synergistic effects of bias voltage and temperature on the SEB of the SiC MOSFETs were discussed. The research results show that the SEB threshold voltage decreases with increases in the absolute value of the negative bias of the gate-source voltage. Before the occurrence of SEB, the leakage current of the device increases, the leakage degradation becomes more severe with increases in the negative bias of the gate-source voltage, and the slope of the relationship curve between the leakage current and the bias voltage increases. The test results for the latent gate damage show that due to the large negative gate-source voltage, the breakdown of the SiC MOSFET oxide layer changes from FN tunneling to PF emission, so the gate latent damage of the device that has an SEB under a large negative bias is also more serious. The research results for the temperature synergy effect show that due to the increase in temperature, the intrinsic carrier concentration increases, the Fermi potential decreases, the conduction threshold of the SiC MOSFETs decreases, and the SEB threshold voltage increases. However, the SiC MOSFETs experience more severe gate latent damage after irradiation at high temperatures.