Ensuring Space-Readiness: NASA’s Radiation Testing of LSK389 & LSK489 JFET Amplifiers

In March 2021, NASA published a pivotal report—NASA/TM‑20210009960—evaluating Linear Systems’ dual N-channel Junction Field-Effect Transistors, the LSK389 and LSK489, under conditions that mimic the harsh radiation environment of space. The study assessed both Total Ionizing Dose (TID) effects at cryogenic temperature (−65 °C) and Single-Event Latchup (SEL) vulnerability under heavy ion exposure. With their application in sensitive space missions like the Ocean Color Instrument (OCI) on the PACE spacecraft, confirming these JFETs’ resilience is critical.

Test Objectives & Rationale

The tests aimed to:

1. Determine if high-energy heavy ions could induce “latchup”—a destructive form of failure—despite the shared substrate between the JFET pairs, which could behave like a parasitic p–n–p–n structure.

2. Evaluate long-term exposure to gamma radiation (TID), up to tens of krad(Si), with the transistors operated at −65 °C to simulate real-world space thermal conditions.

Experimental Setup

• SEL Testing: Conducted at Lawrence Berkeley National Lab’s 88-inch cyclotron using intense 1,039 MeV silver ions (LET 48.15 MeV·cm²/mg). Both “substrate bonded to drain” and “floating substrate” configurations were tested at room temperature—conservatively more challenging than the target −65 °C.

• TID Exposure: Performed at NASA Goddard’s Radiation Effects Facility. Each JFET variant was irradiated in steps (1, 3, 6, 12, 25 krad(Si)), with devices kept biased and cold. Parametric measurements (drain current IDS, gate leakage IGS) were taken before and after each dose.

Key Findings

• SEL Immunity: No high-current latchup occurred during aggressive heavy-ion testing—even with floating substrates and at room temperature. Given the harsher testing conditions, the parts are effectively SEL-immune at actual mission temperatures.

• TID Performance: Up to 25 krad(Si), both JFETs maintained electrical performance within specifications. Minor parameter shifts were attributed to measurement noise rather than degradation.

Detailed Results

The report includes extensive parametric tracking—IDS and IGS over key voltage operating points across each dose step. It notes that variations observed after ~6 krad(Si), which is the qualification dose for OCI, fell within expected noise bounds, with averages used to present robust results. Conductively and non-conductively bonded samples performed statistically the same.

Implications for Space Applications

• SEL Immunity: The absence of latchup in even the most vulnerable substrate configuration under heavy-ion bombardment is compelling evidence of the JFETs’ safety for space missions.

• TID Durability: Stable performance through relevant radiation doses confirms their suitability for long-duration scientific instruments and deep-space missions.

• Design Confidence: Engineers can rely on these JFETs for precision front-end amplification in high-reliability systems, knowing radiation-induced failures are highly unlikely.

Conclusion

NASA’s exhaustive testing of the LSK389 and LSK489 demonstrates that both JFETs are well-suited for demanding space environments. Their combined resilience to SEL and TID effects positions them as strong candidates for use in sensitive instrumentation, such as low-noise photodiode readouts and scientific payloads aboard satellites.

For a deeper dive into the data or experimental methodologies, the full technical memorandum (NASA/TM‑20210009960) is publicly available via the NASA Technical Reports Server (ntrs.nasa.gov).