The deployment of fifth-generation (5G) networks requires more closely spaced wireless infrastructures with a high output power to deal with high-frequency signal attenuation issues. Microwave power limiters have been widely used in the RF front-end in various wireless communication systems. A diode limiter circuit prevents the damage of sensitive receiver components by allowing RF signals below a certain threshold to pass through, while larger signals exceeding the threshold are attenuated. Many studies have been carried out on Si-based diode limiters in recent years; however, they have shown scant room for further improvement as silicon reaches its theoretical limitations. From this perspective, there is a need for new semiconductor materials to satisfy the requirements of devices. Wide-bandgap materials (e.g., gallium nitride) have recently attracted a great deal of interest due to their superior material properties such as wide band-gap, high electron saturation velocity, and high critical electric field.
Although lateral-structure GaN devices are staying ahead of the pace of industrialization, they still face several constraints and do not reach the GaN material limit due to requiring a high epitaxial layer quality and precise processing. A vertical structure is a convenient solution in Si- or SiC-based devices, which are also attractive alternatives to GaN devices. Quasi-vertical GaN devices have the freedom to select substrates (such as silicon, sapphire, and SiC) by using hetero-epitaxial growth technology. A planar structure design is easy to integrate with other RF components. This dissertation aimed to develop a quasi-vertical GaN diode for high-power RF and microwave applications which could operate in a wide frequency band and at high input power levels, with easy integration and low cost. The scope of this dissertation involved three aspects: design and fabrication of a quasi-vertical GaN device with mesa etching optimization; suppression of reverse leakage with an enhanced breakdown voltage; demonstration of microwave power applications (limiters and detectors) based on developed GaN diodes.
First, a literature summary of the state-of-the-art vertical GaN SBDs is presented in Chapter 2. A trade-off between the 𝑅𝑜𝑛,𝑠𝑝 and BV of a diode is analyzed to characterize the performance of diodes. We discuss the benchmark of 𝑅𝑜𝑛,𝑠𝑝 and BV for vertical GaN SBDs with different substrates (Si, sapphire, and GaN) and various edge terminal techniques. The equivalent circuit model of a diode for studying the high-frequency properties is introduced.Second, the optimization of mesa etching for a quasi-vertical GaN SBD by inductively coupled plasma (ICP) etching is comprehensively investigated in this chapter. In particular, the microtrench at the bottom corner of the mesa is eliminated by optimizing etch recipes. For the photoresist (PR) masked GaN samples, high source power is the cause of deteriorated mesa sidewall morphology. Although high-temperature (>140 ℃) hard baking prior to etching can produce a smooth sidewall, the drawbacks are significant and include oblique sidewall profile formation and hard striping. For the 𝑆𝑖𝑂2-masked GaN samples, the micro-trench problem at the bottom corner of the mesa can be reduced or eliminated by reducing the source power or by adding 𝐵𝐶𝑙3 into the 𝐶𝑙2 plasma. After ICP etching, the use of a TMAH wet treatment for samples can obtain a near-90° steep mesa sidewall that is microtrench-free and has a smooth surface. The proposed etching technique can be extended to other GaN nanostructures, such as hexagonal pyramids and nanowire arrays, which is promising for sensors, vertical transistors, optoelectronics, and photovoltaics.Third, a quasi-vertical GaN SBD is developed from the perspective of epilayer design, device layout, device modeling, fabrication, and leakage suppression. The design flow and fabrication process of quasi-vertical GaN diodes for microwave power applications are presented. Three solutions are developed to suppress the leakage current, namely, mesa optimization, argon ion terminations, and post-mesa nitridation. The experiment results show that our diode has the lowest leakage current density at 80% of the BV among the reported vertical GaN SBDs for a BV between 120 and 250 V. Combining mesa optimization and post-mesa nitridation technology effectively enhances the breakdown voltage and achieves excellent conduction characteristics.Fourth, a high-performance quasi-vertical GaN Schottky diode on a sapphire substrate and its application for high-power microwave circuits are investigated. We experimentally demonstrate the use of a vertical GaN SBD for L-band microwave power limiters for the first time ever. The GaN SBD limiter can handle at least 40 dBm of CW input power at 2 GHz without failure, which is comparable to a commercial Si-based diode limiter. Then, we experimentally demonstrate a quasi-vertical GaN SBD with post-mesa nitridation for high-power and broadband microwave detection. The fabricated quasi-vertical GaN diode reaches a high forward current density of 9.19 𝑘𝐴/𝑐𝑚2 at 3 V, and BV of 106 V. An extremely high output current of 400 mA is obtained when the detected power reaches 38.4 dBm at 3 GHz in pulsed-wave mode.Finally, all of the research content mentioned in this thesis is summarized, and the problems needing to be further investigated with lucubrate direction are indicated.