Evaluation of VTH and RON Drifts during Switch-Mode Operation in Packaged SiC MOSFETs

In this paper, we investigate the evolution of threshold voltage (VTH) and on-resistance (RON) drifts in the silicon carbide (SiC) power metal-oxide-semiconductor field-effect transistors (MOSFETs) during the switch-mode operation. A novel measurement setup for performing the required on-the-fly characterization is presented and the experimental results, obtained on commercially available TO-247 packaged SiC devices, are reported. Measurements were performed for 1000 s, during which negative VTH shifts (i.e., VTH decrease) and negative RON drifts (i.e., RON decrease) were observed. To better understand the origin of these parameter drifts and their possible correlation, measurements were performed for different (i) gate-driving voltage (VGH) and (ii) off-state drain voltage (VPH). We found that VTH reduction leads to a current increase, thus yielding RON to decrease. This correlation was explained by the RON dependence on the overdrive voltage (VGS–VTH). We also found that gate-related effects dominate the parameter drifts at low VPH with no observable recovery, due to the repeated switching of the gate signal required for the parameter monitoring. Conversely, the drain-induced instabilities caused by high VPH are completely recoverable within 1000 s from the VPH removal. These results show that the measurement setup is able to discern the gate/drain contributions, clarifying the origin of the observed VTH and RON drifts.


Introduction
Silicon carbide (SiC) is a common technology option for replacing silicon (Si) in power-switching applications that require low switching and conduction losses [1]. Unfortunately, the degradation of key electrical parameters in SiC power devices under repetitive switching stress reduces both device performance and reliability well below specifications [2]. In this scenario, stability issues like threshold voltage (VTH) and dynamic onresistance (RON) drifts are often encountered [3], severely impacting device lifetime and performance. For instance, a positive RON drift (i.e., increase) affects the conduction losses within power-switching converters, yielding a considerable efficiency reduction. Similarly, a positive VTH shift results in a lower current level for a given driving gate voltage (VGS) [4,5], whereas negative VTH variations could impede fully turning off the device, causing unwanted conduction at a large drain bias [6].
Generally, Bias Temperature Instability (BTI) experiments are employed to reveal the mechanisms underlying these issues in SiC technology [7][8][9]. However, BTI tests can only be used to investigate the effects related to gate bias, neglecting the effects related to the drain bias. To get a more complete picture of the detrimental effects produced by repetitive switching stress [2,10], it is necessary to evaluate the VTH and RON evolution when the device is driven in realistic operating conditions, i.e., with both gate and drain potential repeatedly switched on and off. To this end, test circuits should be designed to mimic conventional switching conditions in terms of frequency, voltage, and current level. At the same time, these circuits should also extract the parameters of interest (e.g., VTH and RON) without affecting device operation. Accordingly, a fast stress measurement approach is needed, in order to cope with switching frequencies in the kHz range [11] and also to limit the delay between the stress and readout [12][13][14] phases, thus avoiding ambiguous parameter drifts and/or wrong data interpretation. Nevertheless, achieving these goals is challenging for conventional parameter analyzers, especially when characterizing packaged devices with large parasitic capacitances [15]. The development of custom, ad-hoc systems is thus essential to investigate the stability and performance of commercially available state-of-the-art devices.
In this work, we present a novel measurement setup that allows the simultaneous monitoring of both VTH and RON during the conventional switch-mode operation for packaged SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). The measurement is carried out by means of double-pulsed on-the-fly (OTF) characterization, in which the Device Under Test (DUT) is repeatedly switched between a high voltage off-state condition and a low voltage on-state one in the kHz frequency range. To this end, soft-switching conditions are considered (i.e., turn ON and turn OFF transitions are performed at zero current). During OTF measurements, the device VTH and RON are evaluated in order to monitor their drifts over time with a digital sampling oscilloscope (DSO). Since the parameter instabilities could manifest either after a few or several operating cycles [16], it is important to monitor the parameters of interest over several time decades to capture the whole dynamics. Accordingly, the DSO acquisition was triggered at logarithmically spaced time instants in order to avoid memory saturation and loss of accuracy [17]. By applying the proposed measurement method on commercially available devices (i.e., TO-247 vertical SiC MOSFETs), it was possible to test the effectiveness of the setup and to provide a qualitative interpretation of the physical mechanisms leading to the observed VTH and RON drifts. Particularly, negative drifts (i.e., decrease) were observed for both VTH and RON, that can be ascribed to the presence of traps at the SiC/SiO2 interface [18][19][20].
The paper is organized as follows. Section 2 describes the custom measurement setup and the adopted design strategies to improve the measurement accuracy. In Section 3, the data collected for tested SiC MOSFETs are presented, highlighting the drift effects on the devices' trans-characteristics. Particularly, we investigated the effect of the bias conditions on the observed drifts, emphasizing the effect of the gate-driving voltage applied in onstate (VGH) and of the drain voltage applied during off-state stress (VPH). Finally, Section 4 draws the conclusions of the paper.

Measurement Setup
The simplified schematic of the custom measurement setup is shown in Figure 1. The system was developed starting from the design first presented in [17], with several important modifications as discussed in the following. The gate driver circuit is responsible for repeatedly switching the DUT ON and OFF and is devoted to the generation of the ramp signal required for the extraction of the VTH and RON values. For performing on-wafer measurements, the usage of a simple waveform generator would be sufficient, thanks to the small parasitic capacitances of the DUTs. However, this solution is not viable for characterizing packaged devices due to the gateto-source capacitances (CGS) in the order of few nF [15]. For this reason, a custom gate driver circuit is required to prevent any distortion of the ramp signal, thus ensuring a fast and accurate VTH extraction [14].
In off-state conditions, the DUT is stressed with a large drain voltage (VPH), provided by a DC power supply, inducing both lateral and vertical trapping [21], while the on-state drain bias voltage (VPL) is provided through an isolated DC/DC converter. The drain voltage is pulsed by means of a couple of NMOS (MPH and MPL in Figure 1) arranged in a push-pull configuration, with an additional NMOS device (MPLL) inserted for reducing the series resistance during DUT turn-on. A 0.1 Ω resistor (RSENSE) is connected to the DUT's source terminal in order to extract the drain current (IDS) while a clamping circuit (M1, R1, D1) is connected to the drain terminal of the DUT for improving the measurement accuracy at low voltages. An additional NMOS (MDSC) was placed between the DUT's drain terminal and GND in order to force the drain voltage to 0 V. This condition is required for monitoring the parameter recovery after the VPH removal. The whole setup is controlled by a MicroController Unit (MCU) used for triggering the DSO acquisition with logarithmically spaced time steps [17] and for generating the necessary synchronization signals. Figure 2 illustrates a typical bias scheme for the circuit. Figure 3 shows the time sequence of the trigger signal used to activate the DSO acquisition.  The DSO used in this work was a four-channel PicoScope 5000 Series with 14-bit resolution and a 200 MHz maximum bandwidth. In this study, the rapid block mode was used for data acquisition in order to allow the log-like sampling required for monitoring the drifts over a large time interval.
Measurements were carried out with a 120 µs switching period and 10% Duty Cycle, i.e., by applying a gate bias rising from −5 V to 20 V in 6 µs, and staying at 20 V for the further 6 µs during the low-(drain)-voltage on-state time interval (see Figure 2a). In offstate conditions, a VPH stress voltage is applied to the DUT, whereas in the on-state VPL it is set to 3 V (see Figure 2b). VGL is typically set to −5 V in order to prevent a false turn-on and ensure the safe operation against voltage spikes on the gate [6]. During the 6 µs rising ramp, the IDS-VGS characteristic is reconstructed, allowing the extraction of the device VTH using the extrapolation in the linear regime method [7,22]. To perform a correct RON extraction, at the end of the gate ramp, a stable VGH level is held for 6 µs to compute RON as the ratio between the measured VD1 and IDS averaged over the 6 µs range.
The chosen IDS-VGS sweep rate represents the best compromise between speed and accuracy, since the short time required for the VTH and RON measurement minimizes the fast recovery between stress phases but still provides enough samples for a correct IDS-VGS acquisition.
The DUTs were commercially available TO−247 packaged SiC MOSFETs based on a vertical structure for which a good measurement repeatability was found. Accordingly, we discuss the results obtained on a single device without loss of generality. The fresh IDS-VGS characteristic acquired with the custom setup is shown in Figure 4.

Results
In this section, we discuss the results obtained with the OTF characterization system presented in Section 2. The results are shown in terms of VTH and RON drifts.
A sequence of no-stress acquisitions (i.e., VPH = 0 V) is performed on fresh devices at the beginning of each characterization and the corresponding IDS-VDS curves were acquired to set a reference value (VTH0 and RON0) from which the relative drifts are evaluated. Then, the parameter evolution is monitored for 1000 s. ∆VTH is simply calculated as (VTH-VTH0), whereas the acquired RON values are normalized with respect to the fresh value (RON0).

VTH and RON Drifts
The data obtained during the OTF characterization are shown in Figure 5. As we can see in Figure 5a, a significant negative VTH drift was observed only after several stress cycles (e.g., −0.55 V at 1000 s). The RON parameter evolution is shown in Figure 5b, in which a small benign negative shift of ≈ −3.5% was observed after 1000 s stress. To gain further insights on the origin of these behaviors, we investigated the transcharacteristics (IDS-VGS) evolution during stress. Accordingly, in Figure 6, we report three transfer curves measured at some relevant time instants. That is, the green solid curves, referring to the fresh devices, are used as reference, while the black dotted and red dashed curves correspond to the first (≈110 µs) and to the last (1000 s) stress period, respectively. The parameter drifts shown in Figure 5 find consistent correspondence with the trans-characteristics drifts of Figure 6. No appreciable drifts were observed after the first stress period on both VTH and RON, whereas non-negligible variations appeared after 1000 s of stress. A considerable triode-current variation (i.e., RON drift) is present in correspondence with the last stress cycle, in which a slight IDS increase (see inset of Figure 6b) is a signature of decreased RON. The negative VTH shift observed in Figure 5a is consistent with the IDS curve shift in Figure 6b after 1000 s. These negative drifts could be potentially explained by the presence of traps at the SiC/SiO2 interface [18−20] and the long transients observed could be a signature of charge emission dynamics.

Effects of Measurement Conditions on the Parameter Drifts
In order to better understand the mechanism involved in the observed drifts, it is important to know the influence of the measurement conditions on the parameter drifts. To this end, the effects associated to (i) the gate-driving voltage provided in on-state (VGH) and (ii) off-state drain voltage (VPH) were investigated. With the aim of pursuing a deeper investigation, after 1000 s stress, an additional 1000 s measurement phase was inserted for monitoring the parameter recovery after the VPH removal. During this additional phase, the VPH bias is no longer applied to the DUT and the drain voltage in the off-state condition is set to 0 V. On the other hand, the same gate ramp (VGL; VGH) and low drain voltage (VPL) are provided to the DUT for extracting VTH and measuring RON in the triode region.
To highlight the VGH effect on the observed drifts, the characterization was performed by ramping the gate voltage from −5 V to three different VGH levels (15 V, 17.5 V, and 20 V), whereas the VPH voltage was fixed to 100 V. The results obtained for the VTH and RON parameters are shown in Figures 7 and 8, respectively. The transients observed for both VTH and RON during the stress phase present similar time constants, suggesting a possible correlation between the two phenomena. Particularly, the observed negative VTH drift should lead to an increase in device conductivity (at a fixed driving VGH), thus yielding a RON reduction. The observed RON transient (Figure 8a) could thus be a directly correlated to the VTH ones (Figure 7a), indicating the presence of a single underlying mechanism.
However, a different VGH effect was observed for VTH and RON transients during the stress phase. As shown in Figure 7a, the VTH transients are not affected by the applied VGH, while a reduction in the RON drift is observed for higher VGH. This can actually be ascribed to the fact that at higher VGH, the device transconductance (gm) degrades [23], yielding a negligible impact on the overdrive voltage (i.e., VGS-VTH) variations on the device current and, consequently, on RON. On the other hand, at lower VGH, variations in the VTH affect the device current (i.e., RON), due to a less degraded gm [23]. Accordingly, the RON variation increases with decreasing VGH from 20 V to 15 V (see Figure 8a), even though the associated VTH transients present negligible differences (i.e., the VTH drift is the same). Concerning the recovery phase (see Figures 7b and 8b), both VTH and RON drifts are unrecoverable in the considered time interval (i.e., 1000 s), independently on the applied VGH level. This observation can be explained in two ways. (i) The recovery transients could present time constants longer than 1000 s, thus making the employed time window insufficient for observing them; (ii) the applied VPH stress (i.e., 100 V) does not affect the parameter drifts, and the observed instabilities can be totally ascribed to the gate switching applied during the OTF measurement. In fact, monitoring the device parameters in the recovery phase requires the gate voltage to be repeatedly switched from negative to positive and vice versa. As a consequence, the corresponding stress cannot be removed, making the second hypothesis more likely. To prove this point, the drain bias should be completely removed from the stress phase (VPH = 0 V), thus monitoring the DUT behavior when just the VGS stress is applied. Clearly, the results obtained with VPH = 0 V do not mimic a real operative condition but are nonetheless useful to clarify the origin of the observed drifts.
The results obtained with VPH = 0 V are shown in Figure 9, in which the transients measured during the stress phase are compared with those obtained for VPH = 100 V. The results shown in Figure 9 confirm our hypothesis, since no appreciable variation between the drifts measured with VPH = 0 V and VPH = 100 V was found, yielding nicely overlapped transients. Accordingly, the observed drifts can be totally ascribed to the driving VGS, at least for relatively low VPH (i.e., 100 V). This result is consistent with the vertical structure presented by tested devices. In fact, the influence of the drain voltage on interface states is limited by the physical distance between the oxide and the drain contact, while the gate bias plays a major role on the VTH stability [7−9]. However, thanks to the large breakdown voltage of SiC devices, the drain voltage applied in the off-state could be significantly higher than 100 V during a typical switching operation; thus, the complete absence of drain-induced instabilities cannot be ruled out. Accordingly, the VPH role has to be studied for sufficiently high stress voltages to observe the corresponding effect.
We now investigate the VTH and RON drifts at higher stress voltages (VPH). To this end, it is important to keep the driving VGL and VGH constant in order to discern the gate/drain contributions. Accordingly, a gate voltage between −5 V and 20 V was applied to the DUT, consistently with the waveforms reported in Figure 2, while three different VPH levels were considered (100 V, 250 V, and 500 V). A non-negligible VPH effect is observed on the VTH transients (Figure 10), whereas the corresponding RON drifts present a weak dependence on the applied VPH (see Figure 11). Negligible differences were observed on the VTH transients measured at 100 V and 250 V, whereas the negative VTH shift is more pronounced at VPH = 500 V (Figure 10a), indicating that the off-state stress voltage plays a role to some extent. Interestingly, this additional drift is recoverable within 1000 s from the VPH removal (see Figure 10b), indicating that negative BTI (NBTI) effects are non-permanent and are enhanced by high VPH values. The fact that stress and recovery transients have similar time constants suggests that both mechanisms are governed by two similar and opposite slow processes. Moreover, no recovery was observed for VPH = 100 V and VPH = 250 V, confirming that lower off-state stress voltages do not produce any additional instability with respect to that induced by the gate potential. On the other hand, as shown in Figure 11, the slow RON transients observed in the range between 1 s and 1000 s present negligible VPH dependence and just a weak recovery appears at VPH = 500 V (at least within 1000 s from the stress removal, as discussed in Section 3.1). This result is consistent with the VGH dependence previously investigated, indicating that the large gate bias applied in this phase (i.e., VGH = 20 V) makes the device current less sensitive to the VTH variations. In fact, the maximum RON dynamics was observed at VGH = 15 V (see Figure 7a), indicating that this condition is optimal to highlight the VPH effect even on the RON parameter. Accordingly, a new set of measurements was taken for a gate voltage ranging between VGL = −5 V and VGH = 15 V, exploring the same VPH levels used previously (100 V, 250 V, and 500 V). The corresponding VTH and RON drifts are reported in Figures 12 and 13, respectively. Once again, the negative VTH shift observed at VPH = 500 V is stronger with respect to that measured at lower VPH (Figure 12a). At the same time, the RON drift clearly changes from −10% to −17% while passing from 250 V to 500 V, showing the same VPH dependence observed on VTH (compare Figures 12a and 13a). The fact that similar trends were observed on both VTH and RON degradation confirms that the observed instabilities share the same origin. Particularly, the RON drift is a direct consequence of the VTH one, since at VGH = 15 V, the less degraded gm [23] makes the device current (i.e., RON) highly sensitive on the overdrive voltage (i.e., VGH-VTH). Accordingly, the significant impact of the drain stress voltage observed on VTH yields an additional RON drift at VPH = 500 V, whereas a negligible effect is observed at 100 V and 250 V.
Further evidence of the VPH role is given by the partial recovery observed after 500 V stress. Almost no recovery was observed after the stress performed at 100 V and 250 V, whereas the recovery transients appear for both RON and VTH at 500 V. In fact, after the VPH removal, the drain-induced instabilities can be recovered, whereas the gate-induced ones are still present, yielding a residual drift in VTH and RON. Accordingly, the recovery monitoring can be used to isolate the drain-induced instabilities from the gate-related ones, whereas the effect of the gate-driving voltage can be observed by reducing (or by possibly removing) VPH.

Conclusions and Recommendations
The developed measurement technique is able to highlight VTH and RON drifts in packaged SiC power devices. The observed slow negative VTH shift was associated with the presence of traps at the SiC-SiO2 interface. A direct correlation between VTH and RON drifts was first speculated and then verified. To this end, the impacts of the gate-driving voltage (VGH) and of the off-state stress drain voltage (VPH) were separately investigated. At a relatively low VPH (i.e., 100 V), the drifts can be totally ascribed to the repeated switching of the gate signal, whereas the VPH influence could be observed only at 500 V. This result is consistent with the vertical structure of tested devices, for which the gate potential is expected to mainly affect the dynamics of traps at the oxide-semiconductor interface. In order to observe drain-related effects in vertical devices, it is thus recommended to apply a large off-state stress voltage (VPH), which was limited to 500 V in this study. The outcomes obtained with the developed system are instrumental to reveal the mechanisms limiting device stability and performance, by discerning the gate/drain contribution on the parameter degradation. To get a better insight into gate-related effects, in future works, we will investigate other bias combinations, and we will study the effect of the negative gate voltage applied in the off-state (VGL). Moreover, the capabilities of the designed setup will be further enhanced by including hard-switching characterization, which will be used to highlight the effect of the transitions when SiC devices are driven in more severe switching conditions.