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| Fig. 1: Plot of radiation dose encountered during space missions vs. aluminum shielding thickness. [1] (Image source: Z. Jahan) |
The space radiation environment poses significant challenges to spacecraft electronics through cumulative and transient damage mechanisms that can cause mission failures. Understanding these effects is essential for reliable spacecraft design, particularly as the industry increasingly adopts commercial-off-the-shelf (COTS) components with unknown radiation tolerance. [1]
The near-Earth radiation environment consists of three primary sources. First, trapped radiation belts called Van Allen Belts contain energetic protons in the inner belt and electrons in both inner and outer belts that are confined by Earth's magnetic field. Second, solar particle events produce energetic protons and heavy ions during solar flares. Third, galactic cosmic radiation (GCR) provides omnidirectional high-energy particles (electrons, protons, and heavy ions) that originate outside of the solar system. [1]
These radiation sources damage microelectronics through three main mechanisms: Total Ionizing Dose (TID) effects from cumulative ionization in insulators, displacement damage from atomic displacements in the crystalline lattice, and Single Event Effects (SEE) from individual energetic particles. Each mechanism is quantified differently based on its underlying physics. Understanding the distinct measurements and effects of these mechanisms when designing microelectronics for space applications is essential for mission success. [1]
Total ionizing dose effects result from the cumulative deposition of energy by ionizing radiation in semiconductor materials, particularly in silicon dioxide (SiO2) layers. TID is measured in units of kilorad(Si), where 1 krad(Si) = 10 J/kg of energy deposited in silicon, or equivalently 0.01 J/kg = 1 rad(Si). This measurement involves exposing a thin silicon target to radiation and quantifying the absorbed energy per unit mass, with the thin geometry ensuring most radiation passes through rather than being completely absorbed. When radiation interacts with SiO2, it creates electron-hole pairs at approximately 8.1 × 1012 pairs per rad(SiO2) × cm3. While electrons are quickly swept away due to their high mobility, holes move slowly and some become trapped at Si/SiO2 interfaces. [2] This trapped positive charge alters electric field distributions in metal-oxide-semiconductor (MOS) devices, causing threshold voltage reduction due to additional oxide charge, increased leakage currents, and degraded performance.
The extent of TID effects depends on device design and technology node. Table 1 summarizes experimentally measured threshold voltage shifts for MOSFET and GaN-based devices at different dose levels.
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| Table 1: Threshold voltage shifts for various device types at different total ionizing dose levels. [3,4] |
From Table 1, it can be seen that non-hardened silicon MOSFETs can experience threshold voltage shifts exceeding -3.0 V at 100 krad(Si), with the initial VTH of +3.0 V shifting to below 0 V, transitioning from enhancement mode to depletion mode and making them uncontrollable. In contrast, radiation-hardened MOSFETs show only -0.5 V shift of VTH at 100 krad(Si), maintaining controllability up to 500 krad(Si). [3] Modern fully depleted silicon on insulator (FDSOI) technology at 22 nm nodes also has significantly improved tolerance, with only 0.052 V VTH shift at 400 krad(Si). [4] Wide bandgap semiconductors have notable radiation tolerance at high doses, with GaN p-gate high- electron-mobility transistors (HEMTs) showing only a -0.2 V shift even at 500 krad(Si) due to the absence of gate oxide. [3]
Fig. 1 shows the relationship between aluminum shielding thickness and radiation dose encountered during space missions for a representative 2000 km altitude, 103° inclination orbit over 378 days. The dose depicted primarily represents energetic electrons and protons from trapped radiation belts. Aluminum is effective at shielding these charged particles, rather than highly penetrating photons, demonstrated by the fact 40 mils of aluminum reduces dose by approximately a factor of 100. From the figure, though, it can be seen that the effectiveness of shielding decreases with increasing thickness. Adding 8 mils at 20 mils total reduces dose by half, while achieving the same reduction at 80 mils requires 60 additional mils, making shielding beyond 100 mils cost-ineffective for weight-constrained spacecraft. [1] As a result, proper part selection and radiation hardening of microelectronics have become more important than shielding for achieving high TID tolerance.
Displacement damage occurs when radiation provides sufficient energy to atoms in the crystalline lattice to overcome binding energy, causing displacement from normal lattice positions. This disruption creates defect states that act as recombination centers for minority carriers, reducing carrier lifetime and diffusion length. Unlike TID which measures energy deposited (in krad), displacement damage is primarily determined by the number of particle impacts rather than their total energy. A 1 MeV electron depositing 2 MeV of ionization energy, and thus contributing negligibly to TID, can still create significant displacement damage through atomic collisions. [1] These effects are especially severe in devices dependent on minority carrier properties, such as bipolar transistors and solar cells.
Solar cells are one of the most critical applications that require displacement damage protection. Degradation typically causes reduced short- circuit current, decreased open-circuit voltage, and reduced maximum power output, with power degradation of up to 50% possible over mission lifetimes. [1] Because displacement damage depends on the number of atomic collisions (fluence, measured in particles/cm2) rather than total energy deposited, it is quantified differently than TID. For a polar orbit mission with 6 mil cover glass, the annual particle fluence produces displacement damage equivalent to what approximately 4.2 × 1014 1 MeV electrons/cm2 would create in a solar cell. Increasing cover glass thickness to 12 mils reduces this fluence to 6.4 × 1013 equivalent electrons/cm2. [1] Cover glass thickness is typically optimized between 6-12 mils to balance radiation protection against optical transmission losses.
Single event effects result when a single energetic particle passes through a sensitive region of a microelectronic device. These events are caused by individual cosmic rays, which are high-energy particles from galactic or solar sources, that deposit large amounts of localized energy along their trajectories. When a high-energy ion traverses silicon, it creates a dense column of electron-hole pairs. If sufficient charge is collected at a given node, many different effects can occur depending on device technology and circuit design. [5]
SEE can be split into non-destructive and destructive effects. Non-destructive effects include single event upset (SEU) where bits flip in memory, single event transient (SET) producing temporary voltage pulses, and single event functional interrupt (SEFI) causing recoverable loss of functionality. Destructive effects include single event latchup (SEL) switching devices into a high current state, single event burnout (SEB) in power transistors, and single event gate rupture (SEGR) from dielectric breakdown. [1,4]
Unlike TID and displacement damage which are characterized by cumulative dose averages, single events are so rare and stochastic that they cannot be characterized by average energy deposition or particle counts. Instead, they are measured empirically by exposing devices to particle beams at accelerator facilities and counting the actual bit-flips and transistor destruction events that occur. [5] The critical parameter for heavy ion SEE is linear energy transfer (LET), which measures energy lost by a particle per unit path length in units of MeV×cm2/mg, because energy loss per unit path length (in MeV/cm) is normalized by the density of the target material (in mg/cm3). Modern sub-100 nm technologies have threshold LETs less than 1 MeV×cm2/mg, making them increasingly susceptible to low-LET particles. An important trade-off in space electronics design is that smaller transistors are more susceptible to radiation-induced errors since smaller circuits are more likely to make radiation mistakes from individual particle strikes that would not affect larger devices. [5] Also, unlike for TID and displacement damage, shielding cannot effectively mitigate SEE due to the high energies of GCR and solar particles, but may rather worsen it by causing particles to slow and increasing their LET. [1]
It is clear that radiation mitigation strategies must be tailored to the specific damage mechanism. For TID effects, aluminum shielding provides effective dose reduction for electrons and low-energy protons, but lack of improvement beyond 100 mils make part selection and radiation- hardened design increasingly important for high-dose missions. [1] Some traditional methods for implementing radiation hardening include thinner gate oxides, increased drive currents, and guard rings for latchup prevention. [6] The use of wide bandgap semiconductors, particularly gallium nitride (GaN) and silicon carbide (SiC), is also promising due to their limited threshold voltage shifts in response to TID effects, as well as their low displacement damage susceptibility and high SEE resistance. [3,7]
For addressing SEE in general, circuit-level mitigation is needed since shielding is ineffective. The primary defense against bit-flips and damage is making switches physically larger, but this trade-off means circuits cannot be made arbitrarily small because doing so enables radiation mistakes from individual particle strikes. Additionally, error detection and correction (EDAC) can detect and correct bit errors, and triple modular redundancy (TMR) provides SEU protection at an area and power cost through parallel operation with majority voting. Memory scrubbing can also be done to periodically read and rewrite contents to correct accumulated errors. [1]
Another design approach that considers both the need and cost associated with radiation hardening is to group systems by criticality. Vital systems such as reactor control, life support, or attitude control must use radiation-hardened electronics since single upsets can cause mission failure. Mission-important systems like communications and data processing can use mixtures of rad-hard and commercial parts with appropriate error correction, while on-critical systems may use COTS components with comprehensive testing and mitigation. [1]
Quantitatively understanding radiation effects is essential for ensuring reliable spacecraft electronics design, especially as feature sizes continue to shrink and device densities increase, worsening radiation sensitivity. TID effects result in threshold voltage shifts, displacement damage produces significant solar cell power degradation, and SEE from individual cosmic rays causes bit-flip and destruction events. Successful microelectronics design requires rigorous testing, appropriate hardening and shielding, circuit-level mitigation, and consideration of different semiconductor materials.
© Zara Jahan. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. W. Howard and D. M. Hardage, "Spacecraft Environments Interactions: Space Radiation and Its Effects on Electronic Systems," U.S. National Aeronautics and Space Administration, NASA/TP-199-209373, July 1999.
[2] J. R. Srour and J. M. McGarrity, "Radiation Effects on Microelectronics in Space," IEEE 90114, Proc. IEEE 76, 1443 (1988).
[3] H. Wu et al., "Total Ionizing Dose Effects on the Threshold Voltage of GaN Cascode Devices," Micromachines 14, 1832 (2023).
[4] P. E. Dodd and L. W. Massengill, "Basic Mechanisms and Modeling of Single-Event Upset in Digital Microelectronics," IEEE 1208578, IEEE Trans. Nucl. Sci. 50, 583 (2003).
[5] C. Marques et al., "A Methodology to Estimate Single-Event Effects Induced by Low-Energy Protons," Eng 5, 319 (2024).
[6] Y. Ren et al., "Overview on Radiation Damage Effects and Protection Techniques in Microelectronic Devices," Sci. Technol. Nucl. Install. 2024, 3616902 (2024).
[7] V. Sandeep, J. C. Provin, and S. A. Kumar, "Ionizing Radiation Defects and Reliability of Gallium Nitride-Based III-V Semiconductor Devices: A Comprehensive Review," Microelectron. Reliab. 159, 115445 (2024).