Evaluation of 4h-Sic Photoconductive Switches for Pulsed Power Applications Based on Numerical Simulations

Since the early studies by Auston, photoconductive semiconductor switches (PCSSs) have been investigated intensively for many applications owing to their unique advantages over conventional gas and mechanical switches. These advantages include high speeds, fast rise times, optical isolation, compact geometry, and negligible jitter. Another important requirement is the ability to operate at high repetition rates with long device lifetimes (i.e., good reliability without degradation). Photoconductive semiconductor switches (PCSSs) are low-jitter compact alternatives to traditional gas switches in pulsed power systems. The physical properties of Silicon Carbide (SiC), such as a large bandgap (3.1-3.35 eV), high avalanche breakdown field (~3 MV/cm), and large thermal conductivity (4-5 W/cm-K) with superior radiation hardness and resistance to chemical attack, make SiC an attractive candidate for high voltage, high temperature, and high power device applications. A model-based analysis of the steady-state, current-voltage response of semi-insulating 4H-SiC was carried out to probe the internal mechanisms, focusing on electric field driven effects. Relevant physical processes, such as multiple defects, repulsive potential barriers to electron trapping, band-to-trap impact ionization, and field-dependent detrapping, were comprehensively included. Results of our model matched the available experimental data fairly well over orders of magnitude variation in the current density. A number of important parameters were also extracted in the process through comparisons with available data. Finally, based on our analysis, the possible presence of holes in the samples could be discounted up to applied fields as high as 275 kV/cm. In addition, calculations of electric field distributions in a SiC photoconductive semiconductor switch structure with metal contacts employing contact extensions on a high-k HfO2 dielectric were carried out, with the goal of assessing reductions in the peak electric fields. For completeness, analysis of thermal heating in a lateral PCSS structure with such modified geometries after photoexcitation was also included. The simulation results of the electric field distribution show that peak electric fields, and hence the potential for device failure, can be mitigated by these strategies. A combination of the two approaches was shown to produce up to a ~67% reduction in peak fields. The reduced values were well below the threshold for breakdown in SiC material using biasing close to experimental reports. The field mitigation was shown to depend on the length of the metal overhang. Further, the calculations show that, upon field mitigation, the internal temperature rise would also be controlled. A maximum value of 980 K was obtained here for an 8 ns electrical pulse at a 20 kV external bias, which is well below the limits for generating local stress or cracks or defects

Aging-Aware Design Methods for Reliable Analog Integrated Circuits using Operating Point-Dependent Degradation

The focus of this thesis is on the development and implementation of aging-aware design methods, which are suitable to satisfy current needs of analog circuit design. Based on the well known \gm/\ID sizing methodology, an innovative tool-assisted aging-aware design approach is proposed, which is able to estimate shifts in circuit characteristics using mostly hand calculation schemes. The developed concept of an operating point-dependent degradation leads to the definition of an aging-aware sensitivity, which is compared to currently available degradation simulation flows and proves to be efficient in the estimation of circuit degradation. Using the aging-aware sensitivity, several analog circuits are investigated and optimized towards higher reliability. Finally, results are presented for numerous target specifications

NEGATIVE BIAS TEMPERATURE INSTABILITY STUDIES FOR ANALOG SOC CIRCUITS

Negative Bias Temperature Instability (NBTI) is one of the recent reliability issues in sub threshold CMOS circuits. NBTI effect on analog circuits, which require matched device pairs and mismatches, will cause circuit failure. This work is to assess the NBTI effect considering the voltage and the temperature variations. It also provides a working knowledge of NBTI awareness to the circuit design community for reliable design of the SOC analog circuit. There have been numerous studies to date on the NBTI effect to analog circuits. However, other researchers did not study the implication of NBTI stress on analog circuits utilizing bandgap reference circuit. The reliability performance of all matched pair circuits, particularly the bandgap reference, is at the mercy of aging differential. Reliability simulation is mandatory to obtain realistic risk evaluation for circuit design reliability qualification. It is applicable to all circuit aging problems covering both analog and digital. Failure rate varies as a function of voltage and temperature. It is shown that PMOS is the reliabilitysusceptible device and NBTI is the most vital failure mechanism for analog circuit in sub-micrometer CMOS technology. This study provides a complete reliability simulation analysis of the on-die Thermal Sensor and the Digital Analog Converter (DAC) circuits and analyzes the effect of NBTI using reliability simulation tool. In order to check out the robustness of the NBTI-induced SOC circuit design, a bum-in experiment was conducted on the DAC circuits. The NBTI degradation observed in the reliability simulation analysis has given a clue that under a severe stress condition, a massive voltage threshold mismatch of beyond the 2mV limit was recorded. Bum-in experimental result on DAC proves the reliability sensitivity of NBTI to the DAC circuitry

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

Study of the effects of deuterium implantation upon the performance of thin-oxide CMOS devices

The use of ultra thin oxide films in modem semiconductor devices makes them increasingly susceptible to damage due to the hot carrier damage. Deuterium in place of hydrogen was introduced by ion implantation at the silicon oxide-silicon interface during fabrication to satisfy the dangling bonds. Deuterium was implanted at energies of 15, 25 and 35 keV and at a dose of 1x1014/cm2. Some of the wafers were subjected to N2O annealing following gate oxide growth. It was demonstrated that ion implantation is an effective means of introduction of deuterium. Deuterium implantation brings about a clear enhancement in gate oxide quality by improving the interface characteristics. N2O annealing further improves device performance. A reduction of electron traps with deutenum was also observed. A combination of deuterium implantation at 25 keV and a dose of 1x1015/cm2, followed by annealing in N2O was observed to have the most positive influence on device behavior. Concurrently, MEMS microheaters being fabricated for an integrated VOC sensor were also tested for their temperature response to an applied voltage. Different channel configurations and materials for the conducting film were compared and the best pattern for rapid heating was identified. Temperature rises of upto 390° C were obtained. The temperature responses after coating spin-on glass in the microchannels were also measured

Numerical Simulation of Combustion in the Ironmaking Blast Furnace Raceway

As almost all conversion of raw iron ore to pig iron at the start of the ironmaking process currently takes place in a blast furnace, these furnaces remain a critical component in the iron and steelmaking industry. Enhancements in the efficiency of blast furnace operation have a significant effect on industrial energy consumption, as the process represents nearly 70% of the total energy consumption of the iron and steelmaking process. Over the past several decades, auxiliary fuel injection has been adopted as a method of reducing the total amount of coke necessary for furnace operation. Coke making is both energy intensive and environmentally unfriendly, and as such, any reduction in coke usage by the blast furnace is positive for the iron and steelmaking industry. However, the intricate variations in blast furnace raceway conditions and injected fuel combustion characteristics due to the method and conditions at which auxiliary fuels are injected into the furnace are still not fully understood. The goal of this research is to utilize computational fluid dynamics (CFD) modeling to provide a deeper level of understanding of the complex relationships between blast furnace injection system designs and operating conditions on the combustion processes and phenomena within the raceway. In this vein, a multi-stage 3-D CFD model has been developed and applied to simulate combustion phenomena within several industrial blast furnace raceway regions. The three primary components of focus in this research are the tuyere and injection apparatus, raceway formation, and raceway combustion. A comprehensive CFD methodology for simulation operating conditions and combustion within the blast furnace raceway has been developed. This methodology utilizes CFD modeling to simulate conditions within the raceway region. A revised raceway formation model has been developed to better correspond to industrial observations, and new methodology for analysis and presentation of simulation results from these models have been developed. The models have been validated against industrial observation and measurements from three currently operating industrial blast furnaces. The models have also been utilized to examine varied operating conditions in the aforementioned furnaces. Two new methods of exploring raceway gas temperature using simulation modeling were developed in this research, namely a Topographical Flame Temperature (TOFT) and a Raceway Adiabatic Flame Temperature (RAFT) analogue. These methods allow for both better validation of computational modeling results against industrial observation and measurement, as well as providing a new path to explore raceway gas temperature distribution under unique conditions, including extremely high natural gas injection rates, which may present potential for significantly improving the economic and operational efficiency of the furnace. The analyses of industry blast furnaces provide significant insight into the effects of injection conditions and apparatus designs upon combustion characteristics and reaction phenomena within the raceway. Previously unexplored novel fuel injection techniques were explored within this research, and simulations have indicated that injected fuel burnout rates could be improved by as much as 23% in specific scenarios and production could be increased by roughly 2.5%. While a switch to these injection techniques may pose some difficulties in practice, industrial project partners have already begun trials for implementation on a full-scale furnace. Finally, this modeling revealed significant potential benefits to blast furnace operation through modification of natural gas and pulverized coal injection locations, pulverized coal carrier gas type, injection lance tip design, and other parameters. While these exact parameters cannot be implemented identically across all plant furnaces, they provide a baseline of fundamental understanding from which furnace operators and engineers can draw in their ongoing attempts to optimize combustion efficiency and reduce operational expenditures