A Two-Dimensional Computer-Aided Design Study of Unclamped Inductive Switching in an Improved 4H-SiC VDMOSFET

Due to its high thermal conductivity, high critical breakdown electric field, and high power, the silicon carbide (SiC) metal-oxide-semiconductor field-effect transistor (MOSFET) has been generally used in industry. In industrial applications, a common reliability problem in SiC MOSFET is avalanche failure. For applications in an avalanche environment, an improved, vertical, double-diffused MOSFET (VDMOSFET) device has been proposed. In this article, an unclamped inductive switching (UIS) test circuit has been built using the Mixed-Mode simulator in the TCAD simulation software, and the simulation results for UIS are introduced for a proposed SiC-power VDMOSFET by using Sentaurus TCAD simulation software. The simulation results imply that the improved VDMOSFET has realized a better UIS performance compared with the conventional VDMOSFET with a buffer layer (B-VDMOSFET) in the same conditions. Meanwhile, at room temperature, the modified VDMOSFET has a smaller on-resistance (Ron,sp) than B-VDMOSFET. This study can provide a reference for SiC VDMOSFET in scenarios which have high avalanche reliability requirements.


Introduction
Some important progress towards wide-bandgap semiconductors has been described in [1][2][3].Silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFET) have been widely used in large quantities due to their excellent properties, such as, high thermal conductivity, high critical breakdown electric field, and high power [4][5][6].In the application of SiC MOSFET, a common reliability problem is avalanche failure.
The standard test commonly used to evaluate the avalanche robustness of SiC MOSFET is the unclamped inductive switching (UIS) test [7].So far, in order to ensure the reliability of SiC MOSFET avalanche failure, all kinds of studies have been conducted to assess the avalanche robustness of SiC MOSFET.In [8][9][10][11], the researchers show that UIS failure appears when the temperature of the source aluminum layer surpasses the melting point.In addition, the switching on of parasitic bipolar junction transistor (BJT) is also thought to be a cause of avalanche failure in [12,13].And in [14], the researchers believe that the opening of the channel is also responsible for the avalanche failure.
Moreover, Bai et al. [15] studied the effects of structural parameters on the characteristics of UIS.The researchers discussed the influence of temperature on avalanche-energyprocessing capacity and established a safe avalanche operating boundary model in [16,17].Some researchers have studied the failure mechanism and avalanche ability of different device structures under single-pulse UIS stress, such as planar gate, asymmetric, and double-trench SiC MOSFET [18][19][20][21].There is also some research on the failure mechanism and performance degradation of SiC MOSFET under repetitive avalanche-shock conditions [22][23][24][25][26][27][28].In addition, for the purpose of improving the avalanche robustness of SiC MOSFET, some modified designs were proposed by the authors of [29].
In this paper, a concept for a device structure which can improve avalanche capability is proposed.A buffer layer is added to adjust the high electric field in the N-drift [30].Next, we ensured a decrease in ON resistance by adding the current spread layer (CSL).Then, we added a high concentration P-type region and low concentration N-type regions to the device to suppress the triggering of parasitic BJT.The avalanche energy in the proposed structure can be increased by 20% compared with the traditional structure, and the R on,sp is reduced to 1.0 mΩ•cm 2 by adding the CSL layer.

Description of Device Structure and Simulation
A cross-sectional view of the 4H-SiC VDMOSFET with a buffer layer (B-VDMOSFET) is shown in Figure 1a.A cross-sectional scheme of the improved VDMOSFET with CSL, deep p+ region, n− regions, and a buffer layer (DPNB-VDMOSFET) is shown in Figure 1b.and [17].Some researchers have studied the failure mechanism and avalanche ability of different device structures under single-pulse UIS stress, such as planar gate, asymmetric, and double-trench SiC MOSFET [18][19][20][21].There is also some research on the failure mechanism and performance degradation of SiC MOSFET under repetitive avalanche-shock conditions [22][23][24][25][26][27][28].In addition, for the purpose of improving the avalanche robustness of SiC MOSFET, some modified designs were proposed by the authors of [29].
In this paper, a concept for a device structure which can improve avalanche capability is proposed.A buffer layer is added to adjust the high electric field in the N-drift [30].Next, we ensured a decrease in ON resistance by adding the current spread layer (CSL).Then, we added a high concentration P-type region and low concentration N-type regions to the device to suppress the triggering of parasitic BJT.The avalanche energy in the proposed structure can be increased by 20% compared with the traditional structure, and the Ron,sp is reduced to 1.0 mΩ•cm 2 by adding the CSL layer.

Description of Device Structure and Simulation
A cross-sectional view of the 4H-SiC VDMOSFET with a buffer layer (B-VDMOSFET) is shown in Figure 1a.A cross-sectional scheme of the improved VDMOSFET with CSL, deep p+ region, n− regions, and a buffer layer (DPNB-VDMOSFET) is shown in Figure 1b.The doping concentration of the N-drift is 3 × 10 15 cm −3 .The P-base region is at a constant impurity concentration of 2 × 10 17 cm −3 .The impurity concentration in the substrate is 1 × 10 19 cm −3 .And the area factor is 4 × 10 5 .
The design of the 1.7-kV basic SiC VDMOSFET parameter refers to [31,32].The main parameters are shown in Table 1.The doping concentration of the N-drift is 3 × 10 15 cm −3 .The P-base region is at a constant impurity concentration of 2 × 10 17 cm −3 .The impurity concentration in the substrate is 1 × 10 19 cm −3 .And the area factor is 4 × 10 5 .

Structure
The design of the 1.7-kV basic SiC VDMOSFET parameter refers to [31,32].The main parameters are shown in Table 1.In addition, the UIS test circuit has been built using the Mixed-Mode simulator in TCAD simulation software.Figure 2a In addition, the UIS test circuit has been built using the Mixed-Mode simulator in TCAD simulation software.Figure 2a,b show both the test circuit and the test waveform.

UIS Characteristic
Analyses of the UIS performance of the two SiC VDMOSFET were carried out using the 2D simulator software Sentaurus TCAD.These simulation experiments were carried out using specific physical models and material properties of SiC.Some of the pivotal physical models include:

UIS Simulation Results and Discussion
In an SiC-power MOSFET, the SiC material does not melt but sublimes at temperatures higher than 2200-2300 K,; the metal/SiC surface contact or oxide could be damaged by local overheating at the temperatures above 1500 K.
In order to compare the avalanche ability of the B-VDMOSFET and DPNB-VDMOSFET, UIS simulation experiments were carried out at room temperature with 10 mH of load inductance, 20 Ω of resistance and a Vds of 80 V.The last waveform before

UIS Characteristic
Analyses of the UIS performance of the two SiC VDMOSFET were carried out using the 2D simulator software Sentaurus TCAD.These simulation experiments were carried out using specific physical models and material properties of SiC.Some of the pivotal physical models include:

UIS Simulation Results and Discussion
In an SiC-power MOSFET, the SiC material does not melt but sublimes at temperatures higher than 2200-2300 K, the metal/SiC surface contact or oxide could be damaged by local overheating at the temperatures above 1500 K.
In order to compare the avalanche ability of the B-VDMOSFET and DPNB-VDMOSFET, UIS simulation experiments were carried out at room temperature with 10 mH of load inductance, 20 Ω of resistance and a V ds of 80 V.The last waveform before avalanche failure is usually used to calculate the avalanche tolerance E av of a device.Avalanche tolerance E av is an important parameter used to evaluate the reliability of device UIS.
The last waveforms before avalanche failure in the B-VDMOSFET, and DPNB-VDMOSFET before UIS failure, are shown in Figure 4a,b.The single avalanche energy Eav of SiC MOSFET is calculated as follows: Micromachines 2024, 15, x FOR PEER REVIEW 5 of 13 avalanche failure is usually used to calculate the avalanche tolerance Eav of a device.Avalanche tolerance Eav is an important parameter used to evaluate the reliability of device UIS.
The last waveforms before avalanche failure in the B-VDMOSFET, and DPNB-VDMOSFET before UIS failure, are shown in Figure 4a,b.The single avalanche energy Eav of SiC MOSFET is calculated as follows: The maximum currents before the failure of the B-VDMOSFET and DPNB-VDMOSFET are, respectively, 10.3A and 11.3 A. This translates to a 1.2× higher avalanche capability for the new design.
The simulated UIS waveforms for B-VDMOSFET and DPNB-VDMOSFET after failure are shown in Figure 5a,b.These waveforms differ slightly from the waveforms measured in [37].Whether the avalanche current in the device will eventually return to zero is usually used as a criterion for avalanche failure.It can be seen that the currents of the B-VDMOSFET and DPNB-VDMOSFET have a rising trend, so it can be preliminary judged that the B-VDMOSFET and DPNB-VDMOSFET have failed.The maximum currents before the failure of the B-VDMOSFET and DPNB-VDMOSFET are, respectively, 10.3A and 11.3 A. This translates to a 1.2× higher avalanche capability for the new design.
The simulated UIS waveforms for B-VDMOSFET and DPNB-VDMOSFET after failure are shown in Figure 5a,b.These waveforms differ slightly from the waveforms measured in [37].Whether the avalanche current in the device will eventually return to zero is usually used as a criterion for avalanche failure.It can be seen that the currents of the B-VDMOSFET and DPNB-VDMOSFET have a rising trend, so it can be preliminary judged that the B-VDMOSFET and DPNB-VDMOSFET have failed.It is well known that the avalanche capability of power of SiC MOSFET is closely related to the suppression of parasitic BJT.In order to observe and study the internal physical characteristics of the B-VDMOSFET and DPNB-VDMOSFET, the UIS characteristics of the B-VDMOSFET and DPNB-VDMOSFET were simulated and analyzed by using a 2D computer-aided design (TCAD) physical simulation, and the relevant results were extracted.
Avalanche current density is often used to evaluate the intensity of avalanche breakdown.In order to study the reason for the failure, the distribution of the avalanche current inside B-VDMOSFET and DPNB-VDMOSFET after avalanche failure is extracted by the two-dimensional simulation software Sentaurus TCAD.The avalanche current density of B-VDMOSFET and DPNB-VDMOSFET after avalanche failure is shown in Figure 6a,b.The time shown in the figures is calculated from the discharging phase and the times are taken from beginning of the avalanching.It is well known that the avalanche capability of power of SiC MOSFET is closely related to the suppression of parasitic BJT.In order to observe and study the internal physical characteristics of the B-VDMOSFET and DPNB-VDMOSFET, the UIS characteristics of the B-VDMOSFET and DPNB-VDMOSFET were simulated and analyzed by using a 2D computer-aided design (TCAD) physical simulation, and the relevant results were extracted.
Avalanche current density is often used to evaluate the intensity of avalanche breakdown.In order to study the reason for the failure, the distribution of the avalanche current inside B-VDMOSFET and DPNB-VDMOSFET after avalanche failure is extracted by the two-dimensional simulation software Sentaurus TCAD.The avalanche current density of B-VDMOSFET and DPNB-VDMOSFET after avalanche failure is shown in Figure 6a,b.The time shown in the figures is calculated from the discharging phase and the times are taken from beginning of the avalanching.
The parasitic BJT of N-channel SiC MOSFET is NPN-type.When the base-emitter bias voltage is higher than the base-emitter junction built-in voltage, the NPN-type parasitic BJT turns on.
As shown in the figure, there is a large amount of current flowing out of the channel when the B-VDMOSFET and DPNB-VDMOSFET fail in the avalanche.Therefore, we can preliminarily judge that the parasitic BJT is turned on at this time.At the same time, we can see that there is still a large amount of current flowing out of the channel after a period of time, so it can be considered that the parasitic BJT is not closed at this time.The parasitic BJT of N-channel SiC MOSFET is NPN-type.When the base-emitter bias voltage is higher than the base-emitter junction built-in voltage, the NPN-type parasitic BJT turns on.
As shown in the figure, there is a large amount of current flowing out of the channel when the B-VDMOSFET and DPNB-VDMOSFET fail in the avalanche.Therefore, we can preliminarily judge that the parasitic BJT is turned on at this time.At the same time, we can see that there is still a large amount of current flowing out of the channel after a period of time, so it can be considered that the parasitic BJT is not closed at this time.
In addition, in Figure 6a,b, the maximum avalanche current density of DPNB-VDMOSFET is much smaller than that of B-VDMOSFET.In Figure 6a, the avalanche current in this B-VDMOSFET is mainly concentrated in the parasitic BJT region.However, as shown in Figure 6b, the avalanche current in DPNB-VDMOSFET is mainly concentrated below the deep P+ region and in the channel at the beginning.This may be one of the reasons why DPNB-VDMOSFET performs better than B-VDMOSFET under UIS conditions.
BJT is a bipolar semiconductor device that generates leakage current even in the reverse-bias PN junction.The thermal generation current generated by SRH recombination and the diffusion current formed by carrier differential concentration constitute the leakage current [38].The leakage current can be ignored at the low temperature.But, if the In addition, in Figure 6a,b, the maximum avalanche current density of DPNB-VDMOSFET is much smaller than that of B-VDMOSFET.In Figure 6a, the avalanche current in this B-VDMOSFET is mainly concentrated in the parasitic BJT region.However, as shown in Figure 6b, the avalanche current in DPNB-VDMOSFET is mainly concentrated below the deep P+ region and in the channel at the beginning.This may be one of the reasons why DPNB-VDMOSFET performs better than B-VDMOSFET under UIS conditions.
BJT is a bipolar semiconductor device that generates leakage current even in the reverse-bias PN junction.The thermal generation current generated by SRH recombination and the diffusion current formed by carrier differential concentration constitute the leakage current [38].The leakage current can be ignored at the low temperature.But, if the temperature gradually increases to be close to the intrinsic temperature of SiC, the leakage current will become non-negligible.
To further analyze the cause of avalanche failure in the device, the lattice temperature of B-VDMOSFET and DPNB-VDMOSFET in avalanche mode are shown in Figure 7a,b.The time shown in the figures is calculated from the discharging phase.It can clearly be seen that, after avalanche failure, the B-VDMOSFET first heats from the corner of the P-base region, which gradually diffuses to the entire cell.And in DPNB-VDMOSFET, the temperature changes similarly.The maximum temperatures of B-VDMOSFET and DPNB-VDMOSFET are around 1500 K, and the device may be burned if the temperature inside the device continues to rise.However, at the time of BJT conduction, the temperature of the channel position has exceeded 1300 K.Such a high temperature will increase the leakage current, which will make it non-ignorable.On the basis of the distribution of electron and hole currents, the combined action of electrons and holes produces the total current through the channel, which is characteristic of the bipolar drain current.Therefore, because the high temperature increases the bipolar leakage current, there is current flowing through the channel even in the absence of a base-emitter positive bias voltage.However, at the time of BJT conduction, the temperature of the channel position has exceeded 1300 K.Such a high temperature will increase the leakage current, which will make it non-ignorable.On the basis of the distribution of electron and hole currents, the combined action of electrons and holes produces the total current through the channel, which is characteristic of the bipolar drain current.Therefore, because the high temperature increases the bipolar leakage current, there is current flowing through the channel even in the absence of a base-emitter positive bias voltage.
Meanwhile, because the built-in voltage of the SiC PN junction has a negative temperature coefficient (NTC), it will gradually decrease when the temperature increases.Such a high temperature, as in Figure 7a,b, causes the junction barrier at the given channel location to drop below 0 V.In the meantime, at high temperatures, the P-base region resistance cannot be ignored, which will result in a positive bias voltage between the parasitic BJT base and emitter, which accelerates the increase in the current of parasitic BJT due to the current amplification effect of bipolar transistor.
It can be seen from Figure 6a that the current flowing through the P-base region is large when the B-VDMOSFET fails in avalanche.However, in Figure 6b, the current in DPNB-VDMOSFET that flows through P-base region is smaller.This may also be one of the reasons why the DPNB-VDMOSFET has better UIS reliability.
In addition, the electric field distribution of the B-VDMOSFET and DPNB-VDMOSFET after failure was extracted using TCAD simulation, as shown in Figure 8a,b.The time shown in the figures is calculated from the discharging phase.As shown in the figure, the highest electric field of B-VDMOSFET is in the corner of the P-base region, while the highest electric field of DPNB-VDMOSFET is in the deep P+ region.At this time, both the electric fields in the gate oxide are less than their highest electric fields.The electric field of B-VDMOSFET does not exceed the electric field limit of SiC material of 3 MV/cm.Due to the presence of the N-region, the electric field of DPNB-VDMOSFET does not reach the breakdown value.The reason that the electric field is high is because there is a lot of current flowing through and a lot of carriers are converging here.
location to drop below 0 V.In the meantime, at high temperatures, the P-base region resistance cannot be ignored, which will result in a positive bias voltage between the parasitic BJT base and emitter, which accelerates the increase in the current of parasitic BJT due to the current amplification effect of bipolar transistor.
It can be seen from Figure 6a that the current flowing through the P-base region is large when the B-VDMOSFET fails in avalanche.However, in Figure 6b, the current in DPNB-VDMOSFET that flows through P-base region is smaller.This may also be one of the reasons why the DPNB-VDMOSFET has better UIS reliability.
In addition, the electric field distribution of the B-VDMOSFET and DPNB-VDMOSFET after failure was extracted using TCAD simulation, as shown in Figure 8a,b.The time shown in the figures is calculated from the discharging phase.As shown in the figure, the highest electric field of B-VDMOSFET is in the corner of the P-base region, while the highest electric field of DPNB-VDMOSFET is in the deep P+ region.At this time, both the electric fields in the gate oxide are less than their highest electric fields.The electric field of B-VDMOSFET does not exceed the electric field limit of SiC material of 3 MV/cm.Due to the presence of the N-region, the electric field of DPNB-VDMOSFET does not reach the breakdown value.The reason that the electric field is high is because there is a lot of current flowing through and a lot of carriers are converging here.Figure 9 shows the temperature curve before and after failure of the B-VDMOSFET and DPNB-VDMOSFET.The results in the figure show that the temperature before the failure showed a downward trend, while the temperature of the B-VDMOSFET and DPNB-VDMOSFET both showed a rising trend after the failure.The continuous rise in temperature will eventually lead to thermal runaway of the device.In conclusion, the reason for the failure of the device may be that the parasitic BJT is on and cannot be turned off, thus it is continuously generating a large amount of current, and further, that the temperature of the device continues to rise until it finally burns out.Therefore, DPNB-VDMOSFET that can inhibit parasitic BJT conduction has better UIS reliability compared to B-VDMOSFET.

Conclusions
Table 2 shows correlational research about UIS of power devices in the past three years.In this work, DPNB-VDMOSFET improved the avalanche ability and reduced onresistance.
Table 2. Correlational research about UIS of power devices in the past three years.

Structure Name
The Improvement in Avalanche Energy The Reduction in Ron,sp (mΩ•cm 2 ) BV (V) SiC MOSFETs with varied JFET region width [39] 6.3% 0.5 1200 Asymmetric Trench SiC MOSFET [18] 13.8% 0.1 1200 SiC planar MOSFET [40] 13.3% -900 This work 20% 1.0 1700 In this article, the UIS performance of the 1.7-kV VDMOSFET with the same buffer layers was comparatively studied.The device structure proposed under UIS conditions is validated by numerical simulation.The simulation results show that, when L = 10 mH, R = 20 Ω and Vds = 80 V, the avalanche energy of the proposed structure can be increased by 20% compared with the traditional structure.In the meantime, due to adding the CSL layer, the Ron,sp of the improved structure is reduced to 1.0 mΩ•cm 2 compared with another structure, which can reduce the energy lost from the system.In conclusion, the reason for the failure of the device may be that the parasitic BJT is on and cannot be turned off, thus it is continuously generating a large amount of current, and further, that the temperature of the device continues to rise until it finally burns out.Therefore, DPNB-VDMOSFET that can inhibit parasitic BJT conduction has better UIS reliability compared to B-VDMOSFET.

Conclusions
Table 2 shows correlational research about UIS of power devices in the past three years.In this work, DPNB-VDMOSFET improved the avalanche ability and reduced on-resistance.Table 2. Correlational research about UIS of power devices in the past three years.

Structure Name
The Improvement in Avalanche Energy The Reduction in R on,sp (mΩ•cm 2 )

BV (V)
SiC MOSFETs with varied JFET region width [39] 6.3% 0.5 1200 Asymmetric Trench SiC MOSFET [18] 13.8% 0.1 1200 SiC planar MOSFET [40] 13.3% -900 This work 20% 1.0 1700 In this article, the UIS performance of the 1.7-kV VDMOSFET with the same buffer layers was comparatively studied.The device structure proposed under UIS conditions is validated by numerical simulation.The simulation results show that, when L = 10 mH, R = 20 Ω and V ds = 80 V, the avalanche energy of the proposed structure can be increased by 20% compared with the traditional structure.In the meantime, due to adding the CSL layer, the R on,sp of the improved structure is reduced to 1.0 mΩ•cm 2 compared with another structure, which can reduce the energy lost from the system.

Figure
Figure 3a shows the breakdown voltage characteristics of two MOSFETs.The breakdown voltages in the B-VDMOSFET and DPNB-VDMOSFET are 2526 V and 2538 V. Figure 3b shows the ON-state performance of the B-VDMOSFET and DPNB-VDMOSFET.However, the modified structure has better conduction characteristics due to the current path in the CSL region.The Ron,sp of the B-VDMOSFET and DPNB-VDMOSFET are 5.8 mΩ•cm 2 and 4.8 mΩ•cm 2 at VGS = 20 V.As shown in Figure 3c, because their base regions have an equal doping concentration, the threshold voltages (Vth) of two SiC MOSFETs are identical.

Figure
Figure3ashows the breakdown voltage characteristics of two MOSFETs.The breakdown voltages in the B-VDMOSFET and DPNB-VDMOSFET are 2526 V and 2538 V. Figure3bshows the ON-state performance of the B-VDMOSFET and DPNB-VDMOSFET.However, the modified structure has better conduction characteristics due to the current path in the CSL region.The R on,sp of the B-VDMOSFET and DPNB-VDMOSFET are 5.8 mΩ•cm 2 and 4.8 mΩ•cm 2 at V GS = 20 V.As shown in Figure3c, because their base regions have an equal doping concentration, the threshold voltages (V th ) of two SiC MOSFETs are identical.
Figure3ashows the breakdown voltage characteristics of two MOSFETs.The breakdown voltages in the B-VDMOSFET and DPNB-VDMOSFET are 2526 V and 2538 V. Figure3bshows the ON-state performance of the B-VDMOSFET and DPNB-VDMOSFET.However, the modified structure has better conduction characteristics due to the current path in the CSL region.The R on,sp of the B-VDMOSFET and DPNB-VDMOSFET are 5.8 mΩ•cm 2 and 4.8 mΩ•cm 2 at V GS = 20 V.As shown in Figure3c, because their base regions have an equal doping concentration, the threshold voltages (V th ) of two SiC MOSFETs are identical.

Figure 3 .
Figure 3. (a) The breakdown voltage characteristics of devices (b) ON-state characteristic curves for the devices.(c) Transfer characteristic curves for the devices.
(1) the Fermi model is used to describe the carrier distribution of two devices.(2) the Shockley Read Hall (SRH) and Auger recombination models are concerned with temperature and doping.(3) The incomplete ionization model is used to calculate the impact ionization rate.(4) The Analytic model is used to describe the temperature dependent mobility of carrier distributions [33-36].

Figure 3 .
Figure 3. (a) The breakdown voltage characteristics of devices (b) ON-state characteristic curves for the devices.(c) Transfer characteristic curves for the devices.
(1) the Fermi model is used to describe the carrier distribution of two devices.(2) the Shockley Read Hall (SRH) and Auger recombination models are concerned with temperature and doping.(3) The incomplete ionization model is used to calculate the impact ionization rate.(4) The Analytic model is used to describe the temperature dependent mobility of carrier distributions [33-36].

Figure 6 .
Figure 6.(a) The avalanche current density of B-VDMOSFET after failure.(b) The avalanche current density of DPNB-VDMOSFET after failure.

Figure 6 .
Figure 6.(a) The avalanche current density of B-VDMOSFET after failure.(b) The avalanche current density of DPNB-VDMOSFET after failure.

Figure 7 .
Figure 7. (a) The lattice temperature of B-VDMOSFET after failure.(b) The lattice temperature of DPNB-VDMOSFET after failure.

Figure 7 .
Figure 7. (a) The lattice temperature of B-VDMOSFET after failure.(b) The lattice temperature of DPNB-VDMOSFET after failure.

Figure 8 .
Figure 8.(a) The electric field of B-VDMOSFET after failure.(b) The electric field of DPNB-VDMOSFET after failure.

Figure 9
Figure9shows the temperature curve before and after failure of the B-VDMOSFET and DPNB-VDMOSFET.The results in the figure show that the temperature before the failure showed a downward trend, while the temperature of the B-VDMOSFET and DPNB-VDMOSFET both showed a rising trend after the failure.The continuous rise in temperature will eventually lead to thermal runaway of the device.

Figure 9 .
Figure 9.The lattice temperature of B-VDMOSFET and DPNB-VDMOSFET before and after failure.

Figure 9 .
Figure 9.The lattice temperature of B-VDMOSFET and DPNB-VDMOSFET before and after failure.

Table 1 .
Main Device Parameters in Simulations.

Table 1 .
Main Device Parameters in Simulations.