Categorization of PBTI Mechanisms on 4H-SiC MOSFETs by the Stress Gate Voltage and Channel Plane Orientation

To analyze the mechanism of the positive bias temperature instability (PBTI) of 4H-SiC MOSFETs based on existing models and predict the practical PBTI lifetime in low stress operation, its possible models were classified by the magnitude of the stress gate voltage $(V_{GS\_{}stress})$ under high-temperature gate-bias (HTGB) stress and channel plane orientations: Si-face (0001), C-face (000-1), and a-face (11-20). The test samples were vertical MOSFETs with planar surface channels on the Si- or C-face and trench-sidewall channels on the a-face. The measurement-stress-measurement cycles that alternately repeat the HTGB stress and the conventional drain current - gate voltage $(I_{D}-V_{GS})$ sweep derived the gate threshold voltage shifts $({\Delta }V_{TH}$ ( $t_{s}, sweep$ )) measured after both the stress time $(t_{s})$ and the $V_{TH}$ -shift-recovery with the sweep process. These were more conspicuous on higher $V_{GS\_{}stress}$ ( $>$ approximately 35, 25, and 30 V for the Si-, C-, and a-faces, respectively) at stress temperature $(T_{stress})\,\,=$ 448 K, identified as components trapped in the gate oxide bulk of the Fowler-Nordheim tunneling electrons. For lower $V_{GS\_{}stress}$ ( $=$ 15 V), we measured ${\Delta }V_{TH} (t_{s}, t_{r})$ as a function of $t_{s}$ followed by the relaxation time $(t_{r})$ at $T_{stress}\,\,=$ 293, 448, and 573 K. ${\Delta }V_{TH} (t_{s}, t_{r})$ was analyzed by the capture/emission time (CET) mapping suitable to evaluate the barrier energy beyond which channel electron carriers are (de-)trapped on the MOS interface states. The CET map successfully illustrated the dependence of ${\Delta }V_{TH} (t_{s}, t_{r})$ on $T_{stress}$ , demonstrating negative or positive correlation owing to shallower or deeper traps, early in the relaxation phase or after long-term $t_{s}$ , respectively. This also enabled to estimate the lifetime for a defined allowable limit of ${\Delta }V_{TH} (t_{s}, t_{r})$ . When the lifetime is set to about 30 years, the reaching ${\Delta }V_{TH}$ ( $t_{s}, sweep$ ) was calculated to be approximately 70, 150, 350 mV for the Si-, C-, and a-faces, respectively, after $t_{s}\,\,=$ 1E9 s $(\approx 30$ years) at $T_{stress}\,\,=$ 448 K on $V_{GS\_{}stress}\,\,=$ 15 V.