Dissertations / Theses on the topic 'Current collapse'

A spectrum of phenomena related to the reliability of AlGaN/GaN HEMTs are investigated in this thesis using numerical simulations. The focus is on trap related phenomena that lead to decrease in the power output and failure of devices, i.e. the current collapse and the device degradation. The current collapse phenomenon has been largely suppressed using SiN passivation, but there are gaps in the understanding of the process leading to this effect. Device degradation, on the other side, is a pending problem of current devices and an obstacle to wide penetration of the market. Calibration of I-V measurements of two devices is performed with high accuracy to provide a trustworthy starting point for modelling the phenomena of interest. Traditionally, in simulations of nitride based HEMTs, only direct piezoelectric effect is taken into account and the resulting interface charge is thence independent of the electric field. In this work, the impact of the electric field via the converse piezoelectric effect is taken into account and its impact on the bound charge and the drain current is studied, as a refinement of the simulation methodology. It is widely believed that the current collapse is caused by a virtual gate, i.e. electrons leaked to the surface of the device. We have found a charge distribution that reproduced the I-V measurement that shows current collapse, hence validating the concept of the virtual gate. While it was previously shown that the virtual gate has a similar impact on the I-V curve as is observed during the current collapse, we believe that this is for the first time that a wide range of gate and drain voltages was calibrated. High gate/drain voltage leading to permanent degradation was also investigated. The hypothesis that stress induced defects and dislocations might be responsible for the degradation was tested but not fully confirmed. Finally, the leakage of electrons thought to be responsible for formation of the virtual gate and the current collapse due to the Poole-Frenkel emission, is simulated in order to explain the surface charge distribution responsible for the current collapse and deduced in Chapter 5.

Ce manuscrit présente les résultats d’une analyse exhaustive des mécanismes physiques qui limitent les performances et la fiabilité des transistors à haute mobilité d’électrons (HEMT) sur nitrure de gallium (GaN). En particulier : • Les phénomènes de dégradation à fort champ électrique des HEMT sur GaN sont analysés en comparant les données expérimentales avec les résultats de simulations physiques. Des stresses DC de 150 heures ont été effectués en conditions de canal ouvert et de pincement. Les effets des dégradations qui ont caractérisé ces deux types de stresses sont les suivants: une chute de courant DC de drain, une amplification des effets de gate-lag, et une diminution du courant inverse de grille. Les simulations physiques indiquent que la génération simultanée de piéges de surface (et/ou barrière) et de volume peut expliquer tous les modes de dégradation décrits plus haut. Les mesures expérimentales ont également montré que le stress en canal ouvert a causé une chute de la transconductance seulement pour de fortes valeurs de la tension VGS, alors que le stress au pincement a provoqué une chute de transconductance uniforme pour toutes les valeurs de VGS. Ce comportement peut être reproduit par la simulation physique pourvu que, dans le cas de stress a canal ouvert, on considère que les piéges s’accumulent au long d’une vaste région qui s’étend latéralement du bord de la grille vers le contact de drain, tandis que, dans le cas du stress au pincement, on considère que la génération des pièges ait lieu dans une portion plus petite de la zone d’accès à proximité de la grille et qu’elle soit accompagnée par une grande dégradation des paramètres de transport du canal. Enfin on propose que les électrons chauds et l’augmentation de la contrainte par le champ électrique soient à l’origine des dégradations observées après les stresses a canal ouvert et au pincement respectivement. • Les piéges dans les HEMT sur GaN ont été caractérisés en utilisant les techniques de DLTS et leur comportement associé de charge/décharge est interprété à l’aide des simulations physiques. Sous certaines conditions de polarisation, les piéges du buffer peuvent produire de faux signaux de piéges de surface, c'est-à-dire, le même type de signaux I-DLTS et ICTS attribués généralement aux piéges de surface. Clarifier cet aspect est très important à la fois pour les tests de fiabilité et pour l’optimisation des dispositifs, car il peut provoquer une identification erronée du mécanisme de dégradation, et par conséquent induire une mauvaise correction des procédés technologiques. • Les mécanismes physiques qui provoquent l’effondrement du courant RF dans les HEMT sur GaN sont analysés par le biais de mesures expérimentales et de simulations physiques. Ce travail propose les conditions suivantes : i) les piéges du buffer aussi bien que ceux de surface peuvent contribuer à l’effondrement du courant RF à travers un mécanisme identique qui impliquerait la capture et l’émission des électrons provenant de la grille; ii) la passivation de la surface diminue considérablement l’effondrement du courant RF par la réduction du champ électrique en surface et la diminution qui en découle de l’injection d’ électrons de la grille vers les pièges ; iii) pour des densités de piéges de surface inférieures à 9 × 1012 cm-2 , des barrières de potentiel superficiels dans l’ordre de 1-2 eV peuvent coexister avec des piéges de surface ayant des énergies plus faibles et qui causent l’effondrement du courant RF caractérisé par des constantes de temps relativement courtes. • Les effets de l’effondrement du courant dans les HEMT sur GaN sont étudiés en utilisant les résultats de mesures expérimentales et de simulations physiques. D’après les mesures pulsées, les dispositifs employés montrent un gate-lag considérable et un drain-lag négligeable qui peuvent être attribués à la présence de piéges de surface et de buffer respectivement
This thesis reports the results of an extensive analysis of the physical mechanisms that limit the performance and reliability of gallium nitride (GaN) based High Electron Mobility Transistors (HEMT). In particular: • High electric field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate–source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters. Channel hot electrons and electric-field-induced strain-enhancement are finally suggested to play major roles in power-state and off-state degradation, respectively. • Traps are characterized in AlGaN-GaN HEMTs by means of DLTS techniques and the associated charge/discharge behavior is interpreted with the aid of numerical device simulations. Under specific bias conditions, buffer traps can produce ‘‘false’’ surface-trap signals, i.e. the same type of current-mode DLTS (I DLTS) or gate-lag signals that are generally attributed to surface traps. Clarifying this aspect is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in wrong correction actions on the technological process. • The physical mechanisms underlying RF current collapse effects in AlGaN-GaN high electron mobility transistors are studied by means of measurements and numerical device simulations. This work suggests the following conclusions: i) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; ii) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; iii) for surface-trap densities lower than 9 × 1012 cm-2, surface-potential barriers in the 1–2 eV range can coexist with surface traps having much a shallower energy and, therefore, inducing RF current-collapse effects characterized by relatively short time constants. • Current collapse effects are investigated in AlGaN/GaN HEMTs by means of measurements and numerical device simulations. According to pulsed measurements, the adopted devices exhibit a significant gate-lag and a negligible drain-lag ascribed to the presence of surface and buffer traps, respectively. Furthermore, illumination of the devices with two specific wavelengths can result in either a recovering of current collapse or a decrease in the gate current. On the other hand, numerical device simulations suggest that the kink effect can be explained by electron trapping into barrier traps and the subsequent electron emission after a critical electric-field value is reached
Questa tesi riporta i risultati ottenuti da un’ampia analisi dei meccanismi fisici che limitano le prestazioni e l’affidabilità dei transistor ad alta mobilità di elettroni (HEMT) al nitruro di gallio (GaN). In particolare: • I fenomeni di degradazione ad alto campo elettrico nei GaN/AlGaN/GaN HEMT sono analizzati confrontando i dati sperimentali con i risultati delle simulazioni numeriche. Sono stati effettuati stress DC di 150 ore in condizioni di canale aperto e chiuso. Gli effetti di degradazione che hanno caratterizzato entrambi i tipi di stress sono i seguenti: una caduta nella corrente DC di drain, un’amplificazione degli effetti di gate lag, e una diminuzione della corrente inversa di gate. Le simulazioni numeriche indicano che la generazione simultanea di trappole in superficie (e/o barriera) e buffer può spiegare tutti i suddetti modi di degradazione. Le misure sperimentali hanno mostrato inoltre che lo stress a canale aperto ha causato una caduta della tranconduttanza solo ad alte tensioni VGS, mentre lo stress a canale chiuso ha provocato una caduta della transconduttanza uniforme a tutte le tensioni VGS. Questo comportamento può essere riprodotto con le simulazioni se, nel caso di stress a canale aperto, si assume che le trappole si accumulano lungo un’ampia regione che si estende lateralmente dal bordo di gate verso il contatto di drain, mentre, nel caso di stress a canale chiuso, si suppone che la generazione delle trappole abbia luogo in una più stretta porzione della zona di accesso vicino al bordo di gate e che sia accompagnata da una degradazione significativa dei parametri di trasporto del canale. Infine si propone che gli elettroni caldi del canale e l’aumento di strain indotto dal campo elettrico siano alla base delle degradazioni osservate dopo gli stress a canale aperto e chiuso rispettivamente. • Le trappole in AlGaN-GaN HEMTs sono caratterizzate usando le tecniche di DLTS e il relativo comportamento di carica/scarica é interpretato con l’aiuto delle simulazioni numeriche. Sotto particolari condizioni di polarizzazione, le trappole di buffer possono produrre falsi segnali da trappole di superficie, ossia lo stesso tipo di segnali I-DLTS e forma d’onda di gate lag attribuiti generalmente alle trappole di superficie. Chiarire questo aspetto è molto importante sia per le prove di affidabilità che per l’ottimizzazione dei dispositivi, in quanto può provocare una errata identificazione del meccanismo di degradazione, portando ad azioni correttive sbagliate nell’ottimizzazione del processo tecnologico. • I meccanismi fisici che originano il collasso di corrente RF negli HEMT AlGaN-GaN sono analizzati usando misure sperimentali e simulazioni numeriche. Questo lavoro suggerisce le seguenti condizioni: i) sia le trappole di superficie che quelle di buffer possono contribuire al collasso di corrente RF tramite un simile meccanismo fisico che coinvolge la cattura e l’emissione di elettroni provenienti dal gate; ii) la passivazione della superficie diminuisce fortemente il collasso della corrente RF tramite la riduzione del campo elettrico in superficie e la conseguente diminuzione dell’iniezione di elettroni dal gate alle trappole; iii) per densità di trappole di superficie minori di 9 × 1012 cm-2 , barriere di potenziale superficiale di 1-2 eV possono coesistere con trappole di superficie aventi energie relativamente basse e che provocano effetti di collasso di corrente RF caratterizzati da costanti di tempo relativamente corte. • Gli effetti di collasso di corrente negli HEMT AlGaN-GaN sono studiati usando i risultati delle misure sperimentali e delle simulazioni numeriche. Basandosi sulle misure delle caratteristiche d’uscita impulsate, i dispositivi utilizzati mostrano un evidente gate-lag e un trascurabile drain-lag, attribuiti alla presenza di trappole di superficie e buffer rispettivamente

Abstract This thesis examines magnetic reconnection in the solar corona. Magnetic reconnection is the only mechanism which allows the magnetic topology of magnetized plasmas to be changed. Many of the dynamic processes in the Sun's atmosphere are believed to be driven by magnetic reconnection and studying the behaviour of such phenomena is a key step to understanding the reconnection mechanism. In Chapters 1 to 3, we discuss the physical and mathematical framework on which current magnetohydrodynamic reconnection models are based. The aim of the thesis is to investigate theoretical models of magnetic reconnection using variety of analytic and numerical techniques within the theoretical frame work of magnetohydrodynamics (MHD). In Chapter 4 we use a line-tied X-point collapse model for compressible plasmas to investigate the role of viscosity on the energy release mechanism. This model also provides the basis for the investigation of Chapter 5 which explores the impact of Hall currents in the transient X-point energy dissipation. Chapter 6 is concerned with how reconnection is modified in the presence of generalized Ohm's law which includes both Hall current and electron inertia contributions. In contrast to the closed X-point collapse geometry adopted for compressible plasmas previously, we find it more convenient to explore this problem using an open incompressible geometry in which plasma is continually entering and exiting the reconnection region. Specially, we find the scaling of the Hall-MHD system size analytically, rather than numerically as in the X-point problem of Chapter 5. Chapter 7 summarizes the results of investigations in Chapters 4, 5 and 6.

Cette thèse relève du commerce international, des politiques monétaires et de macroéconomie internationale. Le premier chapitre étudie les différentes caractéristiques des différents régimes de change fixe et évalue les effets de chacune de ces caractéristiques sur le commerce international. Il apparaît que la transparence des prix et les coûts de transaction liés au changement de monnaie n’ont pas d’effet significatif sur le commerce, seule la crédibilité du régime de change fixe (liée à l’absence de possibilité de dévaluation) augmente significativement le commerce. Cela explique pourquoi seule l’union monétaire augmente le commerce entre ses membres dès les premières années du régime de change. Le second chapitre s’intéresse aux effets de l’euro sur le commerce pendant la crise financière de 2008-2009. Il apparaît que l’euro amortit la chute du commerce observée partout dans le monde, grâce à l’absence de volatilité du taux de change entre ses membres. Le troisième chapitre présente un modèle théorique permettant d’étudier la transmission des chocs dans une économie spécialisée verticalement, et propose une politique monétaire optimale pour stabiliser l’économie. Le modèle promeut des politiques monétaires symétriques entre les deux pays partageant un mode de production basé sur les chaînes globales de valeur
This thesis studies international trade, monetary policy and international macroeconomics. Chapter 1 examines the different characteristics of different fixed exchange rate regimes and assesses the effects of each of these characteristics on international trade. It appears that price transparency and transaction costs linked to the currency changes do not have a significant effect on trade; only the credibility of the fixed exchange rate regime (due to the absence of devaluation possibilities) increases trade significantly. This explains why only monetary union increases trade between its members from the first years of the exchange rate regime. Chapter 2 looks at the effects of the euro on trade during the 2008-2009 financial crisis.It seems that the euro amortized the trade drop observed throughout the world, owing to the absence of exchange rate volatility among its members. Chapter 3 presents a theoretical model to study the transmission of shocks in a vertically specialized economy, and proposes an optimal monetary policy to stabilize the economy. The model promotes symmetrical monetary policies between the two countries sharing a production mode based on global value chains

Dans cette thèse, les phénomènes d’auto-échauffement ont été étudié pour guider la conception de circuits intégrés 3D de nouvelle génération. Grâce à des études expérimentales et numériques, la dissipation thermique dans des imageurs 3D par collage hybride a été analysée et l’impact de l’augmentation de température résultante a été évalué. Premièrement, afin de développer des modèles précis, les propriétés thermiques des matériaux utilisés dans les circuits intégrés ont dû être déterminées. Différents films minces diélectriques impliquant des oxydes, des nitrures et des composés low-k ont été étudiés. Pour ce faire, la microscopie thermique à sonde locale (SThM) et la méthode électrothermique 3ω, sensibles à la conductivité thermique effective faible et élevée, ont été mises en œuvre. Dans un deuxième temps, des modèles éléments finis de circuits intégrés 3D ont été développés. Une méthode numérique nécessitant homogénéisations et approches multi-échelles a été proposée pour surmonter des grands rapports de forme inhérents à la microélectronique. La procédure numérique a été validée en comparant les calculs et les mesures expérimentales effectuées par SThM, la thermométrie résistive et la microscopie infrarouge sur une puce de test par collage hybride simplifiée. Il a été montré que la dissipation de chaleur est principalement limitée par la conductance du puit thermique ainsi que les pertes par l'air. Enfin, des études numériques et expérimentales ont été réalisées sur des imageurs 3D par collage hybride fonctionnels. Le champ de température a été mesuré par SThM et comparé aux calculs par éléments finis à la surface de la matrice. Les résultats numériques ont montré que la température de la surface des pixels est égale à celle du Front-End-Of-Line de l’imageur. L'influence de l'échauffement sur les performances optiques de l'imageur a été déduite de cette analyse. Cette étude a permis également d'évaluer les différentes méthodes numériques et expérimentales pour la caractérisation de la dissipation de chaleur en microélectronique
In this PhD thesis, self-heating phenomena are studied for guiding the design of next-generation 3D Integrated Circuits (ICs). By means of experimental and numerical investigations, associated heat dissipation in 3D Hybrid Bonding imagers is analyzed and the impact of the resulting temperature rise is evaluated. First, in order to develop accurate models, the thermal properties of materials used in ICs are to be determined. Different dielectric thin films involving oxides, nitrides, and low-k compounds are investigated. To do so, Scanning Thermal Microscopy (SThM) and the 3ω electrothermal method, sensitive to low and large effective thermal conductivity, are implemented. In a second step, finiteelement models of 3D ICs are developed. A numerical method involving homogenization and a multiscale approach is proposed to overcome the large aspect ratios inherent in microelectronics. The numerical procedure is validated by comparing calculations and experimental measurements performed with SThM, resistive thermometry and infrared microscopy on a simplified Hybrid Bonding test chip. It is shown that heat dissipation is mainly limited by the heat sink conductance and the losses through air. Finally, numerical and experimental studies are performed on fully-functional 3D Hybrid Bonding imagers. The temperature field is measured with SThM and compared with finite-element computations at the die surface. The numerical results show that the temperature of the pixel surface is equal to that of the imager Front-End-Of-Line. The influence of the temperature rise on the optical performance of the imager is deduced from the analysis. The study also allows assessing the various numerical and experimental methods for characterizing heat dissipation in microelectronics

碩士
東海大學
電機工程學系
103
A GaN high-electron-mobility field effect transistor (HEMT) with a two-dimensional electron gas (2DEG) as the channel is often used in high-frequency circuit. Usually lower on-resistance in semiconductor devices can be achieved by increasing element area. But increasing element area will increase capacitance and the switching loss when the device is used in high frequency applications. GaN HEMT can have a very low on-resistance with small capacitance, therefore, it can effectively reduce the power loss and enhance the characteristics of the circuit greatly. But GaN HEMTs have current collapse. After a drain bias stress, electrons may trapped by surface states that will cause the surface depletion and increase This paper developed a simulation method, to imitate the quantized degradation of a buck converter due to the GaN current collapse. This method is applied to different load mode and different switching frequencies. The results can provide circuit designers some reference about how current collapse in GaN HEMT may affect the circuits.

碩士
國立交通大學
工學院半導體材料與製程設備學程
108
High-power transistors and modules are the core devices of power electronics, which can be used for high-current power switching, but high-power operation can cause heat dissipation and affect the performance of devices. The thermal resistance of junction to case is the key specification of the power devices, which is related to heat dissipated performance. In addition, the electrical reliability of the device is also very important, affecting the stability and efficiency of its operation. From the influence scale measurement of the Current Collapse Effect, the ability of various material devices to withstand voltage can be clearly understood. Firstly, the thermal resistance of the power field effect transistor and the power module is measured. The measurement method is to heat the device under test with a certain power and current input, and measure the forward voltage drop between the p-n junctions. These temperature sensitive parameters utilized the relationship between the temperature and the forward voltage drop to calculate the surface temperature of the p-n junction, and combine the transient thermal resistance measurement method and the structure function method to evaluate the thermal resistance of the power transistor and the module. Refer to JESD 51-14 Transient Dual Interface Method (TDIM) method determines the thermal resistance of the device. The smaller the value, the better the heat dissipation capability of the device. The measurement method of replacing the traditional cooling curve with a heating curve can greatly reduce the time required for the measurement. Using the structure function method, we can see the thermal resistance change of each layer structure of the power device and the module, and compare the difference between the thermal resistance and the heat capacity. This method is used to analyze the reliability of structural layer as the basis for device packaging. Secondly, in order to understand the stability difference between the power device and the module, we measure the device current collapse effect, which is a reliability test. The measurement method is to add a negative bias condition to the device gate terminal with a fixed drain voltage and in the initial off state. A high electric field is formed due to the gate negative bias, resulting in a small leakage current at the gate. The electrons in the leakage current are easily captured by surface traps, thus on the surface. A negative potential is formed, and the electrons in the lower channel are depleted. When the device turned on, the trapped electrons cannot be released immediately, and the surface of the device channel will be electronically trap as the device is switched from the off state to the on state. The electron trap will cause the on-resistance to become large, causing the drain current to drop and the threshold voltage to shift, resulting in a decrease in operating power. This current collapse phenomenon hinders digital or analog operation. This experiment can understand the extent to which each device is affected by the current collapse effect and can be used as a reference for device process design.

碩士
國立臺灣大學
光電工程學研究所
104
The applications of gallium nitride high electron mobility transistors (GaN HEMTs) have become more and more important in recent years. Due to the outstanding material properties including wide-band-gap and high electron mobility, GaN HEMTs are widely applied to high voltage electronics and high efficiency power conversion systems. The two dimensional electron gas (2DEG) formed in heterojunction ensures the large operating output current and low on-resistance of the device. However, the leakage current and current collapse phenomenon concerning to the material defects reduce the power conversion efficiency of the device in high speed switching. In this research, enhancement-mode (E-mode) AlGaN/GaN HEMTs are demonstrated; the electrical characteristics and dynamic characteristics are also investigated. In this thesis, dynamic output characteristics are analyzed between the conventional p-GaN HEMT and the HEMT with plasma enhanced chemical vapor deposition (PECVD) SiO2 as the passivation. The current collapse can be suppressed effectively in E-mode HEMT with SiO2 due to the extra electron-accumulating space created by passivation. Electrons that used to accumulate in p-GaN capping layer and deplete 2DEG carriers can be neutralized by electron-accumulating space formed in SiO2. In order to construct metal-insulator-semiconductor (MIS)-HEMTs with optimized performance, high quality of layer interface is the critical factor of current collapse suppression and gate leakage minimization. We propose surface treatments including fluorine plasma, argon plasma and nitrogen plasma to improve interface quality. Based on our previous experience of developing AlGaN/GaN HEMTs, we construct MIS structures with atomic layer deposition (ALD) Al2O3. The Al2O3 can passivate the surface defects formed by plasma bombardment and suppress current collapse in long pulse mode. Also, we investigate the impact of epi structures on electrical characteristics and the phenomenon of current collapse. The power conversion efficiency in p-GaN MIS-HEMTs can be effectively improved by double heterostructure.

碩士
國立交通大學
電子工程學系 電子研究所
102
Gallium nitride-based high-electron-mobility transistors (HEMTs) have demonstrated outstanding performance for high-power and high-frequency applications for defense and communication systems. However, there are many undesirable effects such as current collapse and increase in dynamic ON-resistance due to the surface donor states and the high polarization nature of the material. Nitride-based materials are more desirable for GaN passivation because the oxide-based materials have many oxygen contaminations on GaN. SiN has been proved as an effective passivation dielectric to reduce the surface states and can efficiently suppress current collapse in the GaN HEMTs. However, the bandgap of SiN (∼5 eV) is not high enough to suppress leakage current. AlN has a large bandgap (∼6.2 eV) can effectively reduce leakage current as passivation layer. In this work, we demonstrate GaN MIS-HEMT using AlN/SiN bilayer gate dielectric and passivation layer which combine the advantages of SiN and AlN. For comparing the performance of GaN MIS-HEMT with AlN/SiN bilayer gate dielectric, we also prepared reference devices with single layers dielectric. The wafer was divided into three samples after mesa and ohmic contact process. In conclusion, SiN was been proved that it has many good effects for GaN passivation such as decreasing of channel resistance, low surface state, low interface trapping density and low current collapse effect. AlN with high bandgap nature can suppress the leakage current. However, it would increase channel resistance, and cause severe current collapse effect. In this study, an effective AlN/SiN bilayer dielectric and passivation layer have been demonstrated for reducing current collapse effect and leakage current in GaN MIS-HEMT.

碩士
國立臺灣大學
光電工程學研究所
106
Gallium nitride high electron mobility transistors (GaN HEMTs) have gained more focus in recent years. Due to the outstanding material properties including wide-band-gap and high electron mobility, GaN HEMTs are widely applied to high voltage electronics and high-efficiency power conversion systems. The two dimensional electron gas (2DEG) formed in heterojunction ensures the large operating output current and low on-resistance of the device. However, the inherent normally-on behavior excludes GaN HEMTs from most power electronic applications for reduced circuit complexity and fail-safe operation and current collapse phenomenon concerning the material defects reduce the power conversion efficiency of the device in high speed switching. With the purpose to increase the threshold voltage of the enhancement mode GaN HEMTs (high-electron mobility transistors), we fabricated devices with gate field plates on the p-GaN/AlGaN/GaN/AlGaN double heterostructure. We observed an existence of a subthreshold region from the current –voltage transfer curves. The threshold voltage of the device extracted based on linear extrapolation method is much higher (up to 5.90V) than that of a typical device without a gate field plate. We investigated electrical properties and proposed a model based on carrier distributions with the influence of gate electric field at different regions. Furthermore, we characterized current collapse phenomenon of p-GaN/AlGaN/GaN/AlGaN double heterostructure HEMTs with gate field plates. Our study shows gate field-plate induces additional capacitance that deteriorates current collapse phenomenon at lower off-state drain bias. On the other hand, suppression of current collapse with the existence of gate field plate is observed when the device is switched by a higher off-state drain voltage (>41V in our case for field plate of 3 µm). The results suggest the adoption of gate field plate depends on the application. For low drain voltage (<41V) application, one has to shrink the gate field plate length to reduce gate capacitance and alleviate the corresponding drawback of higher channel resistance. For power electronics operated far above 41V, field plate is advantageous to spread out vertical electric field.

碩士
國立高雄第一科技大學
應用日語研究所
99
Japan in highly economic growth time ⎾ Japanese-style management ⏌, "lifetime-employment system" One of for success primary key aspects. Because of this stable employment practice, employees can relax and work diligently until retirement. The company also has a comprehensive system for recruitment, hiring, training and assignment; thus employees have a high degree of loyalty for the company, and adhere to the common goal of workplace harmony. Colleagues share a strong sense of responsibility to work togrther when completing jobs, and they even hold the faith that the company is their home. employment practice; lifetime-employment is no longer the case. Paid by the hour and contract systems are now adopted. Even the seniority system has been replaced by a merit-based system. Under this system, because things are merit-based, colleagues do not trust each other due to competition. Furthermore, a lack of seniority system caused workplace ethics to undergo an unprecedented change. Therefore, we focus on the changes in However due to the crash of the bubble economy, the company''s traditional personnel system was forced to change. Especially in interpersonal relationships within enterprises after the collapse of the bubble economy .We organize, explore and discuss concrete policies and solutions for improvement. Key words ： Workplace ethics, sense of community, vertical relationships, mutual duty, consensus building