BEYOND “RADIATION TESTED”: WHAT NASA’S LSK389 AND LSK489 RESULTS ACTUALLY PROVE
Saying that a semiconductor has been “radiation tested” sounds definitive.
It is not.
Radiation test results are meaningful only when they are connected to the device samples, radiation source, accumulated dose, particle energy, operating bias, temperature, packaging, monitored parameters and intended application.
NASA tested Linear Systems’ LSK389 and LSK489 dual N-channel JFETs for a specific reason: to evaluate their suitability for the Ocean Color Instrument, or OCI, developed for the Plankton, Aerosol, Cloud, ocean Ecosystem spacecraft.
The results were encouraging—but they must be described accurately.
NASA’s report supports the suitability of the tested devices for the stated OCI readout application under the evaluated conditions. It does not establish an unrestricted radiation-hardness rating for every LSK389 or LSK489 in every circuit, package, mission and radiation environment.
WHY NASA TESTED THE DEVICES
The LSK389 and LSK489 are monolithic dual N-channel JFETs designed for low-noise precision analog applications.
NASA evaluated the devices because the two JFETs share a common substrate. The investigators identified a possible PNPN structure that could potentially be susceptible to single-event latchup, even though the devices are not CMOS integrated circuits.
The test program therefore addressed two different radiation concerns:
Total ionizing dose degradation
Heavy-ion-induced single-event latchup
NASA also evaluated different die-bonding configurations because substrate connection and packaging can influence device behavior.
The purpose was not to assign a universal “space-qualified” label. It was to determine whether the devices were suitable for a particular circuit in a particular NASA instrument.
WHAT IS TOTAL IONIZING DOSE?
Total ionizing dose, usually abbreviated TID, describes the cumulative ionizing energy deposited in a material over time.
It is commonly expressed in rad(Si) or krad(Si), indicating the absorbed dose referenced to silicon.
TID can gradually change semiconductor characteristics by affecting charge within insulating materials and device interfaces. Depending on the technology, possible effects may include changes in:
Leakage current
Threshold or operating voltage
Drain current
Gain
Noise
Matching
Power consumption
Functional behavior
TID is cumulative. It is different from a single energetic particle striking the device at one moment.
WHAT NASA’S TID TEST INVOLVED
NASA performed the TID testing at the Goddard Space Flight Center Radiation Effects Facility using a gamma-radiation source.
The report describes an intermediate dose rate between 1 and 50 rad(Si) per second. Devices were irradiated in a biased configuration and measured between dose steps for functional and parametric changes.
The monitored parameters were selected around the needs of the OCI readout application. Particular attention was given to gate current and drain current as functions of VGS and VDS, including behavior near the application operating point of approximately:
VDS = 3 V
VGS = -1 V
The procedure also examined gate leakage at selected gate voltages and measurements involving a 2-kilohm source resistor.
NASA’s broader test-results compendium summarized the LSK389 and LSK489 as remaining within specification through 25 krad(Si) under the stated test conditions.
The detailed report focused particularly on the OCI qualification dose of approximately 6.6 krad(Si). It stated that the parts showed little degradation at the dose steps and that much of the measured variation appeared comparable to test-system noise.
The report did identify small changes in LSK489 drain current that could matter if an application were sensitive to changes of that magnitude.
NASA’s conclusion was therefore appropriately conditional: unless those small drain-current changes were a concern, both JFET amplifiers were considered suitable for the OCI readout application.
WHAT IS A SINGLE-EVENT EFFECT?
A single-event effect, or SEE, is caused by one energetic particle passing through a sensitive region of a semiconductor.
SEE is a broad category. It can include:
Single-event latchup, or SEL
Single-event transient, or SET
Single-event upset, or SEU
Single-event burnout, or SEB
Single-event gate rupture, or SEGR
Other temporary or destructive responses
These effects are not interchangeable.
A device that shows no single-event latchup under one test condition has not automatically been proven immune to every possible single-event effect.
WHAT NASA’S HEAVY-ION TEST INVOLVED
The LSK389 and LSK489 single-event testing was performed at Lawrence Berkeley National Laboratory using its 88-inch cyclotron.
The devices were exposed to silver ions with:
Energy of 1039 MeV
Linear energy transfer, or LET, of 48.15 MeV-cm2/mg
Normal beam incidence
Room-temperature testing
VDS of 3.5 V
VGS between -0.15 V and -1 V
The devices were monitored during irradiation for high-current conditions and transients, then checked again for functionality.
NASA reported that no high-current conditions were observed in any of the tested parts, regardless of device type or the evaluated bonding configuration.
Because the tested LET exceeded the PACE project requirement and the devices were tested at room temperature while the intended application temperature was -65 degrees C, NASA concluded that the parts could be considered immune to single-event latchup for the purposes of the OCI application.
That final phrase matters:
“For the purposes of the OCI application.”
WHAT THE RESULTS DO PROVE
Within the samples and conditions evaluated, the NASA testing showed that:
No high-current latchup condition was observed during the specified heavy-ion exposure.
The tested LET exceeded the destructive-SEE requirement established for the PACE application.
The evaluated parts exhibited little TID-related degradation over the tested dose range.
NASA’s summary reported parameters remaining within specification through 25 krad(Si) under the stated conditions.
The detailed report found the devices suitable for the OCI readout application, subject to consideration of the small LSK489 drain-current changes.
Both evaluated bonding configurations produced acceptable results for that application.
These are meaningful results from a defined NASA test program.
They provide engineers with valuable evidence about the behavior of the tested LSK389 and LSK489 devices under the reported conditions.
WHAT THE RESULTS DO NOT PROVE
The report does not establish that every LSK389 or LSK489 is universally radiation hard.
It also does not automatically prove:
Immunity at higher total doses
Immunity at higher LET values
Immunity at different drain or gate voltages
Identical performance at every temperature
Identical performance in every package
Identical performance across every production lot
Immunity to every type of single-event effect
Immunity to proton or neutron radiation
Displacement-damage tolerance
Acceptable performance in every orbit or mission duration
Suitability for every spacecraft subsystem
Compliance with every radiation-hardness or space-qualification standard
The report principally evaluated TID degradation and heavy-ion-induced latchup for the OCI application.
It should not be presented as evidence that every possible radiation mechanism was tested.
“NO EFFECT OBSERVED” HAS A SPECIFIC MEANING
A statement such as “no effect observed” means that no qualifying effect was detected:
In the tested samples
During the applied exposure
At the tested LET or dose
At the stated bias conditions
At the stated temperature
Using the defined monitoring criteria
It does not prove that an effect is physically impossible.
The true threshold could be above the maximum tested condition. It could also change with voltage, temperature, lot, packaging, circuit configuration or the type of radiation.
This is why radiation reports must preserve the conditions attached to the result.
WHY BIAS CONDITIONS MATTER
A semiconductor’s radiation response can depend strongly on its electrical state during exposure.
Important conditions may include:
Drain-to-source voltage
Gate-to-source voltage
Drain current
Power dissipation
Whether the device is biased or unbiased
Substrate connection
External resistance
Load conditions
Temperature
NASA selected bias conditions that represented the intended OCI application.
A different circuit might operate the same JFET at a higher drain voltage, a different drain current, a different gate voltage or a different temperature. Those changes could alter the device’s response and may require additional testing.
WHY PACKAGE AND ASSEMBLY MATTER
The NASA test articles were supplied as die and assembled into special test packages.
The investigation considered different die-bonding configurations because the relationship between the substrate, die attach and electrical connections could affect latchup susceptibility.
This is an important reminder: radiation performance is not always determined by the semiconductor die alone.
Engineers may also need to consider:
Package construction
Die attach
Substrate connection
Bond-wire configuration
Shielding
Board layout
Operating temperature
Circuit impedance
Mechanical assembly
A result obtained from one test configuration should not automatically be transferred to a substantially different assembly without engineering review.
TID IS NOT THE SAME AS DISPLACEMENT DAMAGE
TID measures cumulative ionizing energy deposited in the device.
Displacement damage occurs when radiation physically displaces atoms within the semiconductor lattice. It is commonly associated with proton and neutron exposure and can affect semiconductor parameters differently from ionizing dose.
A device’s TID performance does not establish its displacement-damage tolerance.
NASA’s LSK389 and LSK489 report addressed TID and heavy-ion single-event latchup. It should not be interpreted as a complete displacement-damage evaluation.
TESTING IS NOT THE SAME AS QUALIFICATION
Radiation testing generates data under controlled conditions.
Qualification goes further. Depending on the program, it may require:
Defined lot controls
Traceability
Statistical sample plans
Screening
Environmental testing
Radiation-design-margin analysis
Mission-specific dose calculations
Worst-case bias and temperature testing
Destructive physical analysis
Applicable military, aerospace or agency standards
A NASA test report is valuable technical evidence, but its presence does not automatically make a commercial device radiation-hardened, radiation-tolerant for every use or qualified for every space mission.
NASA’s report also states that the use of trade names and trademarks is for identification and does not constitute an official NASA endorsement.
WHAT ENGINEERS SHOULD ASK
When someone says that a component is “radiation tested,” ask:
Which exact part number and production lot were tested?
Was the device tested as packaged product or as die in a special assembly?
What radiation source was used?
Was the test for TID, displacement damage or single-event effects?
Which specific single-event effects were monitored?
What was the accumulated dose?
What was the dose rate?
What particle species, energy and LET were used?
What was the particle fluence?
What were VDS, VGS and drain current during exposure?
Was the device biased or unbiased?
What temperature was used?
Which electrical parameters were measured?
What constituted a failure?
How many devices were tested?
Were multiple lots evaluated?
Were package and substrate configurations representative of the final application?
Does the mission environment remain within the tested envelope?
Was radiation design margin applied?
Does the conclusion apply generally or only to a named application?
Without those details, the phrase “radiation tested” provides very little engineering assurance.
THE VALUE OF NASA’S RESULTS
The correct interpretation does not diminish the importance of the results.
NASA identified a credible latchup concern, created test articles representing the intended application, exposed the devices to controlled gamma and heavy-ion environments, monitored relevant electrical parameters and documented the outcome.
The results support the LSK389 and LSK489 as strong candidates for precision analog applications requiring radiation-performance evidence—provided that engineers compare their own requirements with the actual test conditions.
That is far more useful than an unsupported marketing claim.
ENGINEERING TAKEAWAY
NASA’s testing of the Linear Systems LSK389 and LSK489 produced meaningful and encouraging results.
No high-current latchup conditions were observed during the reported heavy-ion tests. The devices showed little TID-related degradation under the evaluated conditions, and NASA concluded that both JFET amplifiers were suitable for the OCI readout application unless the observed small LSK489 drain-current changes were significant to the circuit.
What the report proves is specific:
Specific device samples
Specific assembly configurations
Specific electrical biases
Specific temperatures
Specific radiation sources
Specific dose and LET levels
Specific monitored parameters
A specific spacecraft application
That is how radiation data should be used.
The strongest engineering statement is not simply:
“These devices are radiation tested.”
It is:
“Under NASA’s documented test conditions, the tested LSK389 and LSK489 devices produced results supporting their use in the PACE OCI readout application.”
Precision in describing the result is just as important as precision in performing the test.
Read NASA Technical Memorandum 20210009960:
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