The Semiconductor's business unit of Raytheon UK in Glenrothes, Scotland (a subsidiary of Raytheon Company of Waltham, MA, USA) and Newcastle University's School of Electrical and Electronic Engineering have collaborated to produce silicon carbide (SiC)-based amplifier circuitry with operational amplifier (op amp)-like characteristics. Once integrated and packaged into a single device, the amplifier has the potential for use in monitoring and closed-loop control circuitry applications within a variety of harsh-environment industries, such as aerospace, oil & gas, geothermal energy and nuclear, says Raytheon.
"To date, the focus on silicon carbide semiconductors has been power electronics and exploiting the material's ability to dissipate internally generated heat," notes Dr Alton Horsfall, reader in Semiconductor Technology at Newcastle University. "For this project though we've focussed on creating circuitry that can operate in high temperature and other harsh environments. This could therefore lead to condition monitoring circuitry mounted on gas turbines or within the primary coolant loop of a nuclear reactor, which runs at about 350oC," he adds.
At the heart of the amplifier circuit is a lateral small-signal junction field-effect transistor (JFET). This offers a significant improvement in reliability in hostile environments, because of the lack of a gate oxide layer. This results in greater stability in the threshold voltage and a reduction in the intrinsic noise, making these structures suitable for the realization of high-temperature, low-noise amplifier circuits. The current circuit is a fully differential, three-stage amplifier, with a source follower final stage, optimized to operate on a ±15V supply. Modifications enable voltage supplies of ±45V to be utilized to increase the voltage headroom of the circuit.
Laboratory tests have shown that the amplifier circuit has an open-circuit gain in excess of 1500 at room temperature. A high-temperature gain of 200 has been recorded at 400°C, but this is limited by the passive components used in the circuit.
The recent monolithic integration of the amplifier into a single chip should deliver the kind of op amp capabilities with which electronics engineers the world over are familiar, says Raytheon.
"Though we're not the only ones to be exploring the suitability of silicon carbide for control and monitoring applications in harsh environments, we believe this amplifier circuit represents the furthest anyone has gone down the lab-to-fab route," states Phil Burnside, business development manager of Raytheon UK's Semiconductors Business Unit. "In this instance, it is Newcastle University's design expertise and understanding of harsh environments, combined with our silicon carbide processing expertise, that have the potential to result in the full commercialization of a high-temperature version of a fundamental electronic building block, the humble op amp."
A technical demonstrator of the amplifier circuit will be on Raytheon UK's stand (134 in Hall 6) at Power Conversion Intelligent Motion (PCIM) Europe 2016 in Nuremberg, Germany (10-12 May). Also at PCIM Europe, on 11 May (10-12am) in the Industry Forum Area (Hall 6, stand 248), Raytheon UK is on a panel session, organised by Yole Developpement, regarding the use of power electronics in high temperature applications.
