RESEARCH REVIEW
SiC extends the temperature range for op-amps
Bipolar 4H-SiC technology allows the construction of analogue circuits operating at 500 °C
RESEARCHERS at KTH Royal Institute of Technology have turned to SiC bipolar technology to set a new benchmark for the operating range of a fully integrated op-amp. The team’s monolithic amplifier, built using 4H-SiC bipolar technology, can operate at up to 500 °C, and should open the door to other analogue integrated circuits constructed from SiC operating at this temperature extreme.
Carl-Mikael Zetterling from KTH says that the performance of the op-amp is good enough to be used for Venus exploration, and also in aviation, automotive, and oil and gas drilling industries.
“If higher open loop amplification is needed, more stages can be added,” adds Zetterling, who points out that it is relatively straightforward to design any analogue circuit block using SiC technology, since it is bipolar.
Thanks to previous efforts by Zetterling and his co-workers that were reported last year, digital circuits built from SiC can also operate at very high temperatures.
Another material system capable of producing devices working at very high temperatures is GaN.
“But this work explores integrated circuits at high temperatures,” explains Zetterling. “So far I have only seen simple GaN ICs like MMICs – on transistor circuits – where as our work opens [the door] for making any integrated circuit at high temperatures.”
Zetterling says that bipolar transistors or MOSFETs for CMOS are the preferred technologies for making integrated circuits. He points out that it is only SiC
Monolithic op-amps made with 4H-SiC bipolar technology are capable of operating at 500 °C
BJTs and JFETs that have demonstrated operation at 500 °C or more, while SiC MOS technology is limited to 400 °C, due to the gate oxide, and silicon-on-insulator technology is only capable of temperatures up to 300 °C.
The building block for the team’s monolithic two-stage op-amp is an npn transistor with a resistive load. This device is formed on a 4-inch SiC wafer accommodating six epilayers with a total thickness of 4.3 μm. Measurements on a transistor reveal a current gain of 38 at 25 °C, falling to 15 at 500 °C.
Op-amp characteristics were determined from on-wafer measurements using a high-temperature probe station. Reliability evaluations were not included, but no degradation in performance was observed during two hours of testing at 500 °C.
At 25 °C and operating in a closed- loop configuration with a 500 Ω resistor connected to the output, the op-amp produced a DC gain of 39.86 dB and a gain bandwidth of 5.92 MHz. At 500 °C, DC gain declined marginally to 39.46 dB,
and the gain bandwidth fell to 4.36 MHz. In an open-loop configuration, op- amp gain fell from 76.3 dB at room temperature to 64 dB at 500 °C. According to the team, this decline is small enough to preserve a relatively constant closed-loop gain over a wide temperature range.
Zetterling and co-workers are trying to improve their devices. “We are presently working on the metallisation for high temperature packaged devices, and are doing on-wafer characterisation in air on a hot stage using needle probes.”
Another goal for the team is to make new circuits. In addition to the npn process, the team has one for making pnp transistors, and this pairing of devices will allow the construction of higher-performance amplifiers that can accommodate active loads. “We also have a sigma delta modulator that we plan to publish,” says Zetterling.
R. Hedayati et. al. IEEE Electron Device Lett. 35 693 (2014)
We are presently working on the metallisation for high temperature packaged devices, and are doing on-wafer characterisation in air on a hot stage using needle probes, was observed during two hours of testing at 500 °C
Copyright Compound Semiconductor Issue VI 2014
www.compoundsemiconductor.net 65
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