Unipolar and Bipolar Pulsed Gate Stresses and Threshold Voltage Shifts in GaN e-HEMTs

In GaN e-HEMTs, Threshold Voltage
$(\boldsymbol{V}_{TH})$
shift from gate voltage
$(\boldsymbol{V}_{GS})$
stress depends on the
$\boldsymbol{V}_{Gs}$
magnitude, stress time, recovery time (time between stress removal and
$\boldsymbol{V}_{TH}$
measurement), temperature and pulse polarity (0 to
$+\boldsymbol{V}_{GS}$
or
$-\boldsymbol{V}_{GS}$
to
$+\boldsymbol{V}_{GS}$
). In this paper, unipolar (0 to
$\boldsymbol{V}_{GS})$
and bipolar
$(-\boldsymbol{V}_{GS}$
to
$+\boldsymbol{V}_{GS})$
pulsed gate stresses have been performed on GaN e-HEMTs with different
$\boldsymbol{V}_{GS}$
magnitudes. The pulse frequency ranges from 25 Hz to 200 kHz. The results show negative
$\boldsymbol{V}_{TH}$
shifts for unipolar
$\boldsymbol{V}_{GS}$
pulses (between 5V and 8V) at long recovery times (above 500 seconds). The
$\boldsymbol{V}_{TH}$
shift is proportional to the
$\boldsymbol{V}_{GS}$
pulse magnitude. When the recovery time is increased from 50 milliseconds to 500 seconds, the measured
$\boldsymbol{V}_{TH}$
shift becomes more negative, indicating a faster release of trapped electrons than holes. In bipolar stresses, results show both positive and negative
$\boldsymbol{V}_{TH}$
shifts with no clear magnitude or frequency dependencies.