Silicon carbide technology for extreme environments

PhD ThesisWith mankind’s ever increasing curiosity to explore the unknown, including a variety of hostile environments where we cannot tread, there exists a need for machines to do work on our behalf. For applications in the most extreme environments and applications silicon based electronics cannot function, and there is a requirement for circuits and sensors to be built from wide band gap materials capable of operation in these domains. This work addresses the initial development of silicon carbide circuits to monitor conditions and transmit information from such hostile environments. The characterisation, simulation and implementation of silicon carbide based circuits utilising proprietary high temperature passives is explored. Silicon carbide is a wide band gap semiconductor material with highly suitable properties for high-power, high frequency and high temperature applications. The bandgap varies depending on polytype, but the most commonly used polytype 4H, has a value of 3.265 eV at room temperature, which reduces as the thermal ionization of electrons from the valence band to the conduction band increases, allowing operation in ambient up to 600°C. Whilst silicon carbide allows for the growth of a native oxide, the quality has limitations and therefore junction field effect transistors (JFETs) have been utilised as the switch in this work. The characteristics of JFET devices are similar to those of early thermionic valve technology and their use in circuits is well known. In conjunction with JFETs, Schottky barrier diodes (SBDs) have been used as both varactors and rectifiers. Simulation models for high temperature components have been created through their characterisation of their electrical parameters at elevated temperatures. The JFETs were characterised at temperatures up to 573K, and values for TO V , β , λ , IS , RS and junction capacitances were extracted and then used to mathematically describe the operation of circuits using SPICE. The transconductance of SiC JFETs at high temperatures has been shown to decrease quadratically indicating a strong dependence upon carrier mobility in the channel. The channel resistance also decreased quadratically as a direct result of both electric field and temperature enhanced trap emission. The JFETs were tested to be operational up to 775K, where they failed due to delamination of an external passivation layer. ii Schottky diodes were characterised up to 573K, across the temperature range and values for ideality factor, capacitance, series resistance and forward voltage drop were extracted to mathematically model the devices. The series resistance of a SiC SBD exhibited a quadratic relationship with temperature indicating that it is dominated by optical phonon scattering of charge carriers. The observed deviation from a temperature independent ideality factor is due to the recombination of carriers in the depletion region affected by both traps and the formation of an interfacial layer at the SiC/metal interface. To compliment the silicon carbide active devices utilised in this work, high temperature passive devices and packaging/circuit boards were developed. Both HfO2 and AlN materials were investigated for use as potential high temperature capacitor dielectrics in metal-insulator-metal (MIM) capacitor structures. The different thicknesses of HfO2 (60nm and 90nm) and 300nm for AlN and the relevance to fabrication techniques are examined and their effective capacitor behaviour at high temperature explored. The HfO2 based capacitor structures exhibited high levels of leakage current at temperatures above 100°C. Along with elevated leakage when subjected to higher electric fields. This current leakage is due to the thin dielectric and high defect density and essentially turns the capacitors into high value resistors in the order of MΩ. This renders the devices unsuitable as capacitors in hostile environments at the scales tested. To address this issue AlN capacitors with a greater dielectric film thickness were fabricated with reduced leakage currents in comparison even at an electric field of 50MV/cm at 600K. The work demonstrated the world’s first high temperature wireless sensor node powered using energy harvesting technology, capable of operation at 573K. The module demonstrated the world’s first amplitude modulation (AM) and frequency modulation (FM) communication techniques at high temperature. It also demonstrated a novel high temperature self oscillating boost converter cable of boosting voltages from a thermoelectric generator also operating at this temperature. The AM oscillator operated at a maximum temperature of 553K and at a frequency of 19.4MHz with a signal amplitude 65dB above background noise. Realised from JFETs and HfO2 capacitors, modulation of the output signal was achieved by varying the load resistance by use of a second SiC JFET. By applying a negative signal voltage of between -2.5 and -3V, a 50% reduction in the signal amplitude and therefore Amplitude Modulation was achieved by modulating the power within the oscillator through the use of this secondary JFET. Temperature drift in the characteristics were also observed, iii with a decrease in oscillation frequency of almost 200 kHz when the temperature changed from 300K to 573K. This decrease is due to the increase in capacitance density of the HfO2 MIM capacitors and increasing junction capacitances of the JFET used as the amplifier within the oscillator circuit. Direct frequency modulation of a SiC Voltage Controlled Oscillator was demonstrated at a temperature of 573K with a oscillation frequency of 17MHz. Realised from an SiC JFET, AlN capacitors and a SiC SBD used as a varactor. It was possible to vary the frequency of oscillations by 100 kHz with an input signal no greater than 1.5V being applied to the SiC SBD. The effects of temperature drift were more dramatic in comparison to the AM circuit at 400 kHz over the entire temperature range, a result of the properties of the AlN film which causes the capacitors to increase in capacitance density by 10%. A novel self oscillating boost converter was commissioned using a counter wound transformer on high temperature ferrite, a SiC JFET and a SiC SBD. Based upon the operation of a free running blocking oscillator, oscillatory behaviour is a result of the electric and magnetic variations in the winding of the transformer and the amplification characteristics of a JFET. It demonstrated the ability to boost an input voltage of 1.3 volts to 3.9 volts at 573K and exhibited an efficiency of 30% at room temperature. The frequency of operation was highly dependent upon the input voltage due to the increased current flow through the primary coil portion of the transformer and the ambient temperature causing an increase in permeability of the ferrite, thus altering the inductance of both primary and secondary windings. However due its simplicity and its ability to boost the input voltage by 250% meant it was capable of powering the transmitters and in conjunction with a Themoelectric Generator so formed the basis for a self powered high temperature silicon carbide sensor node. The demonstration of these high temperature circuits provide the initial stages of being able to produce a high temperature wireless sensor node capable of operation in hostile environments. Utilising the self oscillating boost converter and a high temperature Thermoelectric Generator these prototype circuits were showed the ability to harvest energy from the high temperature ambient and power the silicon carbide circuitry. Along with appropriate sensor technology it demonstrated the feasibility of being able to monitor and transmit information from hazardous locations which is currently unachievable

Wide Band Gap Devices and Their Application in Power Electronics

Power electronic systems have a great impact on modern society. Their applications target a more sustainable future by minimizing the negative impacts of industrialization on the environment, such as global warming effects and greenhouse gas emission. Power devices based on wide band gap (WBG) material have the potential to deliver a paradigm shift in regard to energy efficiency and working with respect to the devices based on mature silicon (Si). Gallium nitride (GaN) and silicon carbide (SiC) have been treated as one of the most promising WBG materials that allow the performance limits of matured Si switching devices to be significantly exceeded. WBG-based power devices enable fast switching with lower power losses at higher switching frequency and hence, allow the development of high power density and high efficiency power converters. This paper reviews popular SiC and GaN power devices, discusses the associated merits and challenges, and finally their applications in power electronics

Silicon carbide based DC-DC converters for deployment in hostile environments

PhD ThesisThe development of power modules for deployment in hostile environments, where the elevated ambient temperatures demand high temperature capability of the entire converter system, requires innovative power electronic circuits to meet stringent requirements in terms of efficiency, power-density and reliability. To simultaneously meet these conflicting requirements in extreme environment applications is quite challenging. To realise these power modules, the relevant control circuitry also needs to operate at elevated temperatures. The recent advances in silicon carbide devices has allowed the realisation of not just high frequency, high efficiency power converters, but also the power electronic converters that can operate at elevated temperatures, beyond those possible using conventional silicon-based technology. High power-density power converters are key components for power supply systems in applications where space and weight are critical parameters. The demand for higher power density requires the use of high-frequency DC-DC converters to overcome the increase in size and power losses due to the use of transformers. The increase in converter switching frequency reduces the size of passive components whilst increasing the electromagnetic interference (EMI) emissions. A performance comparison of SiC MOSFETs and JFETs in a high-power DC-DC converter to form part of a single phase PV inverter system is presented. The drive design requirements for optimum performance in the energy conversion system are also detailed. The converter was tested under continuous conduction mode at frequencies up to 250 kHz. The converter power efficiency, switch power loss and temperature measurements are then compared with the ultra-high speed CoolMOS switches and SiC diodes. The high voltage, high frequency and high temperature operation capability of the SiC DUTs are also demonstrated. The all SiC converters showed more stable efficiencies of 95.5% and 96% for the switching frequency range for the SiC MOSFET and JFET, respectively. A comparison of radiated noise showed the highest noise signature for the SiC JFET and lowest for the SiC MOSFET. The negative gate voltage requirement of the SiC MOSFET introduces up to 6 dBμV increase in radiated noise, due to the induced current in the high frequency resonant stray loop in the gate drive negative power plane. ii A gate driver is an essential part of any power electronic circuitry to control the switching of the power semiconductor devices. The desire to place the gate driver physically close to the power switches in the converter, leads to the necessity of a temperature resilient PWM generator to control the power electronics module. At elevated temperatures, the ability to control electrical systems will be a key enabler for future technology enhancements. Here an SiC/SOI-based PWM gate driver is proposed and designed using a current source technique to accomplish variable duty-cycle PWM generation. The ring oscillator and constant current source stages use low power normally-on, epitaxial SiC-JFETs fabricated at Newcastle University. The amplification and control stages use enhancement-mode signal SOI MOSFETs. Both SOI MOSFETs will be replaced by future high current SiC-JFETs with only minor modification to the clamp-stage circuit design. In the proposed design, the duty cycle can be varied from 10% to 90%. The PWM generator is then evaluated in a 200 kHz step-up converter which results in a 91% efficiency at 81% duty cycle. High temperature environments are incompatible with standard battery technologies, and so, energy harvesting is a suitable technology when remote monitoring of these extreme environments is performed through the use of wireless sensor nodes. Energy harvesting devices often produce voltages which are unusable directly by electronic loads and so require power management circuits to convert the electrical output to a level which is usable by monitoring electronics and sensors. Therefore a DC-DC step-up converter that can handle low input voltages is required. The first demonstration of a novel self-starting DC-DC converter is reported, to supply power to a wireless sensor node for deployment in high temperature environments. Utilising SiC devices a novel boost converter topology has been realised which is suitable for boosting a low voltage to a level sufficient to power a sensor node at temperatures up to 300 °C. The converter operates in the boundary between continuous and discontinuous mode of operation and has a VCR of 3 at 300 °C. This topology is able to self start and so requires no external control circuitry, making it ideal for energy harvesting applications, where the energy supply may be intermittent.EPSRC and BAE SYSTEMS through the Dorothy Hodgkin Postgraduate Awar

Investigation of FACTS devices to improve power quality in distribution networks

Flexible AC transmission system (FACTS) technologies are power electronic solutions that improve power transmission through enhanced power transfer volume and stability, and resolve quality and reliability issues in distribution networks carrying sensitive equipment and non-linear loads. The use of FACTS in distribution systems is still in its infancy. Voltages and power ratings in distribution networks are at a level where realistic FACTS devices can be deployed. Efficient power converters and therefore loss minimisation are crucial prerequisites for deployment of FACTS devices. This thesis investigates high power semiconductor device losses in detail. Analytical closed form equations are developed for conduction loss in power devices as a function of device ratings and operating conditions. These formulae have been shown to predict losses very accurately, in line with manufacturer data. The developed formulae enable circuit designers to quickly estimate circuit losses and determine the sensitivity of those losses to device voltage and current ratings, and thus select the optimal semiconductor device for a specific application. It is shown that in the case of majority carrier devices (such as power MOSFETs), the conduction power loss (at rated current) increases linearly in relation to the varying rated current (at constant blocking voltage), but is a square root of the variable blocking voltage when rated current is fixed. For minority carrier devices (such as a pin diode or IGBT), a similar relationship is observed for varying current, however where the blocking voltage is altered, power losses are derived as a square root with an offset (from the origin). Finally, this thesis conducts a power loss-oriented evaluation of cascade type multilevel converters suited to reactive power compensation in 11kV and 33kV systems. The cascade cell converter is constructed from a series arrangement of cell modules. Two prospective structures of cascade type converters were compared as a case study: the traditional type which uses equal-sized cells in its chain, and a second with a ternary relationship between its dc-link voltages. Modelling (at 81 and 27 levels) was carried out under steady state conditions, with simplified models based on the switching function and using standard circuit simulators. A detailed survey of non punch through (NPT) and punch through (PT) IGBTs was completed for the purpose of designing the two cascaded converters. Results show that conduction losses are dominant in both types of converters in NPT and PT IGBTs for 11kV and 33kV systems. The equal-sized converter is only likely to be useful in one case (27-levels in the 33kV system). The ternary-sequence converter produces lower losses in all other cases, and this is especially noticeable for the 81-level converter operating in an 11kV network

HIGH EFFICIENCY BASE DRIVE DESIGNS FOR POWER CONVERTERS USING SILICON CARBIDE BIPOLAR JUNCTION TRANSISTORS

This thesis explores the base driver designs for Silicon Carbide Bipolar Junction Transistors (SiC BJTs) and their applications for power converters. SiC is a wide bandgap semiconductor which has been the focus of recent researches as it has overcome the several of physical restrictions set by the silicon material. Compared with silicon bipolar devices, SiC BJTs have several advantages including a higher maximum junction temperature, higher current gain and lower switching power losses. Transient power losses are low and temperature-independent in a wide range of junction temperatures. With junction temperature capable of being between 25ºC to 240ºC, SiC BJTs have been of great interest in industry. As a current-driven device, the base driver power consumption is always a major concern. Therefore, high efficiency base drive designs for SiC BJT need to be investigated before this power device can be widely used in industry

Driving and Protection of High Density High Temperature Power Module for Electric Vehicle Application

There has been an increasing trend for the commercialization of electric vehicles (EVs) to reduce greenhouse gas emissions and dependence on petroleum. However, a key technical barrier to their wide application is the development of high power density electric drive systems due to limited space within EVs. High temperature environment inherent in EVs further introduces a new level of complexity. Under high power density and high temperature operation, system reliability and safety also become important. This dissertation deals with the development of advanced driving and protection technologies for high temperature high density power module capable of operating under the harsh environment of electric vehicles, while ensuring system reliability and safety under short circuit conditions. Several related research topics will be discussed in this dissertation. First, an active gate driver (AGD) for IGBT modules is proposed to improve their overall switching performance. The proposed one has the capability of reducing the switching loss, delay time, and Miller plateau duration during turn-on and turn-off transient without sacrificing current and voltage stress. Second, a board-level integrated silicon carbide (SiC) MOSFET power module is developed for high temperature and high power density application. Specifically, a silicon-on-insulator (SOI) based gate driver board is designed and fabricated through chip-on-board (COB) technique. Also, a 1200 V / 100 A SiC MOSFET phase-leg power module is developed utilizing high temperature packaging technologies. Third, a comprehensive short circuit ruggedness evaluation and numerical investigation of up-to-date commercial silicon carbide (SiC) MOSFETs is presented. The short circuit capability of three types of commercial 1200 V SiC MOSFETs is tested under various conditions. The experimental short circuit behaviors are compared and analyzed through numerical thermal dynamic simulation. Finally, according to the short circuit ruggedness evaluation results, three short circuit protection methods are proposed to improve the reliability and overall cost of the SiC MOSFET based converter. A comparison is made in terms of fault response time, temperature dependent characteristics, and applications to help designers select a proper protection method

An ultra-fast protection scheme for normally-on wide bandgap devices

Here, an ultra-fast protection scheme that is dedicated to depletion-mode (d-mode) devices is proposed. The key to the d-mode device gate drive design is the negative supply and overcurrent protection, due to the safety concern for d-mode devices when a failure happens in power conversion applications. This work evaluates specific requirement of d-mode devices, such as the isolated negative power supply and short-circuit protection. Normally-on d-mode GaN devices have lower on-resistance and minimal dead time in comparison with enhancement-mode (e-mode) GaN devices, which can further reduce the switching loss and conduction loss. Both simulation and experimental verification are conducted in this work to evaluate the performance of the proposed protection scheme. The proposed desaturation scheme can wipe out the overcurrent event within 341 ns. Furthermore, the proposed negative power supply scheme can sustain its output for 60.5 ms, providing sufficient action time for the control unit to isolate the converter

The potential of SiC Cascode JFETs in electric vehicle traction inverters

The benefits of implementing SiC devices in EV powertrains has been widely reported in various studies. New generations of SiC devices including planar MOSFETs, trench MOSFETs and more recently, cascode JFETs have been released by various manufacturers. SiC cascode devices comprise of low voltage silicon MOSFETs for gate driving and high voltage depletion mode SiC JFETs for voltage blocking. These devices are particularly interesting because it avoids the known reliability issues of SiC gate oxide traps resulting in threshold voltage drift. In this paper, an EV powertrain is simulated using experimental measurements of conduction and switching energies of various SiC devices including 650V trench, 900V planar and 650V cascode JFETs. Unlike previous papers where losses are calculated using models based on datasheet parameters, here static and dynamic measurements on the power devices at different currents and temperatures are used to calculate losses over simulated driving cycles. Field-stop IGBTs are also evaluated. The 3-phase 2-level inverter model is electrothermal by accounting for the measured temperature dependence of the losses and uses accurate thermal networks derived from datasheets. Converter efficiency and thermal performance are compared for each device technology. Results show that SiC cascode JFETs have great potential in EV powertrain applications

Smart Power Devices and ICs Using GaAs and Wide and Extreme Bandgap Semiconductors

We evaluate and compare the performance and potential of GaAs and of wide and extreme bandgap semiconductors (SiC, GaN, Ga2O3, diamond), relative to silicon, for power electronics applications. We examine their device structures and associated materials/process technologies and selectively review the recent experimental demonstrations of high voltage power devices and IC structures of these semiconductors. We discuss the technical obstacles that still need to be addressed and overcome before large-scale commercialization commences