'Extreme' transistor can withstand heat of 1000-plus F — priming it for use in Venus-bound probes

Scientists in Japan have built a new transistor that can withstand temperatures of 1,110 degrees Fahrenheit (600 degrees Celsius). Components this robust could one day be used in surface probes on Venus — where the thick carbon dioxide atmosphere can reach temperatures of 860 F (460 C).

Most modern tech, including instruments used for deep-space exploration, uses transistors to control the flow of current. But the new device is a type of junction field-effect transistor (JFET), where the strength of an electrical field changes the channel’s conductivity.

JFETs are typically used in specialist applications because they are more difficult to scale down than the more common metal-oxide-semiconductor field-effect transistors (MOSFETs), which are widely used in consumer smartphones and computers. But JFETs can offer lower noise levels because their operation does not rely on an oxide layer, which can introduce interference.

The researchers outlined how the new transistor works in a study published Aug. 17 in the journal APL Electronic Devices.

Can't take the heat

Since at least the start of the century, silicon carbide (SiC) JFETs have been considered a promising option for low-power integrated circuits heading for Venus due to the material's inherent ability to withstand high temperatures.

As the scientists pointed out in the new study: "Past landers have been limited to only a few hours by silicon-based electronics." Venera 13, a Soviet-era lander, holds the world record for the longest time survived on Venus by a spacecraft, at 2 hours, 7 minutes.

"Integrated circuits (ICs) fabricated with SiC are particularly attractive for extreme environments, such as deep-space exploration, geothermal drilling, and aerospace engine control, where conventional silicon-based ICs cannot operate reliably," the researchers added.

But recently developed SiC-JFETs have all met the same two problems: low controllability and large leakage currents. The former is linked to how the SiC substrate is doped with other atoms to alter its electrical properties, defining its gate (where the electrical field is created) and channel (where the current flows) regions.

In a material with a regular crystal structure, like SiC, some dopant atoms may penetrate deeper than expected. This doesn't matter under normal conditions, but when exposed to high temperatures, it causes variations in the field voltage required to open the channel and makes the transistor harder to control reliably. The scientists found that it can throw conventional JFET voltage thresholds off by over 2 volts.

Furthermore, at temperatures above 660 F (350 C), the SiC substrate can become less electrically resistive, allowing current to flow even when the transistor is switched off. This makes it harder for the JFET to control current properly, potentially causing incorrect signals and increased power consumption.

Even the highest-performing JFETs can only operate long-term at 930 F (500 C), but the Kyoto team had a theory as to why these challenges remained unresolved.

"We believe the lack of development is because the research community has been trying to apply silicon-era thinking to a fundamentally different material," said first author of the study Mitsuaki Kaneko, associate professor of engineering at Kyoto University, in a statement.

Turning the transistor on its head

The researchers designed their new SiC-JFET with these two challenges in mind. To improve controllability, they implemented a bottom-gate structure, where the gate is positioned underneath the SiC conducting channel.

Its gate region is heavily doped by design, so when dopant atoms in the channel penetrate deeper into the SiC, it doesn't change the overall doping profile of the channel-gate region, limiting its impact on the threshold voltage even at high temperatures.

The team also used dopants to create two semiconductor regions, or "wells", in the SiC around the JFET. The boundaries between the wells act as barriers to current flow, meaning that even when the SiC becomes more conductive at high temperatures, current cannot bypass the channel when the transistor is switched off.

Researchers measured how well the new JFET could switch current on and off and how closely the actual threshold voltage matched the theoretical value, based on its thickness and level of doping. They recorded these metrics at temperatures ranging from room temperature to 1,110 F (600 C).

"The fabricated devices demonstrated stable, normal transistor operation at temperatures over 873 K [1,110°F], opening up possibilities for ultrahigh-temperature SiC devices," they wrote in the study. Furthermore, at about 750 F (400 C), the threshold-voltage error was found to be less than 0.1 V thanks to the bottom-gate design.

As well as surface probes on Venus, the transistor could be useful inside jet engines, the scientists said. Currently, components connected to gas turbines must be protected from extreme temperatures using thermal shielding, long wires and energy-intensive cooling systems, restricting engine design.

Before it can be launched into space or hooked up to a plane, the team still needs to test and optimise the transistor for practical use. This includes integrating it into more complex circuits, scaling it up to wafer-level and ensuring that the entire circuit package will withstand extreme temperatures and pressures.

Indeed, it may not be too lofty a goal. NASA demonstrated that integrated circuits with SiC-JFETs could withstand temperatures of 860 F (460 C) and 9.3 MPa of pressure for 60 days, and 930°F (500 C) in the air for over a year. Plus, in 2024, a team from the National Institute for Materials Science in Japan developed a MOSFET out of diamond that could operate above 570 F (300 C).