| dc.description.abstract | In this dissertation, the growth mechanisms and device characteristics of AlGaN/GaN-based Schottky barrier diodes (SBDs) with a high breakdown voltage and the AlInN-based high electron mobility transistors (HEMTs) with a high current density have been studied. For the high-voltage GaN SBDs, devices are fabricated on a composite AlGaN/AlN buffer layer with different threading dislocation (TD) densities. The correlation between TDs and the device characteristics could be well linked. The SBDs with an anode-to-cathode distance (LAC) of 30 μm exhibit a low on-state resistance (Ron) of 7.9 mΩ-cm2, a high breakdown voltage (VB) of 3,489 V, and a low leakage current of less than 0.2 μA at -2,000 V, which lead to a high figure-of-merit of 1.54 GW/cm2. Based on the x-ray diffraction, etch pit density, and transmission electron microscopy (TEM) measurements, high breakdown characteristics of the SBDs are attributed to low screw-type and high edge-type dislocations in the AlGaN/GaN buffer layer.
Several measurement are implemented for the in-depth analysis. The surface and buffer leakage current could be recognized successfully by the designed test devices. From capacitance-voltage (C-V) measurement, a large amount of initial occupied fixed charges at zero bias are recognized in the material, demonstrating the trapping effect by the edge-type dislocations. Moreover, the simulation results with the associated trap densities in the structures correlate well with the experimental results, which evidences the VB of the device is associated with their edge-type TDs in the material.
From dynamic Ron measurement, no obvious charging effect as well as the dynamic Ron degradation is observed during the reverse voltage bias. At room temperature (RT), the SBD with a high edge-type dislocation density show a low reverse recovery time of 17 ns. Under a high temperature of 150 oC, the switching curve of the device almost remain the same as RT’s performance. These performances are comparable to the reported GaN-based SBDs and outperform their silicon counterpart especially at high temperature, which demonstrate their potential for high-power low-loss switching circuits.
For the development of AlInN HEMTs, the theoretical calculation of polarization and the growth conditions have been systematically studied. The alloy scattering rate with arbitrary unit could be estimated in both AlGaN and AlInN alloys in comparison. In order to reduce alloy scattering in GaN-based HEMTs, a binary material as an AlN spacer layer with wide bandgap inserting between AlGaN/GaN or AlInN/GaN interface is essential to prevent the electron profile from extending into the barrier layer. The epitaxy growth conditions of the AlInN HEMTs are well investigated. After improving the crystal quality of AlInN HEMTs, the improved surface root-mean-square (RMS) roughness of 0.738 nm and the increased mobility to 1360 cm2/V-s without sacrificing its two-dimensional electron gas (2DEG) density (2.13×1013 cm-2) are successfully demonstrated, leading to a very low sheet resistance (Rsh) of 215 ohm/sq. The benchmark shows the mobility value is one of the best results among AlInN HEMTs grown on silicon substrate.
On the other hand, the electrical characteristics of a series of AlInN HEMTs with GaN cap layer thicknesses ranging from 0 to 26 nm have been investigated. The breakdown voltage, mobility of two-dimensional electron gas, on-state resistance, and dynamic Ron of the HEMTs are improved by increasing the cap layer thickness. The off-state breakdown voltage of AlInN MIS-HEMTs is increased from 530 to 675 V by adding a 13-nm-thick GaN cap layer. Detailed studies on the dynamic Ron of the AlInN HEMTs indicate that the GaN cap layer can greatly reduce the dynamic Ron ratio, and that the devices with a 26-nm-thick GaN cap layer can achieve a dynamic Ron ratio comparable to that of AlGaN MIS-HEMTs. These improved electrical characteristics are attributed to the GaN cap layer, which not only reduces the surface E-field but also raises the conduction band of the barrier layer and effectively prevents electrons from being trapped in the AlInN barrier and above. These results show the talents of AlInN/GaN HEMTs for modern power electronic devices. | en_US |