GaN switches in pulsed power

The expansion of the electric vehicle market is driving the request for efficient and reliable power electronic systems for electric energy conversion and processing. The efficiency, size, and cost of a power system is strongly related to the performance of power semiconductor devices, where massive industrial investments and intense research efforts are being devoted to new wide bandgap (WBG) semiconductors, such as silicon carbide (SiC) and gallium nitride (GaN). The electrical and thermal properties of SiC and GaN enable the fabrication of semiconductor power devices with performance well beyond the limits of silicon. However, a massive migration of the power electronics industry towards WBG materials can be obtained only once the corresponding fabrication technology reaches a sufficient maturity and a competitive cost. In this paper, we present a perspective of power electronics based on WBG semiconductors, from fundamental material characteristics of SiC and GaN to their potential impacts on the power semiconductor device market. Some application cases are also presented, with specific benchmarks against a corresponding implementation realized with silicon devices, focusing on both achievable performance and system cost.

“Ultra” wide-bandgap semiconductors are an emerging class of materials with bandgaps greater than that of gallium nitride (EG > 3.4 eV) that may ultimately benefit a wide range of applications, including switching power conversion, pulsed power, RF electronics, UV optoelectronics, and quantum information. This paper describes the progress made to date at Sandia National Laboratories to develop one of these materials, aluminum gallium nitride, targeted toward high-power devices. The advantageous material properties of AlGaN are reviewed, questions concerning epitaxial growth and defect physics are covered, and the processing and performance of vertical- and lateral-geometry devices are described. The paper concludes with an assessment of the outlook for AlGaN, including outstanding research opportunities and a brief discussion of other potential applications.

A method is presented that determines the carrier lifertime in bipolar transistors from the collector current decay after optical carrier generation. The transient collector current is calculated by solving the one-dimensional diffusion equation. Parameters are the carrier lifetime τ, the ratiod/L of base width to diffusion length and the penetration depth 1/α of the incident light. It is shown that after a transition phase, the collector current decays exponentially with the carrier lifetime of the bulk material as time constant. The smallerd/L, the faster the collector current decays exponentially. Experimental results prove the applicability of the method to determine the low-level lifetime as well as the high-level lifetime in power transistors. Furthermore, the influence of the lateral diffusion is investigated by changing the ratior/L of the radius of the illuminated area and the diffusion length. Within the variation 1.5≦r/L≦6.5, no such influence can be observed.

We report on time-resolved pump and probe characterization of linear and nonlinear optical generation of free carriers in a silicon strip nano-waveguide at the 1550 nm communication band. Analytical expressions were developed to extract the carrier density averaged along the waveguide length from the measured free-carrier absorption for different input pump power levels. This allows us to discriminate the contributions from two-photon absorption (TPA) and single-photon absorption (SPA), obtaining TPA and SPA coefficients of (1.5 ± 0.1) cm/GW and (1.9 ± 0.1) m−1, respectively. Our results reveal that the effective TPA within the waveguide is higher than the value reported for bulk silicon. In addition, we find that for the waveguide under test, the carrier generation via SPA plays an important role up to ∼300 mW, and therefore, it must be taken into account to correctly assess free-carrier effects in silicon photonic devices.

Si power devices are quickly approaching their limits due to the intrinsic material properties. Wide bandgap semiconductors with the potential to allow significant performance improvement of power devices became very promising. With numerous studies and successive breakthroughs in recent years, wide bandgap semiconductors in term of material and device fabrication technologies have achieved huge advances. For example, large-size single crystalline substrate, high-quality epitaxial layer on hetero/homo substrate, and various power devices have been commercially available [1]. However, it is still very far to make the best use of the devices’ potential at low cost due to the difficult fabrication process and the expensive substrate. New innovative fabrication technologies are urgently desired. Wafer bonding, as one key technology capable of novel structure formation, efficient fabrication, device integration and cost reduction, is expected to be able to solve many problems in the fabrication of next-generation power device [2]. However, most of the previous wafer bonding of wide bandgap semiconductors need an annealing process at high temperature, even higher than 1000oC, which would limit its application in device fabrication. A well-developed low temperature wafer bonding method for wide bandgap semiconductors is still absent [3]. In this paper, our recent progress on the wafer bonding of wide bandgap semiconductors at room temperature would be introduced. References H. Okumura, MRS Bull., Vol. 40, pp. 439-444, 2015.S. H. Christiansen, R. Singh, and U. Gosele, Proc. IEEE, Vol. 94, No. 12, pp. 2060-2106, 2006.F. Mu, K. Iguchi, H. Nakazawa, Y. Takahashi, M. Fujino, R. He, and T. Suga, Appl. Phys. Express, Vol. 9, pp. 081302:1-4, 2016.

Here we describe experimental spin stand measurements as well as simulations of the time constants associated with thermal transients in heat-assisted magnetic recording (HAMR) media. The thermal transients associated with both increasing and decreasing steps in laser power contain at least two time constants each, one on the order of 1 ns and another on the order of 10 ns. These time constants are associated with the central thermal peak in the media and the diffuse thermal background, respectively. In addition, the measured thermal fall time is longer than the thermal rise time due to the disk’s motion relative to the head. Whereas in conventional HAMR a constant laser power is applied during writes, a number of proposed advanced recording schemes use pulsed laser power to improve either recording performance or reliability. The thermal time constants reported here suggest care must be taken when implementing such schemes so that transition locations are not shifted to the detriment of areal density capability.

Wide bandgap semiconductors are discussed as materials for electron-beam controlled semiconductor switches. Based on a figure of merit which allows to compare materials with different carrier mobility, dielectric strength, and ionization energy, gallium arsenide (GaAs), zinc selenide (ZnSe) and diamond are identified as the most promising materials for high-power electron-beam switching. A mathematical model is presented which allows the calculation of both the steady-state current voltage characteristics and the transient response of these switches to an electron-beam. The results of the simulation and switching experiments with GaAs, ZnSe, and diamond indicate that an electron-beam switching of wide bandgap semiconductors offers and attractive alternative to conventional photoconductive switching.

Continuous growth of technology has pushed the requirements of power electronics devices beyond the limits possible with Silicon material. Some of the current and near future applications require high efficiency devices to perform at high temperature and high frequency with high current density and high voltage ratings. This has prompted researches in devices based on the wide band gap materials which have superior electronics properties compared to Silicon. Among the wide band gap semiconductors, diamond is regarded as one of the ultimate materials with extreme mechanical and electrical properties such as high breakdown electric field of 10-20 MV/cm (due to large band gap of 5.47 eV), high electron and hole mobility of 4500 cm 2 .V -1 .s - and 3800 cm 2 .V -1 .s - , high saturation velocity of 2.7x10 7 cm/s, and high thermal conductivity of 22-24 W/cm.K. Calculations of different figure of merit (FOM) also show huge potential of diamond compared to the other materials. Recent progress in the fabrication technology of high quality single crystal diamond has enabled laboratory fabrication of diamond Schottky barrier diode with high blocking voltage and high temperature operation. Despite of these, diamond is still in the early research stage and there are still many challenges to overcome. This thesis focuses on the development, characterizations, analysis, modeling and simulations, and design optimizations of diamond Schottky barrier diode (SBD) for power electronics applications. Throughout the study, several p-type vertical diamond SBDs with Molybdenum Schottky contact and different specifications have been fabricated and measured. Various physics models and parameters suitable for numerical simulations of single crystal diamond material were reviewed and adapted for implementation in the finite element physics based Technology Computer Aided Design (TCAD) Sentaurus software. Static and transient characteristics of the diamond SBDs at different temperatures were investigated based on the experimental results, numerical simulations, and theoretical considerations. Measurements results of the experimental Mo/diamond SBDs show increase in the Schottky barrier height (and turn-on voltage) that indicates Fermi level

The most promising class of materials poised to take over the future of microwave power transistors are wide band gap semiconductors among which GaN is most exciting. A high electron mobility transistor (HEMT) made of an AlGaN/GaN heterostructure can take advantage of a sheet charge layer known as a two-dimensional electron gas (2DEG) to achieve superior properties compared to other materials such as Si, SiC and GaAs. Unfortunately AlGaN/GaN HEMTs are plagued by unknown reliability and there is no clear mechanism which describes how these devices degrade. Degradation of AlGaN/GaN HEMTs is a result of multiple mechanisms, and from a materials perspective it is most advantageous to study the physical degradation and microstructural changes of these electronic devices. During high voltage and high frequency operation AlGaN/GaN HEMTs experience irreversible degradation in their electronic properties, and this degradation has been attributed to physical degradation of the device1-7. By performing high resolution transmission electron microscope (HRTEM) imaging and taking advantage of the phase information within a bright field TEM image by performing Geometric Phase Analysis, a higher resolution and greater quantitative understanding of the degradation of AlGaN/GaN HEMTs can be achieved. Establishing the ability to reproducibly perform HRTEM and quantitative analysis on AlGaN/GaN HEMTs of varying amount of bias is only the first step on the road toward to the ultimate goal of performing in-situ TEM on these devices. Much of the ground work for in-situ studies has been laid out and key issues for moving forward are discussed.

In this study, we developed a unified charge collection model using optical cavity dependent charge carrier generation and non-uniform built-in electric field distribution within a bulk heterojunction photovoltaic device. The charge collection model relies on the experimental inputs related to the charge carrier dynamics such as mobilities of charge carriers, recombination lifetime, and junction width of charge carrier species. Optical cavity modes and field strength were calculated using the experimental variable angle ellipsometry analysis of individual components of the devices. In order to evaluate the model, ambient processed PCDTBT:PC71BM based conventional and inverted derive architectures were utilized to underline the effect of unintentional doping and distinct optical cavity modes. The simulated external quantum efficiency and short-circuit current density profiles from the model were compared to the experimental results with differing active layers thicknesses and device architectures. The proposed charge collection model presented a high degree of correlation with the experimental results and underlined its validity for further application on other types of organic photovoltaic devices.

Stationary photoconduction was measured on 100 nm thick films of poly(methylimino-1,4-phenylene-1-phenyl-1,2-ethenylene-2,5-dimethoxy-1,4-phenylene-2-phenyl-1,2-ethenylene-1,4-phenylene) sandwiched between indium tin oxide and aluminium electrodes. Bulk photoionization was observed upon irradiating through the positively biased aluminium electrode. For photon energies 2.6 eV < hv < 3.2 eV either defect catalyzed or intrinsic dissociation of relaxed singlet excitations prevails. For hv > 3.2 eV the carrier yield increases with excess photon energy due to intrinsic photoionization. The functional dependencies on electric field and photon energy are in accord with the 3D version of Onsager's theory of geminate pair dissociation implying a geminate pair distance that increases with hv. As an ionizing event, coupling between the non-relaxed initial Franck-Condon state with an mAg state is involved. Further increase of the yield correlates with the onset of optical absorption to the third optically allowed Bu state.

Photoconductive semiconductor switches (PCSS) have been investigated since the late 1970s. Some devices have been developed that withstand tens of kilovolts and others that switch hundreds of amperes. However, no single device has been developed that can reliably withstand both high voltage and switch high current. Yet, photoconductive switches still hold the promise of reliable high voltage and high current operation with subnanosecond risetimes. Particularly since good quality, bulk, single crystal, wide bandgap semiconductor materials have recently become available. In this chapter we will review the basic operation of PCSS devices, status of PCSS devices and properties of the wide bandgap semiconductors 4H-SiC, 6H-SiC and 2H-GaN.

This state-of-the-art reference text provides comprehensive coverage of the challenges and latest research in wide and ultra-wide bandgap semiconductors. Leading researchers from around the world provide reviews on the latest development of materials and devices in these systems.

Recent advances in the design, fabrication, robustness and reliability of high-voltage vertical and lateral smart power devices in wide and extreme bandgap semiconductors (particularly SiC and GaN) [1] are presented and discussed. The basic device structures and their refinements, with respect to their potential and experimental performance, are evaluated and compared to their Si or GaAs counterparts. Key processing technologies (such as MOS and heterojunction) needed for the performance and manufacturing of these power devices are examined with their compatibility with commercial silicon foundry capabilities. The robustness and reliability of present commercial discrete power transistors (like threshold stability, current collapse, HTRB) are assessed. Future directions and challenges for the widespread adoption of this emerging power device technology are identified.[1] T.P. Chow, I. Omura, M. Higashiwaki, H. Kawarada, and V. Pala, IEEE Trans. Electron Devices, 64, 856 (2017).

Pulsed power systems are continually being designed to increase performance and flexibility in order to meet new needs. Drivers for fusion, equation of state research and radiography are just a few of the scientific areas driving accelerated pulsed power development. Improved system timing and jitter requirements, along with pulse shaping capabilities through independent timing of individual switches, are placing higher demands upon high voltage (HV) switch performance, triggering system performance and cost. Conventional trigger systems require high voltage trigger cables or line-of-sight optics that complicate design, demand space, require extensive maintenance, and significantly impact system cost. With electrical triggers, large diameter, high voltage transmission line cables must be fed through high field regions. With optical triggers, line-of-sight optics must focus high energy laser beams to the interior of the switches with clean, rigidly-mounted, shock-withstanding optics. This paper reports on progress in the development of fiber optically triggered photoconductive semiconductor switches (PCSS) for triggering high voltage pulsed power switches with improved precision. This technology can eliminate the need for large-diameter trigger cables and line-of-sight optics. It also has the potential of significantly reducing the cost of trigger generation systems. It has the potential to improve the performance of prime power switches, diverters, and diagnostics because of the low-jitter sub-nanosecond rise times. Test results will be presented that demonstrate sub-nanosecond jitter from a PCSS, less than 1.