Semiconductor expert: How radiation in space affects transistors

NASA’s $5.2 billion Europa Clipper mission to study Jupiter’s fourth-largest moon—Europa—and assess its potential for harboring life will see the spacecraft travel through the most powerful radiation belt in the solar system.

Concerns surfaced this summer about whether the semiconductor transistors used throughout the on-board spacecraft electronic systems could withstand the radiation. The Jupiter system is particularly harmful to spacecraft because its enormous magnetic field—tens of thousands of times stronger than Earth’s magnetic field—traps charged particles and accelerates them to very high energies, creating intense radiation belts that bombard Europa and other inner moons.

After extensive testing, however, NASA confirmed that the transistors can support the mission. The space agency launched Europa Clipper on Oct. 14, and it should arrive in 2030.

The concerns about transistors, however, raises the question: What happens when electronics are exposed to high levels of radiation?

Dr. Robert Baumann, an expert on radiation effects and reliability in microelectronics, was the former chief technologist for high reliability products at Texas Instruments (TI) and is now director of radiation effects and reliability at the Center for Harsh Environments Semiconductor Systems (CHESS) at The University of Texas at Dallas. He is also lead author of the company’s Radiation Handbook for Electronics.

Baumann said that in this specific case, high-energy protons and electrons can excite electrons in atoms (a process called ionization), creating excess charge carriers.

“This excess charge is accumulated and trapped in the insulating layers and shifts the performance and operation of transistors,” he said.

The damage happens gradually and accumulates as the spacecraft mission progresses.

“Like getting a sunburn, it is not one ultraviolet light ray, or photon, that burns you; it is the accumulation of many UV rays that burns your skin,” Baumann said.

In electronics, one effect of exposure to radiation is the generation, accumulation and trapping of radiation-induced positive charge in the insulator in transistors that leads to an effect called the total ionizing dose, or TID.

Transistors operate as digital on/off switches or analog “volume” controls, where the flow of electric current is controlled by the voltage on the “gate” of the transistor. The transistor acts like a faucet for electrical current, and the gate acts like the handle of a water faucet, Baumann said.

“As transistors are exposed to radiation and start to accumulate TID damage, they start to drift, and eventually, at a high enough dose, the transistor no longer functions as intended,” he said. “Transistors impacted by the TID effect will ultimately either be stuck in the ‘on’ or ‘off’ position. Imagine your home faucet with water gushing out, and no matter how hard you try to turn the handle, nothing happens.”

The Europa Clipper is expected to spend only one day of each of its 21-day orbits in the harshest radiation environment around Jupiter. NASA determined that the Europa Clipper’s transistors will be able to restore themselves through a process called annealing after the spacecraft moves through the part of its orbit outside of Jupiter’s high-radiation environment.

“Looking for life on Europa is a big deal,” said Baumann, who will be keeping tabs on the mission as it progresses. “It is just too bad about the high radiation levels around Jupiter; it makes the whole job significantly more challenging.”