AlGaN/GaN-based heterojunction transistors are being developed with intensive efforts
for next-generation high-efficiency electric power converters, owing to their capabilities to
deliver low ON-resistance, high breakdown voltage, high switching frequency, and high-temperature
operation. The early AlGaN/GaN high electron mobility transistors (HEMTs) with
relatively simpler structures such as Schottky gate and unpassivated access region were faced
with challenges in reducing gate leakage and lowering the dynamic ON-resistance during
switching operation. Consequently, AlGaN/GaN HEMTs have evolved with more advanced
structures such as the gate and passivation dielectrics. With the gate dielectric, metal-insulator-semiconductor
HEMTs (MIS-HEMTs) exhibit lower gate leakage and larger gate voltage swing.
With advanced passivation dielectric structures, the current collapse phenomenon has been
greatly suppressed to deliver low dynamic ON-resistance.
The incorporation of gate dielectric and passivation dielectric in AlGaN/GaN
heterojunction transistors has its own challenges. In MIS-HEMTs, the dynamic
charging/discharging processes associated with the dielectric/semiconductor interface traps and
the gate dielectric’s bulk traps could induce instabilities in the threshold voltage (V
th). The gate
dielectric is also subject to the risk of degradation or breakdown under high electric field.
Furthermore, although very effective passivation dielectrics have been demonstrated, such as
SiN
x/AlN and Al
2O
3/AlN stacks featuring AlN epitaxy grown by plasma-enhanced atomic layer
deposition, the underlying mechanism still needs more in-depth investigation.
This thesis is devoted to the characterization of: (1) instability of V
th in dynamic
operation, (2) the degradation of gate dielectric, especially its spatial location, and (3) the
physical mechanisms of current collapse suppression by the SiN
x/AlN dielectric stack.
The instability of V
th includes hysteresis and long-term V
th-shift under stress. In this
thesis, the commonly-used quasi-static measurement of transfer characteristics is found to
underestimate V
th hysteresis that can be encountered in high-frequency power switching
because the dynamic behavior of interface traps is not captured. As a more accurate method for
evaluating the hysteresis, a pulsed characterization technique is proposed and demonstrated in
AlGaN/GaN power devices for the first time, which captures the dynamic charging/discharging
behavior of the fast interface traps. The characterization technique for long-term V
th-shift under
both dc and ac gate biases is also developed, exposing the effect of the bulk traps.
For characterizing the degradation of the gate dielectric, a new opto-electrical technique
is developed. For this purpose, transparent-gate MIS-HEMTs are realized in this thesis for the
first time. By combining electroluminescence (EL) from under the transparent gate with I-V
characteristics, the new method enables direct and non-destructive observation of the spatial
location of the degradation spots, in contrast to previous techniques that could only reveal a
change in the electrical behavior. Using this technique, the degradation sites are clearly shown
to occur at the overhang’s edge of the T-shaped gate where the dielectric stack underneath is
thick, rather than at the gate footprint where only a thin layer of gate dielectric is present.
To further investigate the passivation mechanism of the SiN
x/AlN stack, a
characterization combining electrical, microscopic, and spectroscopic measurements is carried
out in this thesis which revealed that the SiN
x/AlN passivation stack features a better interface
with less trap states, lower oxidation, and a monocrystal-like lattice quality compared with the
Al
2O
3/AlN stack, which explains its better effect in current collapse suppression.
Post a Comment