Silicon carbide technology for extreme environments

Abstract

With 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.