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.@en

We report a high-performance GaN Schottky barrier diode (SBD) on a sapphire substrate with a novel post-mesa nitridation technique and its application in a high-power microwave detection circuit. The fabricated SBD achieved a very high forward current density of 9.19 kA cm-2 at 3 V, a low specific on-resistance (RON,sp) of 0.22 mO cm2 and breakdown voltage of 106 V. An extremely high output current of 400 mA was obtained when the detected power reached 38.4 dBm@3 GHz in pulsed-wave mode with a small anode diameter of 70 μm. Meanwhile, broadband detection at frequencies ranging from 1 to 6 GHz was achieved at 33 dBm in continuous-wave mode.

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In this brief, a high-performance quasi-vertical GaN Schottky barrier diode (SBD) on sapphire substrate with post-mesa nitridation process is reported, featuring a low damaged sidewall with extremely low leakage current. The fabricated SBD with a drift layer of 1 μm has achieved a very high ON/OFF current ratio (Iscriptscriptstyle ON/Iscriptscriptstyle OFF of 1012 with a low leakage current of ∼ 10-9 A/cm2@-10 V, high forward current density of 5.2 kA/cm2 at 3 V in dc, a low differential specific ON-resistance (Rscriptscriptstyle ONsp) of 0.3 m Ω cm2, and ideality factor of 1.04. In addition, a transmission-line-pulse (TLP) I-V test was carried out and 53 kA/cm2 at 30 V in pulsed measurement was obtained without device failure, exhibiting a great potential for high power applications.

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Gallium nitride (GaN) has attracted increased attention because of superior material properties, such as high electron saturation velocity and high electrical field strength, which are promising for high-power microwave applications. We report on a high-performance vertical GaN-based Schottky barrier diode (SBD) and its demonstration in a microwave power limiter for the first time. The fabricated SBD achieved a very low differential specific on-resistance (R_ON,sp) of 0.21 mΩ·cm², attributed to the steep-mesa technology, which assists in reducing the spacing between the edge of the anode and cathode to 2 µm. Meanwhile, a low leakage current of ~10⁻⁹ A/cm²@−10 V, a high forward current density of 9.4 kA/cm² at 3 V in DC, and an ideality factor of 1.04 were achieved. Scattering parameter measurements showed that the insertion loss (S₂₁ ) was lower than −3 dB until 3 GHz. In addition, a microwave power limiter circuit with two anti-parallel diodes was built and measured on an alumina substrate. The input power level reached 40 dBm (10 watts) in continuous-wave mode at 2 GHz, with a corresponding leakage power of 27.2 dBm (0.5 watts) at the output port of the limiter, exhibiting the great potential of GaN SBD in microwave power limiters.

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In this study, novel AlGaN/GaN Schottky barrier diodes (SBDs) are fabricated with thin-barrier (5 nm) AlGaN/GaN heterostructures, featuring recess-free technology, eliminating bombardment plasma damage, and leading to high device uniformity. Combining a gated-edge termination (GET) design and assistance with high-quality low-pressure chemical vapor deposition SiN x, a low reverse leakage current (∼10 nA mm-1@-600 V) and a high reverse breakdown voltage of over 1.78 kV (@1 μA mm-1) are obtained. At the same time, we achieve a low turn-on voltage of 0.57 V and a low differential on-state resistance R on,sp of 1.49 mΩ cm2 for thin-barrier GET SBDs with an anode-to-cathode distance (L AC) of 15 μm, yielding a Baliga's figure of merit of 2120 MW cm-2. Moreover, this proposed diode process flow is compatible with AlGaN/GaN high-electron-mobility transistors, which is promising for its integration in the smart GaN platform.

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The optimization of mesa etch for a quasi-vertical gallium nitride (GaN) Schottky barrier diode (SBD) by inductively coupled plasma (ICP) etching was comprehensively investigated in this work, including selection of the etching mask, ICP power, radio frequency (RF) power, ratio of mixed gas, flow rate, and chamber pressure, etc. In particular, the microtrench at the bottom corner of the mesa sidewall was eliminated by a combination of ICP dry etching and tetramethylammonium hydroxide (TMAH) wet treatment. Finally, a highly anisotropic profile of the mesa sidewall was realized by using the optimized etch recipe, and a quasi-vertical GaN SBD was demonstrated, achieving a low reverse current density of 10⁻⁸ A/cm² at −10 V.

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Gallium nitride (GaN)-based vertical power Schottky barrier diode (SBD) has demonstrated outstanding features in high-frequency and high-power applications. This paper reviews recent progress on GaN-based vertical power SBDs, including the following sections. First, the benchmark for GaN vertical SBDs with different substrates (Si, sapphire, and GaN) are presented. Then, the latest progress in the edge terminal techniques are discussed. Finally, a typical fabrication flow of vertical GaN SBDs is also illustrated briefly.@en

The diversified system requirements have been continuously growing to drive the development of a variety of new package styles. Wafer-Level Packaging (WLP) technology has drawn attention with its thinner package due to the removal of substrate and thus higher performance. With the greater design flexibility in having more I/Os, Fan-out Wafer Level Packaging (FOWLP) technology has proven to be a more optimal and promising solution. Infineon's eWLB was introduced as the first generation FOWLP technology. In the eWLB technology, die shift is the processing defect that the die moves from its default position result in the wire disconnection. Placing the dies at the preset pitch can compensate the die shift before die attach process. In this paper, the key factors to controlling die shift will be discussed, and the complex process capability index (Cpk) of process is 2.06 which represents the die shift can be well minimized after compensation.

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