Yearlong 500 C Operational Demonstration of Up-Scaled 4H-SiC JFET Integrated Circuits

This work describes recent progress in the design, processing, and testing of significantly up-scaled 500 C durable 4H-SiC junction field effect transistor (JFET) integrated circuit (IC) technology with two-level interconnect undergoing development at NASA Glenn Research Center. For the first time, stable electrical operation of semiconductor ICs for over one year at 500 C in air atmosphere is reported. These groundbreaking durability results were attained on two-level interconnect JFET demonstration ICs with 175 or more transistors on each chip. This corresponds to a more than 7-fold increase in 500 C-durable circuit complexity from the 24 transistor ring oscillator ICs reported at HiTEC 2016. These results advance the technology foundation for realizing long-term durable 500 C ICs with increased functional capability for combustion engine sensing and control, planetary exploration, deep-well drilling monitoring, and other harsh-environment applications

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

6H-SiC Transistor Integrated Circuits Demonstrating Prolonged Operation at 500 C

The NASA Glenn Research Center is developing very high temperature semiconductor integrated circuits (ICs) for use in the hot sections of aircraft engines and for Venus exploration where ambient temperatures are well above the approximately 300 degrees Centigrade effective limit of silicon-on-insulator IC technology. In order for beneficial technology insertion to occur, such transistor ICs must be capable of prolonged operation in such harsh environments. This paper reports on the fabrication and long-term 500 degrees Centigrade operation of 6H-SiC integrated circuits based on epitaxial 6H-SiC junction field effect transistors (JFETs). Simple analog amplifier and digital logic gate ICs have now demonstrated thousands of hours of continuous 500 degrees Centigrade operation in oxidizing air atmosphere with minimal changes in relevant electrical parameters. Electrical characterization and modeling of transistors and circuits at temperatures from 24 degrees Centigrade to 500 degrees Centigrade is also described. Desired analog and digital IC functionality spanning this temperature range was demonstrated without changing the input signals or power supply voltages

Stable Electrical Operation of 6H-SiC JFETs and ICs for Thousands of Hours at 500 C

The fabrication and testing of the first semiconductor transistors and small-scale integrated circuits (ICs) to achieve up to 3000 h of stable electrical operation at 500 C in air ambient is reported. These devices are based on an epitaxial 6H-SiC junction field-effect transistor process that successfully integrated high temperature ohmic contacts, dielectric passivation, and ceramic packaging. Important device and circuit parameters exhibited less than 10% of change over the course of the 500 C operational testing. These results establish a new technology foundation for realizing durable 500 C ICs for combustion-engine sensing and control, deep-well drilling, and other harsh-environment applications

Assessment of Durable SiC JFET Technology for +600 C to -125 C Integrated Circuit Operation

Electrical characteristics and circuit design considerations for prototype 6H-SiC JFET integrated circuits (ICs) operating over the broad temperature range of -125 C to +600 C are described. Strategic implementation of circuits with transistors and resistors in the same 6H-SiC n-channel layer enabled ICs with nearly temperature-independent functionality to be achieved. The frequency performance of the circuits declined at temperatures increasingly below or above room temperature, roughly corresponding to the change in 6H-SiC n-channel resistance arising from incomplete carrier ionization at low temperature and decreased electron mobility at high temperature. In addition to very broad temperature functionality, these simple digital and analog demonstration integrated circuits successfully operated with little change in functional characteristics over the course of thousands of hours at 500 C before experiencing interconnect-related failures. With appropriate further development, these initial results establish a new technology foundation for realizing durable 500 C ICs for combustion engine sensing and control, deep-well drilling, and other harsh-environment applications

Silicon carbide junction field effect transistor integrated circuits for hostile environments

PhD ThesisSilicon carbide (SiC), in particular its 4H polytype, has long been recognised as an appropriate semiconductor for producing hostile environment electronics due to its wide energy band gap, large chemical bond strength and high mechanical hardness. A strong research foundation has facilitated the development of numerous sensor structures capable of operating at high temperatures and in corrosive atmospheres. Front-end electronics suitable for in situ signal conditioning are however lacking. Junction field effect transistors (JFETs) circumvent the pitfalls of contemporary alternative SiC transistor variants and have been found to operate predictably and consistently under such extreme conditions. This thesis demonstrates for the first time the capability of producing the necessary stable and high-performance interface circuits from n-channel lateral depletion-mode (NLDM) JFETs. The temperature dependence of pertinent bulk 4H–SiC material parameters relevant for describing the operation of macroscopic JFETs were initially studied. An accurate phenomenological model was developed to account for the variation of the thermal equilibrium free carrier concentrations. The position of the electrochemical potential and the distribution of free electron energies were found to change markedly when conduction band nonparabolicity, higher energy intrinsic bands and extrinsic effects were accounted for. These in turn were found to influence the determination of p-n junction contact potentials. The worst case error introduced through use of the Boltzmann approximation when applied to the channel and gate regions of the JFETs under study, having nominal doping concentrations of 1 1017 cm3 and 2 1019 cm3, respectively, were approximately 0:1% and 2%, respectively. A set of efficient and well behaved closed form expressions were subsequently developed for the free carrier concentrations in the framework of the Joyce- Dixon approximation (JDA) which are ideally suited for use in circuit simulations. Expressions for the electron conductively effective mass and an appropriate weighting function for the momentum relaxation time were subsequently identified. While the conductivity effective mass along the basal plane remained almost independent of temperature the non-parabolic band dispersion in the direction of principle axis introduced a temperature variation of 19% and 21% between 25 C and 400 C in the first and second conduction bands, respectively. Monolithically integrated 4H–SiC signal-level homo-epitaxial NLDM JFETs, p-n junction diodes and resistors were electrically characterised between room temperature and 400 C and their static and dynamic properties studied. Their behaviours were found to be well represented by macroscopic drift-diffusion models and were in agreement with predictions based on the bulk material properties. The intrinsic voltage gain of the fabricated JFET structures with nominal 9 μm gate length, 300nm channel depth and 250 μm gate width, under typical bias conditions, was roughly 100. As a consequence of the finite doping concentration in the buffer layer beneath the active device channel, with an experimentally determined value of approximately 3 1015 cm3, the devices under study were found to exhibit a strong body-effect. The thermal performance of the utilised tungsten capped annealed nickel-titanium and aluminium-titanium contacts, on highly doped n- and p-type regions, respectively, were investigated and appropriate methods for their characterisation described. The lowest recorded value of specific contact resistance was 1:90(50) 105 cm2 with a corresponding sheet resistance of 7:89(9) 102 = . Lateral current flow through the contact side wall and the difference in sheet resistance under the contact were found to increase the value of the specific contact resistance determined from transfer length method (TLM) test structures by as much as 10% for n-type contacts. While exhibiting much larger contact resistance, the p-type contacts were found to have negligible impact on device performance due to the high impedance of the gate-channel and body-channel p-n junctions under typical operation. Physics based, Simulation Program with Integrated Circuit Emphasis (SPICE) compatible, integrated circuit (IC) consistent compact models were developed that are congruent with experimental measurements over the aforementioned range of temperature and across all essential bias levels. Most notably, a self-contained, asymmetric double-gated, non-selfaligned JFET model was developed that accurately accounts for the body-effect, voltage dependent mobility and temperature. An accurate yet efficient solver of the charge neutrality equation within each region of the device is utilised to account for incomplete ionisation of dopants and the temperature dependence of the p-n junction contact potentials. Meticulous agreement with experimental measurements was attained from a minimal number of input parameters. The modelled devices were used to simulate pertinent IC building blocks, including single stage and differential amplifiers, level-shifters and voltage buffers. The finite bodytransconductance of active load transistors were identified as a major degrading factor for the voltage gain. Practical methods to circumvent this are discussed with the aid of appropriate small-signal equivalent models. Finally, a design was presented for a two-stage 4H–SiC operational amplifier (op-amp) with direct current (DC) stability over the entire temperature range of study. Low-frequency small-signal voltage gains of 80 dB and 70 dB were achieved at 25 C and 400 C, respectively when utilising a 30V supply. A closed-loop non-inverting op-amp configuration with an ideal gain of 11 was then simulated and found to vary by just 1% between 25 C and 400 C. Such amplifiers are of great utility and form the cornerstone of numerous useful and important electronic systems.Engineering and Physical Sciences Research Council (EPSRC) and BAE Systems Maritime for financially supporting this research project

N channel JFET based digital logic gate structure

A circuit topography is presented which is used to create usable digital logic gates using N (negatively doped) channel Junction Field Effect Transistors (JFETs) and load resistors, level shifting resistors, and supply rails whose values are based on the direct current (DC) parametric distributions of those JFETs. This method has direct application to the current state of the art in high temperature, for example 300.degree. C. to 500.degree. C. and higher, silicon carbide (SiC) device production. The ability to produce inverting and combinatorial logic enables the production of pulse and edge triggered latches. This scale of logic synthesis would bring digital logic and state machine capabilities to devices operating in extremely hot environments, such as the surface of Venus, near hydrothermal vents, within nuclear reactors (SiC is inherently radiation hardened), and within internal combustion engines. The basic logic gate can be configured as a driver for oscillator circuits allowing for time bases and simple digitizers for resistive or reactive sensors. The basic structure of this innovation, the inverter, can be reconfigured into various analog circuit topographies through the use of feedback structures

Extreme Temperature Switch Mode Power Supply Based on Vee-square Control Using Silicon Carbide, Silicon on Sapphire, Hybrid Technology

Switch mode power supplies, commonly known as SMPS are basic building blocks of the electronic systems. SMPS performs power regulation by accepting a raw input voltage and transforming it to required voltage at output with desired characteristics. Electronic systems used in applications such as deep well oil drilling, geothermal wells and deep space explorations is expected to operate under extremely harsh conditions like elevated temperature, high pressure and radiation prone environments. To support the onboard electronics in these applications, SMPS capable of operating at extreme temperatures are of high interest.This research work deals with the design and development of a switch mode power supply capable of operating over the temperature range of 300 degree centigrade (�C). Silicon carbide field effect transistors are used as power devices in the design to tolerate these extreme high ambient temperatures without compromising power handling capability. The simplest yet robust vee square control architecture is adopted for control mechanism. The control electronics are implemented as an integrated circuit in 0.5 �m silicon on sapphire process. The supporting components like high temperature tolerant inductors and capacitors are identified by evaluating various samples at elevated temperature. This is the first demonstration of SMPS capable of operating at 275�C as a standalone component. Also for the first time, a gate drive mechanism based on planar transformer architecture is studied and presented for high temperature operation. A low cost packaging technique suited for harsh environment operation is proposed based on gold on aluminum nitride thin film technology. The basic analog building blocks of the system, such as comparator, voltage reference and rail-to-rail amplifiers are made available in discrete packages for use at temperatures above 275�C. A SMPS prototype on a 1.8 square inches substrate is developed and tested. Test results indicate that the system is capable of operating continuously at 275�C for extended period of time, providing the desired performance characteristics.School of Electrical & Computer Engineerin

An Overview of Wide Bandgap Silicon Carbide Sensors and Electronics Development at NASA Glenn Research Center

A brief overview is presented of the sensors and electronics development work ongoing at NASA Glenn Research Center which is intended to meet the needs of future aerospace applications. Three major technology areas are discussed: 1) high temperature SiC electronics, 2) SiC gas sensor technology development, and 3) packaging of harsh environment devices. Highlights of this work include world-record operation of SiC electronic devices including 500?C JFET transistor operation with excellent properties, atomically flat SiC gas sensors integrated with an on-chip temperature detector/heater, and operation of a packaged AC amplifier. A description of the state-of-the-art is given for each topic. It is concluded that significant progress has been made and that given recent advancements the development of high temperature smart sensors is envisioned

A Silicon Carbide Linear Voltage Regulator for High Temperature Applications

Current market demands have pushed the capabilities of silicon to the edge. High temperature and high power applications require a semiconductor device to operate reliably in very harsh environments. This situation has awakened interests in other types of semiconductors, usually with a higher bandgap than silicon\u27s, as the next venue for the fabrication of integrated circuits (IC) and power devices. Silicon Carbide (SiC) has so far proven to be one of the best options in the power devices field. This dissertation presents the first attempt to fabricate a SiC linear voltage regulator. This circuit would provide a power management option for developing SiC processes due to its relatively simple implementation and yet, a performance acceptable to today\u27s systems applications. This document details the challenges faced and methods needed to design and fabricate the circuit as well as measured data corroborating design simulation results