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 large...[ Read more ]

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 Al₂O₃/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 Al₂O₃/AlN stack, which explains its better effect in current collapse suppression.