| dc.description.abstract | Power electronics is the branch that connects electricity and the world. Power devices play a significant role in reducing CO2 emissions and saving energy. As technology develops, the main focus is reducing energy consumption, increasing efficiency, reliability, and reducing costs. Typical power electronics applications include power supplies, chargers, renewable energy systems, electric vehicles, high-voltage transmission systems, and others, categorized by their used power ranges. These include low voltage (below 600V), medium voltage (600V to 6kV), high voltage (6kV to 20kV), and extra-high voltage (above 20kV). Semiconductor-based switches used to convert the power at these levels, such as Thyristors, Power diodes, MOSFETs, GTOs, BJTs, and IGBTs. Recently, GaN-based wide-bandgap semiconductor devices joined this group. The properties of GaN-based heterojunction devices, named High Electron Mobility Transistors (HEMTs), such as polarization-based 2D electron gas and high bandgap, enhance mobility, electron velocity, and breakdown voltage. As a result, HEMTs provide highly efficient, low-cost, and reliable power switches for a wide range of voltage applications. GaN power device development started a long time ago; however, the dynamic properties and reliability of the devices are still unclear, and more studies are required. This research focused on the in-house development of a HEMT for power-switching devices, from design, fabrication, packaging, and static and dynamic testing. The devices were developed for normally on and normally off operation. For practical applications, normally-off devices are more appreciated.
To achieve a 10A-600V current and voltage range, scaling of the device is required, and it is quite challenging. The figure of merit for a power switch is low Ron in the on-state and high blocking voltage with very low off-state leakage in a small area. The GaN devices are lateral and surface-sensitive. One of the advantages of silicon-based devices over any semiconductor device is the perfect interface of SiO2/Si. The interface is crucial for the device's dynamic and reliability. The fabrication step of MESA isolation using a dry etch chemistry creates dangling bonds and N-deficiency on the surface. The post-passivation process, the SiN/defect buffer interface, leads to high leakage in the device. It was observed that the 2-dimensional variable hopping mechanism is responsible for this surface conduction. To solve this issue, a bi-layer SiN using PECVD with low hydrogen contents was developed. The first layer of SiN was deposited by the PECVD tool with High frequency, and subsequent annealing helped to get a good interface between SiN and the etched surface. The second layer, SiN, deposited by low frequency and high frequency, gives excellent passivation for the devices. This bilayer method could show a better leakage-controlled and reliable GaN HEMT device on Silicon. With this baseline, we scaled the device to a 30mm gate-width multi-finger power device.
The winding gate structure with a 30x1mm gate fingers device layout was implemented for the fabrication. The device fabrication involves a multi-layer process in which gold electroplating thickens the metal to carry a high current. A gate field plate was implemented for high blocking. The fabricated device was diced and packaged on a TO-254 three-pin package. AlGaN/GaN HEMTs are normally-on devices due to the formation of 2DEG at the interface. The fabricated devices were measured at 8A current at VD=10V in the on-state. The device's threshold voltage is -7V, and the off-state and gate leakage was less than 500 uA. The wafer buffer determines the vertical breakdown of the device. A superlattice buffer, 5.2um thick, was used for fabrication, and the breakdown voltage was measured at 500V. The devices were diced, and packaged on a PCB, and their double pulse switching performance was tested, with results showing they could switch up to 5A-50V. Energy loss was also estimated for these devices. D-mode devices measuring up to 10A at VD=10V were integrated with commercially available low-voltage silicon MOSFET, and the resulting devices could measure up to 8A-200V with gate leakage of less than 1mA. These packaged devices were tested for their switching performance using double pulse measurements. Coss, Ciss, and Crss were also measured and compared to those of silicon devices in the same range. For testing purposes, the cascode device was used as a freewheeling diode due to the advantage of the third quadrant operation of cascode devices, which have a turn-on voltage of 0.7V. The double pulse test data calculated the energy loss, di/dt, and dv/dt.
For practical applications, enhancement mode devices are required, and for monolithic IC fabrication, E-mode devices made with p-GaN are more suitable. A selective etching recipe with a selectivity of 28:1 for GaN over AlGaN was optimized to fabricate p-GaN devices. To understand the gate leakage and threshold voltage reliability, we compared two types of metal stacks: Ti/Au and TaN. Ti/Au and TaN gated devices showed threshold voltages of +1V and +2.41V, respectively. The on-current of the device was ~200mA/mm for both devices. In temperature-dependent and other reliability studies, the TaN devices demonstrated superior performance over Ti/Au. Positive gate swing was measured up to +8V at room temperature and 150C with less than 100uA leakage. | en_US |
