UK Researchers Make High Temperature Op Amp in SiC

Raytheon UK has collaborated with Newcastle University to produce silicon carbide (SiC) based amplifier circuitry with operational amplifier like characteristics.

The research has used the thermal characteristics of SiC not in a high power device, but in small-signal circuitry that can operate in high temperatures.

"To date, the focus on Silicon Carbide semiconductors has been power electronics and exploiting the material's ability to dissipate internally-generated heat. For this project though we've focussed on creating circuitry that can operate in high temperature and other harsh environments," said Dr Alton Horsfall, the Reader in Semiconductor Technology at Newcastle University.

"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 350 deg C," he said.

The amplifier circuit is based on a lateral small-signal junction field effect transistor (JFET) which improves reliability in hostile environments, because of the lack of a gate oxide layer.

According to the researchers, this results in a greater stability in the threshold voltage and a reduction in the intrinsic noise, making these structures suitable for the realisation of high temperature, low noise amplifier circuits.

The current circuit is a fully differential, three stage amplifier, with a source follower final stage, optimised to operate on a +/-15V supply. Modifications enable voltage supplies of +/-45V to be utilised to increase the voltage headroom of the circuit.

Laboratory tests have shown the amplifier circuit has an open circuit gain in excess of 1,500 at room temperature. A high temperature gain of 200 has been recorded at 400 deg C, but this is limited by the passive components used in the circuit.

The amplifier is likely to be used in monitoring and closed-loop control circuitry applications within a variety of harsh environment industries; such as aerospace, oil and gas, geothermal energy and nuclear.

Phil Burnside, business development manager of Raytheon UK's semiconductors business unit, said:

"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. 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 commercialisation 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 PCIM Europe - Nuremberg, 10th to 12th May.