Comparison Analysis of Radiation Effects on 1.2 kV SiC Metal-Oxide-Semiconductor Field-Effect Transistors with Gamma-Ray and Proton Irradiation

Abstract

TID effects occur in MOS-gated transistors in radiation environments where proton and gamma-rays irradiate the devices. TID effects seriously affect the electrical characteristics of Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET). They can eventually result in the malfunction of power systems when exposed to long-term radiation conditions. We irradiated gamma-rays and protons into 1.2 kV SiC MOSFETs and evaluated the change in electrical properties to analyze the TID’s effects. As a result of the experiment, the threshold voltage (V_T) and on-resistance (R_on) of 1.2 kV SiC MOSFETs decreased because positive fixed charges inside the oxide increased depending on the radiation dose of the gamma-ray and fluence of the proton irradiations. The degradation of breakdown voltage (BV) occurred owing to a change in the depletion curvature at the edge of termination regions owing to the trapping of the charge in the field’s oxide.

1. Introduction

Silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) have garnered attention in the field of power electronics, particularly for their high-voltage applications and utilization in radiation environments [1,2]. SiC MOSFETs have various merits, such as low on-resistance, small gate charge, outstanding dv/dt capability, and high-temperature reliability owing to their wide bandgap properties.

However, when the MOSFET is exposed to radiation, positive fixed charges in the oxide increase owing to the effects of the total ionizing dose (TID) [3,4]. The TID affects the change in electrical characteristics, such as the threshold voltage (V_T) and on-resistance (R_on). In addition, the curvature of the depletion region becomes narrow due to positive fixed charges generated in the thick field oxide layer of the edge termination region. As the electric field is concentrated in this region, the blocking characteristics deteriorate after exposure to radiation. Furthermore, radiation-induced damage leads to the formation of interfacial defects, resulting in a reduction in channel mobility and an increase in the leakage current [5].

Radiation poses a significant challenge to the reliability of power systems [6,7,8]. Given that radiation exists both in the atmosphere and the space environment, it is imperative to account for its effects in aerospace applications [9,10,11,12]. Several research groups reported numerous investigations into the radiation effects on Si MOSFETs [13,14,15,16,17,18,19,20]. In this study, the SiC MOSFETs were irradiated with gamma-rays and protons to analyze the radiation effects by evaluating V_T, R_on, and breakdown voltage (BV). In addition, the TID’s effects were investigated by analyzing the capacitance–voltage (C-V) characteristics of the n-type SiC MOS capacitors to extract the fixed charge density (Q_F) inside the oxide. Additionally, degradation mechanisms of SiC MOSFETs were elucidated through the Sentaurus TCAD simulations.

2. Experimental Details

In this study, we employed 1.2 kV/189 of mΩ n-channel SiC MOSFETs and n-type SiC MOS capacitors. Figure 1 shows the cross-sectional views of the conventional SiC MOSFET power devices. The studied devices are commercial SiC MOSFETs from STMicroelectronics. The rated voltage of the devices was 1200 V. The SiC MOSFETs and MOS capacitor TEGs were irradiated by a gamma radiation source of 1.33 MeV of Co-60 gamma-rays at the Advanced Radiation Technology Institute (ARTI) in Korea.

The SiC MOSFETs and n-type MOS capacitors were irradiated using a gamma radiation source of 1.33 MeV of Co-60 gamma-rays at the Advanced Radiation Technology Institute (ARTI) in Korea. Each device was irradiated with the gamma-ray doses of 1, 3, and 5 Mrad for each device. In addition, we irradiated 1.2 kV MOSFETs and n-type MOS capacitors with protons containing 45 MeV of energy with a fluence of 1 × 10¹² cm⁻² and 1 × 10¹³ cm⁻², respectively, at the Korea Multi-Purpose Accelerator Complex (KOMAC). Specific irradiation conditions in this experiment are summarized in

Table 1. Type and conditions of radiation in the experiment.

Sample ID	Radiation Type	Energy (MeV)	Dose (Mrad)	Fluence (cm−2)
Sample 1	Gamma-ray	1.33	1	-
Sample 2			3	-
Sample 3			5	-
Sample 4	Proton	45	-	1 × 1012
Sample 5			-	1 × 1013

.

We analyzed the electrical characteristics of SiC MOSFETs by analyzing their current–voltage (I–V) characteristics before and after gamma irradiation. Additionally, we analyzed n-type SiC MOS capacitors to verify the TID effect caused by gamma irradiation. We fabricated an N-type MOS capacitor and used it in our experiments to extract the capacitance per unit area of the oxide to accurately analyze the fixed charge inside the oxide. The C-V characteristics of n-type SiC MOS capacitors were analyzed to evaluate the oxide characteristics caused by irradiation. Commercial SiC MOSFETs were measured with Keysight’s B1505A (Santa Rosa, CA, USA) (power device analyzer), and n-type SiC MOS capacitors were measured with Keysight’s B1500A (semiconductor device parametric analyzer).

3. Experimental Results

In Figure 2, the transfer characteristics of 1.2 kV of SiC MOSFETs before and after gamma-ray irradiation are depicted. The irradiated samples exhibit a higher drain current in both sub-threshold and saturation regions, which can be attributed to a reduction from the V_T. The extracted ΔV_T (=V_T,before − V_T,after) for various irradiation conditions are 0.498 V (sample 1), 0.697 V (sample 2), and 0.797 V (sample 3). Hole traps were created inside the oxide because of the TID effects caused by gamma irradiation. The increase in Q_F affects the charge distribution at the SiC regions near the interface and causes more electrons to accumulate in the JFET region, reducing the V_T.

Figure 3 shows the breakdown characteristics of 1.2 kV SiC MOSFETs before and after gamma-ray irradiation. The BV decrease depends on the gamma-ray irradiation dose. Consequently, the BV decreased by 58 V (sample 1), 64 V (sample 2), and 71 V (sample 3). This was due to the creation of hole traps in the thick oxide at the edge of the termination region, which reduces the curvature of the depletion region. The electric field is concentrated in the depletion region with a small curvature, causing the breakdown characteristics to deteriorate. Since SiC MOSFETs are designed with better breakdown characteristics in the edge termination region than in the active region, the degradation of breakdown characteristics has a fatal effect on electrical characteristics. The mechanism of breakdown degradation is discussed in detail through the simulation results.

Figure 4 shows the transfer characteristics of 1.2 kV SiC MOSFETs before and after the proton irradiation. As the proton irradiation fluence (φ_proton) increased, the V_T decreased so that samples 4 and 5 had ΔV_T of 0.398 V and 0.776 V, respectively. The V_T decreases induced by the hole traps were created due to the TID effects caused by high-energy proton irradiation. Decreasing V_T results in unintended turn-on, increased leakage current, and degraded breakdown characteristics, which could lead to power loss or reduced reliability.

As shown in Figure 5, the BV for samples 4 and 5 are decreased by 30 V and 44 V, respectively. The TID effects lead to the creation of hole traps within the thick oxide layer at the edge termination region. This causes a decrease in surface depletion, which leads to an increase in the surface electric field, consequently diminishing the BV.

Our experimental results show that both gamma-ray and proton irradiation result in the degradation of blocking capability with an increased leakage current. The comparative data for the electric characteristics of the SiC MOSFETs before and after gamma-ray or proton irradiation are summarized in

Table 2. Summary of electrical characteristics of 1.2 kV SiC MOSFETs according to the dose of gamma-ray and proton fluences.

Radiation Type	Dose (Mrad)	Fluence (cm−2)	ΔVT (V)	ΔRon (mΩ)	ΔBV (V)
Gamma-ray	Virgin	-	0	0	0
	1	-	−0.498	−10	−58
	3	-	−0.697	−14	−64
	5	-	−0.797	−19	−71
Proton	-	Virgin	0	0	0
	-	1 × 1012	−0.398	−9	−30
	-	1 × 1013	−0.776	−23	−44

Virgin: The devices that were not irradiated with gamma rays and protons.

.

When gamma-rays and protons irradiate the MOS structure, electron-hole pairs (EHPs) are produced within the oxide owing to the high-energy radiation by TID effects. Subsequently, holes tend to remain within the oxide layer, primarily due to the relatively higher carrier mobility of electrons compared to that of holes, as discussed in [21,22]. The presence of holes has a notable impact on the charge distribution at the surface of SiC, influencing V_T [23,24,25]. Additionally, more electrons are accumulated in the JFET regions, leading to a reduction in on-resistance SiC MOSFETs [26]. Furthermore, the hole traps generated within thick field oxide at the edge of the termination regions induced by TID effects influence a reduction in the surface depletion and the concentration of the electric field at the surface, causing a decrease in BV [27]. We fabricated n-type MOS capacitors to investigate the effects of gamma-ray and proton irradiation on the oxide layer. We used a 10 µm thick drift layer with a doping concentration of 1.1 × 10¹⁶ cm^–3. Dry oxidation was performed at 1350 °C to form a 50 nm thick gate oxide, followed by NO annealing at 1250 °C. Subsequently, 4 µm thick Al was patterned as a pad metal. The fabricated n-type MOS capacitors were subjected to gamma-ray irradiation at doses of 1, 3, and 5 Mrad, as well as proton irradiation at fluences of 1 × 10¹² cm⁻² and 1 × 10¹³ cm⁻².

Figure 6a,b illustrate the C-V curves of the n-type MOS capacitors as a function of the gamma-ray irradiation dose and proton fluence, respectively. The gate voltage was swept from –5 V to +5 V at a frequency of 1 MHz. We realized that the C–V curves tended to shift to the left direction after gamma-ray irradiation.

We extracted the shifts of flat band voltage (ΔV_FB = V_FB,ideal − V_FB,after) and Q_F. The extracted ΔV_FB values were 1.806 V (1 Mrad sample), 2.636 V (3 Mrad sample), and 2.965 V (5 Mrad sample), which increased in proportion to the gamma-ray irradiation dose. The ΔV_FB was also extracted through the relationship between Q_F and ΔV_FB

$$Q_F=\frac{\mathsf{\Delta }{V}_{FB}·C_{ox}}{q}$$

(1)

where q is 1.6 × 10⁻¹⁹ C [28]. Our experimental results demonstrate how Q_F increased from 5.42 × 10¹¹ cm⁻² (virgin sample) to 7.04 × 10¹¹ cm⁻² (1 Mrad sample), 9.04 × 10¹¹ cm⁻² (3 Mrad sample) and 9.78 × 10¹¹ cm⁻² (5 Mrad sample), respectively. This tendency agrees well with the reduction in V_T and BV based on the gamma-ray irradiation dose. However, a distinct leftward ΔV_FB can be observed, which is similar to gamma-ray irradiation. The extracted ΔV_FB values were 2.566 V for the sample irradiated with φ_proton = 1 × 10¹² cm⁻² and 3.466 V for the sample irradiated with φ_proton = 1 × 10¹³ cm⁻². The ΔV_FB, according to the proton fluence, resulted in a Q_F difference. Thus, we obtained Q_F values of 1.03 × 10¹² and 1.40 × 10¹² cm⁻² for the sample irradiated with φ_proton = 1 × 10¹² cm⁻² and the sample irradiated with φ_proton = 1 × 10¹³ cm⁻². The characteristics of the oxides in the n-type SiC MOS capacitors before and after gamma-ray and proton irradiation are summarized in

Table 3. Summary of Q_F extracted from n-type MOS capacitors according to the doses of gamma-ray and proton fluences.

Radiation Type	Dose (Mrad)	Fluence (cm−2)	ΔVFB (V)	QF (cm−2)
Gamma-ray	Virgin	-	1.306	5.42 × 1011
	1	-	1.806	7.04 × 1011
	3	-	2.636	9.04 × 1011
	5	-	2.065	9.78 × 1011
Proton	-	Virgin	1.876	5.61 × 1011
	-	1 × 1012	2.986	1.03 × 1012
	-	1 × 1013	3.966	1.40 × 1012

.

We investigated the TID effects by radiation, considering oxide electrical characteristics, such as the Q_F. To verify the breakdown mechanism, the parameters extracted from the n-type MOS capacitors, with gamma-ray experiments, were used for the TCAD simulation. We used a simple edge termination structure of a Single-Zone Junction Terminal Extension (SZ-JTE). The specific design parameters are summarized in

Table 4. Design parameters of TCAD simulation.

LJTE (μm)	tox (μm)	QF (cm−2)	Sample ID
50	0.865	5.42 × 1011	Device A
		7.04 × 1011	Device B
		9.04 × 1011	Device C
		9.78 × 1011	Device D

[29].

Figure 7 depicts the electric field distribution at cut-line A within each SZ-JTE structure under a V_DS of 1200 V. The measured E_max are 9.92 × 10⁵ V/cm, 1.16 × 10⁶ V/cm, 1.32 × 10⁶ V/cm, and 1.73 × 10⁶ V/cm for the 1, 3, and 5 Mrad conditions, respectively. It is noted that when a high Q_F was applied, a higher electric field tended to concentrate at the edge of the main junction.

It can be attributed to the fact that positive charges in the oxide layer have a reduced impact on the extension of the depletion regions, such that BV decreased, as depicted in Figure 8. The simulated BV values were 1486, 1446, 1325, and 1298 V for gamma-ray doses of 0, 1, 3, and 5 Mrad, respectively [30].

4. Conclusions

In this study, we performed a measurement and TCAD simulation in order to analyze the degradation mechanism of the 1.2 kV SiC MOSFETs by gamma-ray and proton irradiation. Following gamma-ray and proton irradiation, the TID effects occurred due to the high-energy impact on the SiC MOSFETs, causing additional electron accumulation at the SiC surface and reducing the V_T. After 5 Mrad of gamma-ray irradiation, the V_T decreased by 22%, and the BV decreased by 5%. Additionally, after proton irradiation (fluence = 1 × 10¹³ cm⁻²), the V_T decreased by 20%, and the BV decreased by 3%. This was a change in electrical characteristics due to the increase in Q_F caused by the occurrence of the TID effect owing to irradiation with high-energy gamma rays and protons. We observed an increase in the Q_F after both gamma-ray and proton irradiation through C-V analysis at the n-type MOS capacitor. In addition, we verified the effects of Q_F on the BV of SZ-JTE in 1.2 kV SiC MOSFETs using a TCAD simulation.