Gate stability of GaN-Based HEMTs with P-Type Gate

status: publishe

When self-consistency makes a difference

Compound semiconductor power RF and microwave device modeling requires, in many cases, the use of selfconsistent electrothermal equivalent circuits. The slow thermal dynamics and the thermal nonlinearity should be accurately included in the model; otherwise, some response features subtly related to the detailed frequency behavior of the slow thermal dynamics would be inaccurately reproduced or completely distorted. In this contribution we show two examples, concerning current collapse in HBTs and modeling of IMPs in GaN HEMTs. Accurate thermal modeling is proved to be be made compatible with circuit-oriented CAD tools through a proper choice of system-level approximations; in the discussion we exploit a Wiener approach, but of course the strategy should be tailored to the specific problem under consideratio

Gallium nitride high electron mobility transistors in chip scale packaging: evaluation of performance in radio frequency power amplifiers and thermomechanical reliability characterization

2017 Summer.Includes bibliographical references.Wide bandgap semiconductors such as Gallium Nitride (GaN) have many advantages over their Si counterparts, such as a higher energy bandgap, critical electric field, and saturated electron drift velocity. These parameters translate into devices which operate at higher frequency, voltage, and efficiency than comparable Si devices, and have been utilized in varying degrees for power amplification purposes at >1 MHz for years. Previously, these devices required costly substrates such as sapphire (Al2O3), limiting applications to little more than aerospace and military. Furthermore, the typical breakdown voltage ratings of these parts have historically been below ~200 V, with many targeted as replacements for 50 V Si LDMOS as used in cellular infrastructure and industrial, scientific, and medical (ISM) applications between 1 MHz and 1 GHz. Fortunately within the past five years, devices have become commercially available with attractive key specifications: GaN on Si subtrates, with breakdown voltages of over 600 V, realized in cost effective chip scale packages, and with inherently low parasitic capacitances and inductances. In this work, two types of inexpensive commercially available AlGaN/GaN high electron mobility transistors (HEMTs) in chip scale packages are evaluated in a set of three interconnected experiments. The first explores the feasibility of creating a radio frequency power amplifier for use in the ISM bands of 2 MHz and 13.56 MHz, at power levels of up to 1 kW, using a Class E topology. Experiments confirm that a DC to RF efficiency of 94% is easily achievable using these devices. The second group of experiments considers both the steady state and transient thermal characterization of the HEMTs when installed in a typical industrial application. It is shown that both types of devices have acceptable steady state thermal resistance performance; approximately 5.27 °C/W and 0.93 °C/W are achievable for the source pad (bottom) cooled and top thermal pad cooled device types, respectively. Transient thermal behavior was found to exceed industry recommended maximum dT/dt by over 80x for the bottom cooled devices; a factor of 20x was noted with the top cooled devices. Extrapolations using the lumped capacitance method for transient conduction support even higher initial channel dT/dt rates. Although this rate of change decays to recommended levels within one second, it was hypothesized that the accumulated mechanical strain on the HEMTs would cause early life failures if left uncontrolled. The third set of experiments uses the thermal data to design a set of experiments with the goal of quantifying the cycles to failure under power cycling. It is confirmed that to achieve a high number of thermal cycles to failure as required in high reliability industrial systems, the devices under test require significant thermal parameter derating to levels on the order of 50%

Wide-field Magnetic Field and Temperature Imaging using Nanoscale Quantum Sensors

The simultaneous imaging of magnetic fields and temperature (MT) is important in a range of applications, including studies of carrier transport, solid-state material dynamics, and semiconductor device characterization. Techniques exist for separately measuring temperature (e.g., infrared (IR) microscopy, micro-Raman spectroscopy, and thermo-reflectance microscopy) and magnetic fields (e.g., scanning probe magnetic force microscopy and superconducting quantum interference devices). However, these techniques cannot measure magnetic fields and temperature simultaneously. Here, we use the exceptional temperature and magnetic field sensitivity of nitrogen vacancy (NV) spins in conformally-coated nanodiamonds to realize simultaneous wide-field MT imaging. Our "quantum conformally-attached thermo-magnetic" (Q-CAT) imaging enables (i) wide-field, high-frame-rate imaging (100 - 1000 Hz); (ii) high sensitivity; and (iii) compatibility with standard microscopes. We apply this technique to study the industrially important problem of characterizing multifinger gallium nitride high-electron-mobility transistors (GaN HEMTs). We spatially and temporally resolve the electric current distribution and resulting temperature rise, elucidating functional device behavior at the microscopic level. The general applicability of Q-CAT imaging serves as an important tool for understanding complex MT phenomena in material science, device physics, and related fields

Characterization and Utilization of 600 V GaN GITs for 4.5 kW Single Phase Inverter Design

Superior properties allow for faster switching and higher power density converters. However, the fast switching capability of GaN, while theoretically beneficial to converter design, presents several challenges due to the presence of printed circuit board (PCB) and device parasitics. Therefore, it is imperative that the results of device characterization reflect actual device behavior in order to adequately model the device for converter design. This thesis focuses on characterization and utilization of 600 V/30 A Gallium Nitride gate injection transistors, or GaN GITs. The experimental data from static and dynamic characterization was used to maximize the performance of the devices in each phase leg of a 4.5 kW, single-phase, full-bridge inverter. The impact of PCB and device parasitics on switching behavior was also investigated, and a trade-off study of switching loss, overshoot voltage, and dead time loss is presented. Device packaging is also of interest regarding the design of high-frequency devices. This thesis compares the impact of two package designs for the GIT device by designing two separate inverters with the same specifications utilizing the different packages. Finally, due to the lower critical energy of the GaN HEMT during a short circuit, this thesis studies the short-circuit robustness of the devices. The performance of a unique gate sensing protection scheme is compared between two different packages, and the impact of the gate drive and protection circuit design parameters on performance is evaluated

ANALYSIS OF FAILURE MECHANISMS THAT IMPACT SAFE OPERATION OF ALGAN/GAN HEMTS

The reliability of AlGaN/GaN high electron mobility transistors (HEMTs) is tra- ditionally determined via thermal lifetime acceleration stress tests. More recently it has been proposed that electric field has a prominent role in limiting lifetimes. Multi- ple failure mechanisms have been proposed as a result of device degradation observed when stressed under high applied electric fields, as typical when the device is biased into the OFF-state. One potential reason for multiple mechanisms could be due to varying levels of quality and maturity of the GaN processes in the reported literature. The work presented in this dissertation seeks to provide clarity and understanding into the failure mechanism of AlGaN/GaN HEMT devices under high electric fields. The devices in this study were fabricated in a commercial GaN process, notable for exceptional ruggedness and industry leading 65V qualified operational bias for RF power amplifiers. A series of OFF-state, high electric field step-stress experiments, as described in literature, were performed to assess if any were applicable to this process. It was discovered that device degradation could only be induced when stressed close to the breakdown limits. This lead to the development of a unique stress method that enables the device to be held close to catastrophic breakdown, while avoiding an over stress event that would prevent the device from being studied at the conclusion of the experiment. It was discovered via careful electrical and optical analysis that failure was due to a localized degradation of the Schottky gate diode properties. The physical analysis found the failure inconsistent with the widely reported inverse piezoelectric effect. Instead the failures resemble recently proposed time dependent dielectric breakdown of the AlGaN barrier laye