Significant Differences in BTI and TDDB Characteristics of Commercial Planar SiC-MOSFETs

Silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) have been produced by several vendors for commercial applications. SiC-MOSFET reliability was assessed using bias-temperature instability (BTI) and time-dependent dielectric breakdown (TDDB) characteristics. Here, we compared two planar SiC-MOSFET samples (A and B) from different vendors. The samples exhibited significantly different positive and negative BTI, time-dependent gate-current, TDDB lifetime statistics, and temperature dependence. These differences suggest NO (nitric oxide)-annealing variations.


Introduction
Silicon carbide metal-oxide-semiconductor field-effect transistors (SiC-MOSFETs) have been successfully applied to railway vehicles. They are also being applied to electric vehicles (EVs). As EVs lack a railway vehicle-like redundancy system, EVs require SiC-MOSFETs to reduce the extrinsic defects. Their reliability needs to be understood. The commercial SiC-MOSFET reliability was compared in bias-temperature instability (BTI) [1], and time-dependent dielectric breakdown (TDDB) [2,3]. Further, gate oxide integrity (GOI) [4,5] was compared for automotive applications.
Several vendors produce SiC-MOSFETs. Here, we compared the reliability of two SiC-MOSFET samples (A and B) in BTI and TDDB.

Experimental
Both the MOSFETs exhibited a conventional planar vertical structure with a 45-46 nm-thick gate oxide when observed under a transmission electron microscope. Positive and negative BTI values (PBTI and NBTI) were measured at 200 °C using spot Ids monitoring during the stress conditions as a JED0 pattern [6]. TDDBs were measured at room temperature (RT, 20-27 ℃), -60 °C, and 200 °C, under a constant voltage stress (CVS, Vgs = 46 or 47 V). The gate current was monitored during the stress. We employed B2902A PC-controlled source measuring units (Keysight Technologies Inc., Santa Rosa, CA), STH-120 temperature-controlled furnaces (ESPEC CORP., Osaka, Japan), and LTF-70 cold plates (Graphtec Corporation, Yokohama, Japan).

Results and Discussion
Fig. 1 depicts the PBTI (a) and NBTI (b) threshold voltage shifts (ΔVth). Sample A exhibited a lower PBTI and a higher NBTI than Sample B, as reported earlier [1]. Moreover, both the samples exhibited Vth hysteresis [7] around 0.4 V by sweeping the gate voltage (Vgs: -10→25→-10 V) at 200℃. NO (nitric oxide)-annealing studies suggested higher nitrogen concentration in Sample A [8].  for each condition here, we present Weibull plots for tBD at RT in Fig. 3(a), which were obtained from another experimental set. Sample A exhibited smaller Weibull slope than Sample B.   [10] proposed charging-induced dynamic stress in SiO2/Si system. This elucidated TDDB anomaly upon using Ig(t) behavioral pattern.

Silicon Carbide and Related Materials 2021
When Ig is in the decreasing phase, a sample with a larger QBD shows a much larger tBD by CVS than expected. On the contrary, in the increasing phase, a sample with larger QBD suffers stronger stress than expected; thus, reducing its tBD. Consequently, while the former resulted in larger tBD variations (as in Sample A), the latter resulted in smaller tBD variations (as in Sample B).
Moreover, we elucidated the TDDB's NO-annealing dependence using test element group (TEG) chips [11]. The samples A and B corresponded to heavy and light NO-annealing situations, respectively. This argument was consistent with the BTI characteristics (Fig. 1). We hypothesized that holes and electrons were trapped near the SiO2/SiC interface [11]. The trapped location remains to be investigated [2,3,5,9]. Fig. 2 (b) depicts the calculated QBDs. Larger QBD values and normal QBD temperature dependence were observed in Sample A, as reported for SiO2/Si system [12]. Although higher temperatures resulted in lower hot-carrier generation, the defect formed upon thermal activation. Therefore, the TDDB temperature coefficient remained positive [13]. However, an anomalous temperature dependence for tBD and QBD was observed in Sample B, which resulted in the largest RT values. Weibull plots from a different experimental set are presented ( Fig. 3(b)) to support this anomaly. The extrinsic mode was observed to a greater extent at -60 ℃. This phenomenon should be investigated further.
Understanding the basis of this anomaly requires further investigation. The presence of residual carbon at the SiO2/SiC interface in an oxidized SiC sample was demonstrated in a carbon-ejection study [14]. The residual carbon might alter thermal activation during the defect formation process. The NO-annealing dependence is being investigated using the TEG chips [11].

Summary
The two SiC MOSFETs exhibited different PBTI, NBTI, TDDB statistics, and temperature dependence in commercial planar structures. These differences are probably due to NO-annealing variations.