Design, Fabrication and Characterization of GaN HEMTs for Power Switching Applications

The unique properties of the III-nitride heterostructure, consisting of gallium nitride (GaN), aluminium nitride (AlN) and their ternary compounds (e.g. AlGaN, InAlN), allow for the fabrication of high electron mobility transistors (HEMTs). These devices exhibit high breakdown fields, high electron mobilities and small parasitic capacitances, making them suitable for wireless communication and power electronic applications. In this work, GaN-based power switching HEMTs and low voltage, short-channel HEMTs were designed, fabricated, and characterized.In the first part of the thesis, AlGaN/GaN-on-SiC high voltage metal-insulator-semiconductor (MIS)HEMTs fabricated on a novel ‘buffer-free’ heterostructure are presented. This heterostructure effectively suppresses buffer-related trapping effects while maintaining high electron confinement and low leakage currents, making it a viable material for high voltage, power electronic HEMTs. This part of the thesis covers device processing techniques to minimize leakage currents and maximize breakdown voltages in these ‘buffer-free’ MISHEMTs. Additionally, a recess-etched, Ta-based, ohmic contact process was utilized to form low-resistive ohmic contacts with contact resistances of 0.44-0.47 Ω∙mm. High voltage operation can be achieved by employing a temperature-stable nitrogen implantation isolation process, which results in three-terminal breakdown fields of 98-123 V/μm. By contrast, mesa isolation techniques exhibit breakdown fields below 85 V/μm and higher off-state leakage currents. Stoichiometric low-pressure chemical vapor deposition (LPCVD) SiNx passivation layers suppress gate currents through the AlGaN barrier below 10 nA/mm over 1000 V, which is more than two orders of magnitude lower compared to Si-rich SiNx passivation layers. A 10% dynamic on-resistance increase at 240 V was measured in HEMTs with stoichiometric SiNx passivation, which is likely caused by slow traps with time constants over 100 ms. SiNx gate dielectrics display better electrical isolation at high voltages compared to HfO2 and Ta2O5. However, the two gate oxides exhibit threshold voltages (Vth) above -2 V, making them a promising alternative for the fabrication of recess-etched normally-off MISHEMTs.Reducing the gate length (Lg) to minimize losses and increase the operating frequency in GaN HEMTs also entails more severe short-channel effects (SCEs), limiting gain, output power and the maximum off-state voltage. In the second part of the thesis, SCEs were studied in short-channel GaN HEMTs using a drain-current injection technique (DCIT). The proposed method allows Vth to be obtained for a wide range of drain-source voltages (Vds) in one measurement, which then can be used to calculate the drain-induced barrier lowering (DIBL) as a rate-of-change of Vth with respect to Vds. The method was validated using HEMTs with a Fe-doped GaN buffer layer and a C-doped AlGaN back-barrier with thin channel layers. Supporting technology computer-aided design (TCAD) simulations indicate that the large increase in DIBL is caused by buffer leakage. This method could be utilized to optimize buffer design and gate lengths to minimize on-state losses and buffer leakage currents in power switching HEMTs

Estudio de la fiabilidad de los dispositivos HEMT de GaN

El presente trabajo de tesis en Ingeniería Electrónica tiene como objetivo el estudio de la fiabilidad de dispositivos HEMT (High Electron Mobility Transistor) de GaN (Gallium Nitride). Gracias a las ventajas teóricas en el uso de dispositivos GaN frente a dispositivos de Silicio y a la aparición de dispositivos GaN que mejoran las prestaciones de sus homónimos de Silicio, el siguiente paso, es el estudio de la fiabilidad que presentan estos dispositivos en la actualidad, con el objetivo de mejorarlos e investigar los límites actuales en determinadas aplicaciones de potencia. La investigación realizada en esta tesis se divide en tres partes claramente diferenciadas. La primera de ellas trata el estudio del principal problema asociado a la tecnología de los HEMT de GaN de cara a su funcionamiento en conmutación: la resistencia dinámica. Este ha sido un problema muy estudiado desde los inicios de la realización de los HEMT de GaN para aplicaciones de potencia, que a día de hoy sigue siendo uno de los principales problemas a resolver en estos dispositivos. En esta tesis se han estudiado distintos dispositivos comerciales, donde se demuestran que un buen diseño estructural del dispositivo GaN HEMT es imprescindible para evitar problemas asociados a la resistencia dinámica. Mientras unos fabricantes continúan presentando este problema en sus dispositivos, otros fabricantes han conseguido minimizarlo mediante modificaciones en la estructura. La segunda parte engloba el análisis y los resultados obtenidos desde un punto de vista de la fiabilidad eléctrica, o también conocido como robustez de estos dispositivos. Esta segunda parte engloba las pruebas para determinar la capacidad de soportar cortocircuitos y avalanchas por sobretensión. Análisis, que es de aplicación directa de cara al funcionamiento real de los dispositivos GaN HEMT como transistores de potencia en el campo industrial. Los resultados obtenidos en esta tesis muestran que los dispositivos HEMT de GaN no tienen un buen comportamiento frente a eventos de avalancha y a priori no parece un problema con una solución inmediata. Sin embargo, los dispositivos HEMT de GaN analizados en cortocircuito si demuestran un buen comportamiento ante condiciones de cortocircuito. A pesar de la necesidad de mejoras, que están relacionadas con el efecto de los electrones calientes (hot-electron) los dispositivos HEMT de GaN han conseguido soportar tiempos de cortocircuito mucho mayores que sus competidores de Silicio. Por último, la tercera parte engloba las pruebas relacionadas con la necesidad asociada al uso de estos dispositivos en la industria aeroespacial, analizando la robustez y comportamiento ante radiación electromagnética. Esta última parte de estudio, demuestra que el diseño y geometría de la estructura juega un papel esencial en el comportamiento de los dispositivos HEMT de GaN frente a radiación. Además, todos los cambios observados tras la radiación de los dispositivos HEMT de GaN están relacionados con efectos de atrapamiento/desatrapamiento de cargas en los dispositivos. Por este motivo, los dispositivos que han demostrado estar libres de este atrapamiento durante los test de resistencia dinámica, han demostrado ser robustos frente a radiación y no han sufrido cambios tras la misma, independientemente del uso de dosis altas o bajas de radiación gamma. Por tanto, los resultados obtenidos durante esta tesis muestran a los HEMT de GaN como dispositivos prometedores, que se encuentran ya muy cerca de su uso en aplicaciones reales, pero que todavía tiene aspectos tecnológicos, de cara a su implantación en los diseños de potencia actuales (tales como el encapsulado, circuitos de disparo, etc.), que se espera, vayan mejorándose con la evolución de los procesos de diseño y fabricación.The theoretical advantages of GaN devices compared to its Silicon counterparts has been extensively studied and confirmed over the last decade. Following these studies, the current thesis is made with the aim of studying the current reliability of commercial GaN devices to contribute the future improvement of these devices from the point of view of its application in the future of power electronics. The research carried out in this thesis is divided into three clearly differentiated parts. The first part is about the main problem associated with the GaN HEMT technology from its beginning and is known as dynamic resistance. This problem has been extensively studied in the literature over the last decades and still being today one of the main problems associated to the GaN HEMTs. This thesis studies this phenomenon over different commercial GaN devices. These studies show that a good structural design of the GaN HEMT is mandatory to avoid problems causing dynamic resistance. While some manufacturers still showing this problem on its HEMTs, others have managed to reduce it with new structural designs. The second part of this thesis corresponds to the analysis and results from a point of view of the electrical reliability, also known as robustness of the GaN HEMT devices. This second part it’s about the capability of the device to endure short-circuits and over-voltages. This part is the closest to the real application of the GaN HEMT devices as power semiconductor in the industrial environment. The results obtained in this thesis show that GaN HEMT devices do not show a good behavior with avalanche events and it does not seem to have an immediate solution. However, GaN HEMT devices analyzed in short circuit demonstrate good behavior under short circuit conditions. Despite of the need for improvements related with hot-electron effect, the GaN HEMTs have endured short circuit events for times much greater than its Silicon counterparts. As third and last part, this thesis shows the tests related with the needed of the use of these devices at the aerospace industry by analyzing the robustness and behavior against electromagnetic radiation. These studies demonstrate that the design and internal structure of the GaN HEMTs play a main role in the behavior of these devices against radiation. Besides, all the observed changes of the GaN HEMTs are related with charge trapping/detrapping effects. For this reason, the devices that have demonstrated being free current collapse during the dynamic resistance tests have also demonstrated a good behavior against radiation, and its electrical characteristics has not been modified after low and high doses of gamma radiation. Therefore, the obtained results over this thesis show the GaN HEMTs as promising devices for the future of power electronics. They are close to its use in final applications but still being necessary to improve some of their technological aspects for its use in the actual power designs (such as encapsulated, drive circuits …). These aspects are expected to improve with the evolution and improvement of design and manufacturing processes

OFF-State Reliability of pGaN Power HEMTs

The concern for climate changes and the increase in the electricity demand turned the attention towards the production, sorting and use of electric energy through zero emission (CO2) and highly efficient solutions (e.g. for electric vehicle), respectively. As a consequence, the need for high performance, reliable and low cost power transistors adopted for power applications is increasing as well. Gallium nitride seems to be the most promising candidate for the next generation of devices for power electronics, thanks to its excellent properties and comparable cost with respect to Si counterpart. The main and most adopted GaN-based device is the high electron mobility transistor (HEMT). In particular, in the case of switching power applications, HEMTs repeatedly are switched between high current on-state and high voltage off-state operation. For both operation modes a good reliability must be guaranteed. This thesis is focused on the reliability issues related to the off-state operation. The results have been obtained during a six months research period at imec (Belgium) on 200V p-GaN gate AlGaN/GaN HEMTS. Different devices have been investigated, differing for gate-to-drain distance, field plates lengths, AlGaN and GaN layers properties. Time-dependent dielectric breakdown and hard breakdown tests have been performed in combination with TCAD simulations. It has been demonstrated that the gate-to-drain distance (LGD) impacts the breakdown voltage and the kind of failure mechanism. If LGD ≤3um the breakdown occurs through the GaN channel layer due to short channel effects. In this case, by reducing the thickness of the GaN channel layer such behaviour can be attenuated, eventually leading to longer time-to-failure. If LGD≥ 4um the breakdown occurs between the 2DEG and the source field plates, where the properties of the AlGaN barrier layer (i.e. thickness and Al concentration) and the field plates configuration play the main role on the time-to-failure

Distributed Modeling Approach for Electrical and Thermal Analysis of High-Frequency Transistors

The research conducted in this dissertation is focused on developing modeling approaches for analyzing high-frequency transistors and present solutions for optimizing the device output power and gain. First, a literature review of different transistor types utilized in high-frequency regions is conducted and gallium nitride high electron mobility transistor is identified as the promising device for these bands. Different structural configurations and operating modes of these transistors are explained, and their applications are discussed. Equivalent circuit models and physics-based models are also introduced and their limitations for analyzing the small-signal and large-signal behavior of these devices are explained. Next, a model is developed to investigate the thermal properties of different semiconductor substrates. Heat dissipation issues associated with some substrate materials, such as sapphire, silicon, and silicon carbide are identified, and thinning the substrates is proposed as a preliminary solution for addressing them. This leads to a comprehensive and universal approach to increase the heat dissipation capabilities of any substrate material and 2X-3X improvement is achieved according to this novel technique. Moreover, for analyzing the electrical behavior of these devices, a small-signal model is developed to examine the operation of transistors in the linear regions. This model is obtained based on an equivalent circuit which includes the distributed effects of the device at higher frequency bands. In other words, the wave propagation effects and phase velocity mismatches are considered when developing the model. The obtained results from the developed simulation tool are then compared with the measurements and excellent agreement is achieved between the two cases, which serves as the proof for validation. Additionally, this model is extended to predict and analyze the nonlinear behavior of these transistors and the developed tool is validated according to the obtained large-signal analysis results from measurement. Based on the developed modeling approach, a novel fabrication technique is also proposed which ensures the high-frequency operability of current devices with the available fabrication technologies, without forfeiting the gain and output power. The technical details regarding this approach and a sample configuration of the electrode model for the transistor based on the proposed design are also provided

Micro- and Nanotechnology of Wide Bandgap Semiconductors

Owing to their unique characteristics, direct wide bandgap energy, large breakdown field, and excellent electron transport properties, including operation at high temperature environments and low sensitivity to ionizing radiation, gallium nitride (GaN) and related group III-nitride heterostructures proved to be enabling materials for advanced optoelectronic and electronic devices and systems. Today, they are widely used in high performing short wavelength light emitting diodes (LEDs) and laser diodes (LDs), high performing radar, wireless telecommunications, as well ‘green’ power electronics. Impressive progress in GaN technology over the last 25 years has been driven by a continuously growing need for more advanced systems, and still new challenges arise and need to be solved. Actually, lighting industry, RF defene industry, and 5G mmWave telecommunication systems are driving forces for further intense research in order to reach full potential of GaN-based semiconductors. In the literature, there is a number of review papers and publications reporting technology progress and indicating future trends. In this Special Issue of Electronics, eight papers are published, the majority of them focusing materials and process technology of GaN-based devices fabricated on native GaN substrates. The specific topics include: GaN single crystalline substrates for electronic devices by ammonothermal and HVPE methods, Selective – Area Metalorganic Vapour – Phase Epitaxy of GaN and AlGaN/GaN hetereostructures for HEMTs, Advances in Ion Implantation of GaN and Related Materials including high pressure processing (lattice reconstruction) of ion implanted GaN (Mg and Be) and III-Nitride Nanowires for electronic and optoelectronic devices

Towards a More Flexible, Sustainable, Efficient and Reliable Induction Cooking: A Power Semiconductor Device Perspective

Esta tesis tiene como objetivo fundamental la mejora de la flexibilidad, sostenibilidad, eficiencia y fiabilidad de las cocinas de inducción por medio de la utilización de dispositivos semiconductores de potencia: Dentro de este marco, existe una funcionalidad que presenta un amplio rango de mejora. Se trata de la función de multiplexación de potencia, la cual pretende resolverse de una manera más eficaz por medio de la sustitución de los comúnmente utilizados relés electromecánicos por dispositivos de estado sólido. De entre todas las posibles implementaciones, se ha identificado entre las más prometedoras a aquellas basadas en dispositivos de alta movilidad de electrones (HEMT) de Nitruro de Galio (GaN) y de aquellas basadas en Carburo de Silicio (SiC), pues presentan unas características muy superiores a los relés a los que se pretende sustituir. Por el contrario, otras soluciones que inicialmente parecían ser muy prometedoras, como los MOSFETs de Súper-Unión, han presentado una serie de comportamientos anómalos, que han sido estudiados minuciosamente por medio de simulaciones físicas a nivel de chip. Además, se analiza en distintas condiciones la capacidad en cortocircuito de dispositivos convencionalmente empleados en cocinas de inducción, como son los IGBTs, tratándose de encontrar el equilibrio entre un comportamiento robusto al tiempo que se mantienen bajas las pérdidas de potencia. Por otra parte, también se estudia la robustez y fiabilidad de varios GaN HEMT de 600- 650 V tanto de forma experimental como por medio de simulaciones físicas. Finalmente se aborda el cálculo de las pérdidas de potencia en convertidores de potencia resonantes empleando técnicas de termografía infrarroja. Por medio de esta técnica no solo es posible medir de forma precisa las diferentes contribuciones de las pérdidas, sino que también es posible apreciar cómo se distribuye la corriente a nivel de chip cuando, por ejemplo, el componente opera en modo de conmutación dura. Como resultado, se obtiene información relevante relacionada con modos de fallo. Además, también ha sido aprovechar las caracterizaciones realizadas para obtener un modelo térmico de simulación.This thesis is focused on addressing a more flexible, sustainable, efficient and reliable induction cooking approach from a power semiconductor device perspective. In this framework, this PhD Thesis has identified the following activities to cover such demands: In view of the growing interest for an effective power multiplexing in Induction Heating (IH) applications, improved and efficient Solid State Relays (SSRs) as an alternative to the electromechanical relays (EMRs) are deeply investigated. In this context, emerging Gallium Nitride (GaN) High‐Electron‐Mobility Transistors (GaN HEMTs) and Silicon Carbide (SiC) based devices are identified as potential candidates for the mentioned application, featuring several improved characteristics over EMRs. On the contrary, other solutions, which seemed to be very promising, resulted to suffer from anomalous behaviors; i.e. SJ MOSFETs are thoroughly analysed by electro‐thermal physical simulations at the device level. Additionally, the Short Circuit (SC) capability of power semiconductor devices employed or with potential to be used in IH appliances is also analysed. On the one hand, conventional IGBTs SC behavior is evaluated under different test conditions so that to obtain the trade‐off between ruggedness and low power losses. Moreover, ruggedness and reliability of several normally‐off 600‐650 V GaN HEMTs are deeply investigated by experimentation and physics‐based simulation. Finally, power losses calculation at die‐level is performed for resonant power converters by means of using Infrared Thermography (IRT). This method assists to determine, at the die‐level, the power losses and current distribution in IGBTs used in resonant soft‐switching power converters when functioning within or outside the Zero Voltage Switching (ZVS) condition. As a result, relevant information is obtained related to decreasing the power losses during commutation in the final application, and a thermal model is extracted for simulation purposes.<br /

A novel AlGaN/GaN based enhancement-mode high electron mobility transistor with sub-critical barrier thickness

Power-switching devices require low on-state conduction losses, high-switching speed, high thermal stability, and high input impedance. Using gallium nitride (GaN) based field-effect transistors, these properties for switching devices can be satisfied. GaN-based High Electron Mobility Transistors (HEMTs) are emerging as promising candidates for high-temperature, high-power (power electronics) and radio-frequency (RF) electronics due to their unique capabilities of achieving higher current density, higher breakdown voltage, higher operating temperatures and higher cut-off frequencies compared to silicon (Si). Conventional GaN HEMTs with an aluminium gallium nitride (AlGaN) barrier are of depletion-mode (d-mode) or normally-on which require a negative polarity power supply to turn off. On the other hand, enhancement-mode (e-mode) or normally-off AlGaN/GaN HEMTs are attracting increasing interest in recent years because no negative gate voltage is necessary to turn off the devices. This leads to the advantage of simple circuit design and low stand-by power dissipation. For power electronics applications, power switches which incorporate e-mode devices provide the highly desirable essential fail-safe operation. In this research, a new high performance normally-off GaN-based metal-oxide-semiconductor (MOS) high electron mobility transistor (HEMT) that employs an ultrathin sub-critical 3nm Al_0.25Ga_0.75N barrier layer and relies on an induced two dimensional electron gas (2DEG) for operation was designed, fabricated and characterized. The device consists of source and drain Ohmic contacts nominally overlapped by the gate contact and employs a gate dielectric. With no or low gate-to-source voltage (V_GS), there is no two dimensional electron gas (2DEG) channel at the AlGaN/GaN interface to allow conduction of current between the drain and source contacts as the AlGaN barrier thickness is below the critical thickness required for the formation of such channel. However, if a large enough positive bias voltage V_GS is applied, it causes the formation of a quantum well at the AlGaN/GaN interface into which electrons from the source and drain Ohmic regions are attracted (by the positive gate voltage), effectively creating a 2DEG channel, and so the structure is a normally-off field effect transistor. Normally-off GaN MOS-HEMT devices were fabricated using plasma enhanced chemical vapour-deposited (PECVD) silicon dioxide (SiO_2) as the gate dielectric. They demonstrated positive threshold voltages (V_th) in the range of +1V to +3 V, and very high maximum drain currents (I_DSmax) in the range of 450mA/mm to 650mA/mm, at high gate voltage (V_GS) of around 6 V. The devices also exhibited breakdown voltages in the range of 9V and 17V depending on the gate dielectric thickness, making them suitable for realising high current low voltage power devices required, for instance, for buck converters for mobile phones, tablets, laptop chargers, etc

The 2018 GaN Power Electronics Roadmap

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here