Investigation of Gallium Nitride Transistor Reliability through Accelerated Life Testing and Modeling

Gallium nitride high electron mobility transistors are attractive to the DoD for their ability to operate at high frequencies, voltages, temperatures, and power. Yet, there are concerns about the reliability of these devices. Various degradation mechanisms and their causes are proposed in the literature. A variety of reliability tests were conducted to understand these mechanisms and causes. A multi-stressor experiment revealed different failure mechanisms than are in the literature. In particular, the devices tested at high voltage in the OFF state did not degrade significantly as suggested by others\u27 reports. The validity of temperature-accelerated life testing when applied to GaN HEMT lifetime assessments is questioned. Temperature alone could not explain the differences in observed degradation. The tested devices showed excellent robustness to high forward gate stress, exhibiting only a slight change in gate diode characteristic, little decrease in maximum drain current, and a persisting breakdown voltage exceeding 200 V. The time-dependence of degradation was analyzed, and results of continuous- and pulsed-direct current stressing were compared

Development of InAlN HEMTs for space application

This thesis investigates the emerging InAlN high electron mobility transistor (HEMT) technology with respect to its application in the space industry. The manufacturing processes and device performance of InAlN HEMTs were compared to AlGaN HEMTs, also produced as part of this work. RF gain up to 4 GHz was demonstrated in both InAlN and AlGaN HEMTs with gate lengths of 1 μm, with InAlN HEMTs generally showing higher channel currents (~150 c.f. 60 mA/mm) but also degraded leakage properties (~ 1 x 10-4 c.f. < 1 x 10-8 A/mm) with respect to AlGaN. An analysis of device reliability was undertaken using thermal stability, radiation hardness and off-state breakdown measurements. Both InAlN and AlGaN HEMTs showed excellent stability under space-like conditions, with electrical operation maintained after exposure to 9.2 Mrad of gamma radiation at a dose rate of 6.6 krad/hour over two months and after storage at 250°C for four weeks. Furthermore a link was established between the optimisation of device performance (RF gain, power handling capabilities and leakage properties) and reliability (radiation hardness, thermal stability and breakdown properties), particularly with respect to surface passivation. Following analysis of performance and reliability data, the InAlN HEMT device fabrication process was optimised by adjusting the metal Ohmic contact formation process (specifically metal stack thicknesses and anneal conditions) and surface passivation techniques (plasma power during dielectric layer deposition), based on an existing AlGaN HEMT process. This resulted in both a reduction of the contact resistivity to around 1 x 10-4 Ω.cm2 and the suppression of degrading trap-related effects, bringing the measured gate-lag close to zero. These discoveries fostered a greater understanding of the physical mechanisms involved in device operation and manufacture, which is elaborated upon in the final chapter

Physics of electrical degradation in GaN high electron mobility transistors

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2009.Cataloged from PDF version of thesis.Includes bibliographical references (p. 147-153).The deployment of GaN high electron mobility transistors (HEMT) in RF power applications is currently bottlenecked by their limited reliability. Obtaining the required reliability is a difficult issue due to the high voltage of operation. In order to improve reliability, it is essential to develop detailed physical understanding of the fundamental degradation mechanisms. In this thesis, we investigate the physical mechanisms behind the electrical degradation of GaN HEMTs by performing systematic stress experiments on devices provided by our industrial collaborators. These devices are electrically stressed under various bias conditions while regularly characterized by a benign characterization suite. We observe that electrical stress beyond a critical voltage results in an increase in drain resistance, a decrease in maximum drain current, and a sharp increase in reverse gate current. We show that this mode of degradation is driven by electric field and that current is less relevant. Behind this degradation is trap formation that occurs at the critical voltage. To understand this, we have developed a new trap-analysis methodology. It is found that under stress, the density of traps increases in the AlGaN barrier layer in the proximity to the gate edge on the drain side of the device. We show that this degradation is enhanced under mechanical uniaxial tensile strain that is externally applied to the device. From our experiments, we propose a degradation mechanism of defect formation through the inverse piezoelectric. In this mechanism, high vertical electric field at the gate edge under high voltage increases tensile stress in the AlGaN layer due to piezoelectricity of the material.(cont.) When the elastic energy in the crystal exceeds a critical value, crystallographic defects are formed. These defects trap electrons and reduce drain current as well as provide leakage paths and increase gate current. We theoretically validate the plausibility of this hypothesis and provide a model for the critical voltage that agrees with experimental observations. Unlike conventional wisdom, hot electrons do not appear to be the direct cause of electrical degradation in the devices that we study. Our studies suggest several possibilities to improving the electrical reliability of GaN HEMTs.by Jungwoo Joh.Ph.D

Trapping and Reliability Investigations in GaN-based HEMTs

GaN-based high electron mobility transistors (HEMTs) are promising candidates for future microwave equipment, such as new solid state power amplifiers (SSPAs), thanks to their excellent performance. A first demonstration of GaN-MMIC transmitter has been developed and put on board the PROBA-V mission. But this technology still suffers from the trapping phenomena, principally due to lattice defects. Thus, the aim of this research is to investigate the trapping effects and the reliability aspects of the GH50 power transistors for C-band applications. A new trap investigation protocol to obtain a complete overview of trap behavior from DC to radio-frequency operation modes, based on combined pulsed I/V measurements, DC and RF drain current measurements, and low-frequency dispersion measurements, is proposed. Furthermore, a nonlinear electro-thermal AlGaN/GaN model with a new additive thermal-trap model including the dynamic behavior of these trap states and their associated temperature variations is presented, in order to correctly predict the RF performance during real RF operating conditions. Finally, an advanced time-domain methodology is presented in order to investigate the device’s reliability and to determine its safe operating area. This methodology is based on the continual monitoring of the RF waveforms and DC parameters under overdrive conditions in order to assess the degradation of the transistor characteristics in the RF power amplifier

Characterization of Charge Trapping Phenomena in GaN-based HEMTs

This dissertation reports on charge-trapping phenomena and related parasitic effects in AlGaN/GaN high electron mobility transistors. By means of static and pulsed I-V measurements and deep-level transient spectroscopy, the main charge-trapping mechanisms affecting the dynamic performance of GaN-based HEMTs devoted to microwave and power switching applications have been comprehensively characterized, identifying the nature and the localization of the deep-levels responsible for the electrically active trap-states. A high-voltage measurement system capable for double-pulsed ID-VD, ID-VG and drain-current transient spectroscopy has been successfully designed and implemented. A characterization methodology, including the analysis of static I-V measurements, pulsed I-V measurements, and deep-level transient spectroscopy, has been developed to investigate the impact of voltage, current, and temperature on the parasitic effects of charge-trapping (threshold voltage instabilities, dynamic on-resistance increase, and transconductance reduction), and on trapping/detrapping kinetics. Experimental results gathered on transistor structures are supported by complementary capacitance deep-level transient spectroscopy (C-DLTS) performed on 2-terminal diode (FATFET) structures. Two main case-studies have been investigated. Schottky-gated AlGaN/GaN HEMTs grown on silicon carbide substrate employing iron and/or carbon doped buffers devoted to microwave applications, and MIS-gated double-heterostructure AlGaN/GaN/AlGaN HEMTs grown on silicon substrate devoted to power switching applications. The devices under test have been exposed to the complete set of current-voltage regimes experienced during the real life operations, including off-state, semi-on-state, and on-state. The main novel results are reported in the following: • Identification of a charge-trapping mechanism promoted by hot-electrons. This mechanism is critical in semi-on-state, with the combination of relatively high electric-field and relatively high drain-source current. • Identification of a positive temperature-dependent charge-trapping mechanism localized in the buffer-layer, potentially promoted by the vertical drain to substrate potential. This mechanism is critical in high drain-voltage off-state bias in high temperature operations. • Identification of deep-levels and charge-trapping related to the presence of doping compensation agents (iron and carbon) within the GaN buffer layer. • Identification of charge-trapping mechanism ascribed to the SiNX and/or Al2O3 insulating layers of MIS-gated HEMTs. This mechanism is promoted in the on-state with positive gate-voltage and positive gate leakage current. • Identification of a potential charge-trapping mechanism ascribed to reverse gate leakage current in Schottky-gate HEMTs exposed to high-voltage off-state. • The characterization of surface-traps in ungated and unpassivated devices by means of drain-current transient spectroscopy reveals a non-exponential and weakly thermally-activated detrapping behaviour. • Preliminary synthesis of a degradation mechanism characterized by the generation of defect-states, the worsening of parasitic charge-trapping effects, and the degradation of rf performance of AlGaN/GaN HEMTs devoted to microwave operations. The evidence of this degradation mechanism is appreciable only by means of rf or pulsed I-V measurements: no apparent degradation is found by means of DC analysis

INVESTIGATION OF RELIABILITY IN GALLIUM NITRIDE HIGH ELECTRON MOBILITY TRANSISTORS USING EQUIVALENT CIRCUIT MODELS FOR USE IN HIGH POWER, HIGH FREQUENCY MICROWAVE AMPLIFIERS

Gallium Nitride (GaN) is beginning to emerge as an alternative to the Gallium Arsenide in high power, high frequency microwave communications. Other novel semiconductors show potential at higher frequency applications. The largest obstacles to GaN emerging as the dominant microwave semiconductor are the issue of cost, which could be reduced through volume, and question of reliability. A new approach to the analysis of reliability has been developed based on the periodic generation of equivalent circuit models while a device is stressed in a manner that is similar to performance likely to be seen during commercial operation. Care was made in this research to ensure that the stress measurements used to induce degradation are as close as possible to those that would degrade a device in real world applications. Equivalent circuit models (ECM) can be used to simulate a device in computer aided design (CAD) software, but these models also provide a picture of the physical properties within the device at a specific point in time. The periodic generation of ECMs allows the researcher to understand the physical changes in the device over time by performing non-destructive electronic measurements. By analyzing the changes in device performance, the physical mechanism of device degradation can be determined. A system was developed to induce degradation and perform measurements of sufficient detail to produce a large signal ECM. Software for producing the ECM was also created. The changes in the ECM were analyzed to diagnose the physical changes in the device under test (DUT) and to identify a method of degradation. The information acquired from this system can be used to improve the device manufacturing process at the foundry. It can also be used to incorporate device degradation into the operation of systems

DESIGN TECHNIQUES FOR HIGH-EFFICIENCY MICROWAVE POWER AMPLIFIERS

The increasingly diffusion of wireless devices during the last years has established a sort of “second youth” of analog electronics related to telecommunication systems. Nowadays, in fact, electronic equipments for wireless communication are exploited not only for niche sectors as strategic applications (e.g., military, satellite and so on): as a matter of fact, a large number of commercial devices exploit wireless transmitting systems operating at RF and microwave frequencies. As a consequence, increasing interest has been focused by academic and industrial communities on RF and microwave circuits and in particular on power amplifiers, that represent the core of a wireless transmitting system. In this context, more and more challenging performance are demanded to such a kind of circuit, especially in terms of output power, bandwidth and efficiency. The present thesis work has been focused on RF and microwave power amplifier design that, as said before, represents one of most actual and attractive research theme. Several aspects of such topic have been covered, from the analysis of different design techniques available in literature to the development of an innovative design approach, providing many experimental results related to realized power amplifiers. Particular emphasis has been given to high-efficiency power amplifier classes of operation, that represent an hot issue in a world more and more devoted to the energy conservation. Moreover, electron device degradation phenomena were investigated, that although not directly accounted for, represent a key issue in microwave power amplifier design. In particular, the first chapter of this thesis provides an overview of commonly adopted design methodologies for microwave power amplifier, analyzing the advantages and the critical aspects of such approaches. Moreover, nonlinear device modeling issues oriented to microwave power amplifier design have been dealt with. In the second chapter, an innovative design technique is presented. It is based on experimental electron device nonlinear characterization, carried out by means of a low-frequency large signal measurement setup, in conjunction with the modeling of high-frequency nonlinear dynamic phenomena. Several design examples have been carried out by exploiting the proposed approach that confirm the effectiveness of the design technique. In the third chapter, the proposed design methodology has been applied to high-efficiency power amplifier classes of operations, that need to control the device terminations not only at the fundamental frequency, but also at harmonics. Two high-efficiency power amplifiers have been realized by adopting such a technique, demonstrating performance in terms of output power and efficiency comparable with the state of the art. Finally, in chapter four an important power amplifier design aspect has been dealt with, related to degradation and performance limitation of microwave electron devices. Several experimental results have been carried out by exploiting a new measurement setup, oriented to the characterization of degradation phenomena of microwave electron devices

Caractérisation des effets parasites dans les HEMTs GaN : développement d'un banc de mesure 3ω

This report is devoted to the development of a new measurement bench for thermal impedance characterization of GaN HEMTs. This measurement test set uses the so-called « 3ω » technique, which consists to measure the electrical signal at third harmonic real image of the thermal magnitude variations of the device. A sweep in function of the excitation frequency allows extracting of the thermal impedance. The measurement results have been validated by electrical simulation. Other complementary studies were performed to identify the trapping effects using different methods to extract the traps signature. The realization of nonlinear models is presented for AlGaN HEMT / GaN and InAIN / GaN to the power amplification applications in frequency bands X and K.Ce document porte sur le développement d’un nouveau banc de mesure pour la caractérisation de l’impédance thermique des HEMTs GaN. Le banc développé repose sur la méthode dite « 3ω » qui consiste à mesurer l’harmonique 3 d’un signal électrique véritable image des variations thermiques du composant. Un balayage en fonction de la fréquence d’excitation conduit à l’extraction de l’impédance thermique. Les résultats de mesures ont été validés par les simulations électriques. Des études complémentaires ont été réalisées pour l’identification des effets de pièges en utilisant différentes méthodes permettant l’extraction de la signature des pièges. La réalisation des modèles non-linéaires est présentée pour les transistors HEMT AlGaN/GaN et InAIN/GaN pour des applications d’amplificateur de puissance dans les bandes de fréquences X et K

Zuverlässigkeit von AlGaN/GaN-Leistungsbauelementen

Zur Ermittlung der Zuverlässigkeit von leistungselektronischen Bauelementen sind eine Reihe von Testverfahren etabliert. In Lastwechseltests ist die Temperatur der dominierende Parameter für bekannte Lebensdauermodelle. Aufgrund des Aufbaus und der Eigenschaften von AlGaN/GaN-Bauelementen ist es notwendig, neue Methoden zur Temperaturbestimmung zu etablieren. Die Untersuchungen berücksichtigen dabei verschiedene Bauteilkonzepte. Dazu gehören High Electron Mobility Transistors (HEMT) mit Schottky/p-Gate, für die eine Verwendung des Gateleckstromes als temperatursensitiver elektrischer Parameter (TSEP) untersucht und zur Temperaturbestimmung empfohlen wird. Für Gate Injection Transistors (GIT) wird ein ähnlicher Ansatz verfolgt. Aufgrund der Gatestruktur dieser stromgesteuerten Bauelemente wird vorgeschlagen, den vorhandenen pn-Übergang am Gate des GIT HEMT unter Verwendung der Gate-Source-Spannung als TSEP zu nutzen. In beiden Fällen erreichen die temperatursensitiven Parameter eine Messauflösung, die mindestens der des pn-Übergangs bei Si-Bauelementen entspricht. Im Lastwechseltest bestimmt im Wesentlichen die verwendete Aufbau- und Verbindungstechnik außerhalb des diskreten Packages die mögliche Zyklenzahl. Werden SMD-Bauelemente auf PCB gelötet, dominiert die Lotverbindung zwischen Bauteil und PCB den Ausfall. Durch ein neues Aufbaukonzept mit in Module gesinterten AlGaN/GaN Packages sind Zyklenzahlen möglich, die bis Faktor 10 über dem Erwartungswert für vergleichbare Si-Bauelemente mit Standard AVT liegen