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.