Electronics Guide

Space Radiation Environment

Space presents a complex radiation environment consisting of multiple sources:

• Galactic cosmic rays: High-energy particles originating outside our solar system, consisting primarily of protons and heavier ions with energies up to GeV levels
• Solar particle events: Sporadic bursts of protons and heavy ions ejected during solar flares and coronal mass ejections
• Trapped radiation belts: Energetic protons and electrons trapped in Earth's magnetic field, particularly intense in the Van Allen belts
• Secondary particles: Particles generated when primary radiation interacts with spacecraft shielding or other materials

Orbital parameters significantly influence the radiation exposure. Low Earth orbit missions experience lower total dose but transit the South Atlantic Anomaly, while geosynchronous orbits face higher trapped radiation exposure.

Terrestrial Radiation Environments

Ground-based systems may also require radiation hardening in certain applications:

• Nuclear power plants: Gamma radiation and neutron flux near reactor cores and during maintenance operations
• Particle accelerators: Intense localized radiation fields near beam lines and interaction points
• Medical facilities: Radiation therapy equipment and diagnostic imaging areas
• High-altitude aviation: Increased cosmic ray flux at flight altitudes
• Atmospheric neutrons: Secondary neutrons from cosmic ray interactions can affect sensitive electronics even at sea level

Radiation Units and Dosimetry

Quantifying radiation exposure requires standardized units and measurement techniques:

• Rad (radiation absorbed dose): Unit of absorbed energy, equal to 100 ergs per gram of material; often specified as rad(Si) for silicon
• Gray (Gy): SI unit equal to 1 joule per kilogram, equivalent to 100 rad
• Linear energy transfer (LET): Energy deposited per unit path length, typically expressed in MeV-cm2/mg
• Fluence: Number of particles passing through a unit area, particles per cm2
• Dose rate: Rate of energy deposition, rad per second or rad per hour

Mission radiation requirements typically specify total ionizing dose for the mission duration, single-event upset rate thresholds, and maximum acceptable LET for latch-up immunity.

Oxide Charge Trapping

When ionizing radiation passes through silicon dioxide, it creates electron-hole pairs. While electrons are quickly swept away by electric fields, holes move slowly and can become trapped at the Si-SiO2 interface or within the oxide itself:

• Oxide trapped charge: Positive charge trapped in the bulk oxide causes threshold voltage shifts in MOS transistors
• Interface traps: Defects at the Si-SiO2 interface create energy states within the bandgap, affecting mobility and subthreshold characteristics
• Field oxide effects: Thick field oxides can develop parasitic leakage paths between adjacent transistors

The rate of charge buildup depends on oxide thickness, electric field strength, temperature, and radiation dose rate. Modern thin gate oxides are inherently more radiation tolerant than older, thicker oxide processes.

Threshold Voltage Shifts

Trapped positive charge in the gate oxide shifts transistor threshold voltages:

• NMOS transistors: Threshold voltage decreases (shifts negative), increasing off-state leakage and potentially causing the transistor to remain conducting even with zero gate voltage
• PMOS transistors: Threshold voltage becomes more negative (larger magnitude), increasing drive requirements and potentially degrading circuit speed
• Depletion mode operation: Severe threshold shifts can convert enhancement-mode devices to depletion mode

Interface traps cause additional threshold shifts that depend on the gate bias during irradiation and can partially compensate or add to oxide charge effects.

Leakage Current Increases

TID induces multiple leakage mechanisms that increase power consumption and degrade circuit performance:

• Subthreshold leakage: Reduced threshold voltage increases leakage current exponentially
• Field oxide leakage: Parasitic transistors formed by trapped charge in field oxides create paths between source and drain regions of adjacent transistors
• Edge leakage: Charge buildup at transistor edges creates leakage paths around the intended channel
• Junction leakage: Interface damage increases generation-recombination currents in reverse-biased junctions

Analog Parameter Degradation

TID affects key analog parameters through multiple mechanisms:

• Transconductance reduction: Interface traps reduce carrier mobility, decreasing gain
• Increased noise: Interface traps generate additional 1/f noise, degrading signal-to-noise ratio
• Matching degradation: Non-uniform threshold shifts degrade transistor matching critical for precision analog circuits
• Offset drift: Differential pairs experience offset shifts as devices degrade differently
• Bandwidth changes: Altered device characteristics affect frequency response

Dose Rate Effects

The rate at which radiation is delivered affects the final damage state:

• Enhanced low dose rate sensitivity (ELDRS): Some bipolar devices exhibit greater degradation at low dose rates typical of space environments compared to high-rate laboratory testing
• Annealing competition: At low dose rates, interface trap formation may outpace oxide charge annealing, resulting in worse degradation
• Test implications: Laboratory testing at high dose rates may underestimate degradation in actual low-rate environments for susceptible devices

Single-Event Transients

Single-event transients (SET) are temporary perturbations in circuit behavior caused by charge collection at sensitive nodes:

• Mechanism: An ionizing particle creates a charge track along its path through silicon, which is collected by nearby junctions
• Current pulse: The collected charge produces a current pulse that can propagate through the circuit
• Analog response: SET in analog circuits may cause output glitches, oscillations, or temporary parameter shifts
• Duration: Transients typically last nanoseconds to microseconds depending on circuit bandwidth

SET are particularly problematic in analog circuits because they can be amplified by subsequent gain stages and may trigger incorrect system responses.

Single-Event Upset Mechanisms

While traditionally associated with digital memory, single-event upset (SEU) mechanisms also affect analog circuits:

• Comparator switching: SET can cause comparators to produce incorrect output states
• Sample-and-hold corruption: Charge injection can corrupt stored analog values
• ADC errors: SEU in successive approximation registers or SET on internal nodes cause conversion errors
• DAC glitches: Bit errors in digital control produce incorrect analog outputs

Single-Event Latch-up

Single-event latch-up (SEL) is a potentially destructive condition triggered by energetic particles:

• PNPN structure activation: Parasitic thyristor structures inherent in CMOS bulk processes can be triggered by charge deposition
• Regenerative current path: Once triggered, the thyristor creates a low-impedance path from power supply to ground
• Potential damage: High currents can cause thermal damage unless power is quickly removed
• Recovery: Power cycling is required to clear the latch-up condition

SEL immunity is often specified by a threshold LET below which latch-up will not occur and a maximum operating temperature.

Single-Event Burnout and Gate Rupture

Power devices are susceptible to destructive single-event effects:

• Single-event burnout (SEB): Heavy ion strikes can trigger destructive avalanche breakdown in power MOSFETs and BJTs operating at high voltage
• Single-event gate rupture (SEGR): Charge deposition can cause localized breakdown of the gate oxide, permanently damaging the device
• Derating requirements: Power devices must be operated well below rated voltage to maintain acceptable SEB/SEGR immunity

Analog Circuit Susceptibility

Several factors make analog circuits particularly susceptible to SEE:

• High-impedance nodes: Small charge deposits create large voltage transients
• Wide bandwidth: Fast circuits do not attenuate rapid SET pulses
• Single-ended configurations: Cannot reject common-mode charge injection
• Sensitive thresholds: Comparators and ADCs have defined decision points easily crossed by transients
• Lack of error correction: Unlike digital systems, analog circuits have no inherent error detection or correction

Process Selection and Hardening

The choice of semiconductor process significantly influences radiation tolerance:

• Thin gate oxides: Modern deep submicron processes with thin gate oxides are inherently more TID tolerant due to reduced charge trapping volume
• Silicon-on-insulator (SOI): Buried oxide isolates devices, eliminating latch-up paths and reducing charge collection volume
• Hardened processes: Dedicated rad-hard foundries use special oxide growth, implant, and anneal processes to minimize charge trapping
• Silicon-germanium (SiGe): Offers excellent TID tolerance and is often used for high-performance rad-hard applications
• Gallium arsenide: Inherently radiation tolerant due to higher bandgap and reduced oxide charge trapping issues

Enclosed Transistor Layouts

Layout modifications can significantly improve TID tolerance:

• Enclosed layout transistors (ELT): Gate completely surrounds the drain, eliminating edge leakage paths
• Annular gates: Ring-shaped gate structures prevent parasitic edge channels
• H-gate layouts: Modified structures that balance area efficiency with edge leakage prevention
• Area penalty: Enclosed layouts typically require 2-4 times more area than standard layouts

Guard Rings and Isolation

Physical isolation structures improve both TID tolerance and SEL immunity:

• N+ guard rings: Surrounding NMOS devices collect minority carriers before they reach active regions
• P+ guard rings: Isolate PMOS devices and prevent lateral carrier injection
• Deep trench isolation: Physical barrier extending through the substrate provides superior isolation
• Triple-well processes: Additional deep N-well isolates PMOS devices from the substrate

Biasing and Operating Point Considerations

Careful selection of operating conditions improves radiation tolerance:

• Threshold margin: Design circuits to tolerate expected threshold voltage shifts without functional failure
• Current source headroom: Allow margin for increased leakage currents consuming bias current
• Temperature compensation: Radiation effects are temperature dependent; ensure operation over combined radiation and temperature extremes
• Supply voltage margin: Degraded device characteristics may require adjusted supply voltages

Feedback and Compensation Techniques

Circuit-level techniques can compensate for radiation-induced parameter drift:

• Negative feedback: High loop gain reduces sensitivity to device parameter variations
• Matched device pairs: Differential circuits cancel common-mode radiation effects
• Current-mode circuits: Less sensitive to threshold shifts than voltage-mode designs
• Calibration and trimming: Periodic calibration can compensate for gradual TID drift

Transistor Layout for TID Tolerance

Specific layout techniques address oxide charge-related leakage:

• Enclosed drain structures: Complete enclosure of the drain by the gate eliminates bird's beak edge leakage paths
• Minimum channel length avoidance: Longer channels reduce sensitivity to threshold shifts
• Wide channel devices: Distribute width over multiple fingers to reduce individual finger sensitivity
• Consistent orientation: All transistors oriented identically for predictable radiation response

Single-Event Effect Mitigation Layout

Layout strategies to reduce SEE sensitivity:

• Reduced node capacitance: Smaller junction areas reduce charge collection cross-section
• Distributed structures: Spreading critical nodes over larger areas reduces the probability of a single strike affecting multiple nodes
• Physical separation: Redundant elements spaced apart to prevent single particles from affecting multiple copies
• Minimized sensitive volume: Reduce the physical extent of charge collection regions

Latch-up Prevention Layout

Layout rules specifically targeting SEL immunity:

• Guard ring requirements: Continuous, properly contacted guard rings around all devices
• Substrate contacts: Frequent substrate ties reduce substrate resistance and latch-up holding current
• N-well contacts: Abundant well taps reduce well resistance and parasitic thyristor gain
• Device spacing: Increased spacing between NMOS and PMOS reduces parasitic thyristor coupling
• I/O layout: Special attention to high-current I/O structures that can trigger latch-up

Metal and Interconnect Considerations

Interconnect design affects SEE response and reliability:

• RC filtering: Distributed resistance and capacitance naturally attenuates fast transients
• Critical node shielding: Metal shields over sensitive nodes reduce direct charge deposition
• Redundant interconnects: Multiple parallel paths protect against localized damage
• Current density limits: Radiation can exacerbate electromigration; conservative current densities improve reliability

Basic TMR Architecture

The fundamental TMR concept applied to analog circuits:

• Three parallel channels: Three identical analog circuits process the same input signal independently
• Analog voter: A voting circuit selects or averages the outputs, rejecting the erroneous channel
• Error masking: A single channel experiencing a SET produces an incorrect output that is overridden by the two correct channels
• Overhead: TMR requires at least three times the circuit area, power, and cost

Analog Voting Circuits

Several approaches implement analog voting:

• Median selection: Selects the middle value of three inputs, automatically rejecting high or low outliers
• Majority voting with comparators: Comparators determine which two outputs agree, switching to select the majority
• Weighted averaging: Combines outputs with weights based on consistency with other channels
• Current averaging: Current-mode outputs naturally average when connected together

The voter circuit itself must be hardened against radiation effects, as a voter failure can defeat the protection provided by redundancy.

Physical Implementation Requirements

Effective TMR requires careful physical design:

• Physical separation: Channels must be far enough apart that a single particle cannot affect multiple channels simultaneously
• Minimum spacing: Typically several tens of micrometers minimum separation
• Independent power domains: Separate power distribution prevents common-mode failures from supply transients
• Separate bias generation: Independent bias circuits for each channel
• Voter placement: Voter circuitry must also be hardened and potentially distributed

Alternative Redundancy Approaches

Variations on TMR address specific requirements:

• Dual redundancy with comparison: Two channels with mismatch detection; lower overhead but requires system-level response to detected errors
• N-modular redundancy: More than three channels for higher reliability requirements
• Temporal redundancy: Single circuit with multiple samples over time; effective only if SET duration is shorter than sampling interval
• Information redundancy: Error detection codes applied to digitized analog values

Limitations of TMR for Analog Circuits

TMR has specific challenges in analog applications:

• Continuous output variations: Unlike digital circuits with discrete states, analog outputs vary continuously, complicating error detection
• Threshold selection: Determining what constitutes agreement between channels requires careful threshold setting
• Matching requirements: Channel outputs must match well enough that normal variations are not interpreted as errors
• Bandwidth limitations: Voter circuits may limit overall bandwidth
• Voter susceptibility: The voter represents a potential single point of failure

RADFET Dosimeters

Radiation-sensitive field-effect transistors (RADFETs) are purpose-designed dosimeters:

• Thick gate oxide: Specially grown thick oxide maximizes charge trapping and sensitivity
• Threshold shift measurement: Accumulated dose causes measurable threshold voltage shift
• Read-out circuitry: Simple circuits measure threshold voltage change over time
• Sensitivity ranges: Different RADFET designs cover dose ranges from millirads to megarads
• Bias dependence: Gate bias during irradiation affects sensitivity and must be controlled

PIN Diode Dosimeters

PIN diodes provide real-time dose rate measurement:

• Displacement damage: Radiation creates recombination centers in the intrinsic region
• Forward voltage shift: Increased recombination raises forward voltage at constant current
• Current-mode operation: Reverse-biased operation provides dose-rate-dependent current
• Wide dynamic range: Different diode sizes and configurations cover various dose rates

Integrated Dosimetry Systems

Complete dosimetry systems integrate sensing, signal conditioning, and data handling:

• Multi-range capability: Automatic range selection covers wide dose ranges
• Temperature compensation: Built-in correction for temperature-dependent sensor response
• Non-volatile storage: Accumulated dose data preserved through power cycles
• Telemetry interface: Standard interfaces for system integration

On-Chip Dose Monitoring

Integrating dosimetry capability directly on functional circuits provides real-time health monitoring:

• Sentinel transistors: Test structures that degrade predictably with dose
• Parameter monitoring: Tracking threshold voltage, leakage, or speed of reference structures
• Predictive maintenance: Enables component replacement before functional failure
• Mission planning: Real-time dose data supports dynamic mission management

Thermal Annealing Mechanisms

Temperature-activated processes allow damage recovery:

• Oxide charge annealing: Trapped holes can escape or be neutralized by electron tunneling, reducing threshold shifts
• Interface trap formation and annealing: Interface traps can both form and anneal at elevated temperatures
• Activation energy: Different trap types have different thermal activation energies, affecting annealing rates
• Time-temperature relationship: Higher temperatures accelerate annealing; equivalent recovery can occur at lower temperatures over longer times

Bias-Dependent Annealing

Electric fields during and after irradiation affect damage evolution:

• Field-enhanced annealing: Electric fields can accelerate charge neutralization through tunneling
• Bias conditions during irradiation: The electric field present during exposure affects initial charge distribution
• Post-irradiation bias: Continued operation can either enhance or reduce recovery depending on conditions
• Rebound effects: Some processes show initial recovery followed by degradation as interface traps continue forming

Practical Implications

Annealing affects both testing and operational strategies:

• Test conditions: Annealing between irradiation steps can affect test results; standards specify time and temperature limits
• Worst-case prediction: Must consider both the maximum damage immediately after irradiation and long-term annealed condition
• Operational duty cycle: Systems that are periodically powered down may experience different total degradation than continuously operating systems
• Accelerated testing correlation: High dose rate testing must account for reduced annealing time compared to actual low-rate environments

Controlled Annealing in System Design

Some system architectures deliberately incorporate annealing:

• Periodic heating: Intentional temperature cycling to anneal accumulated damage
• Bias cycling: Alternating bias conditions to promote recovery
• Mission planning: Scheduling non-critical periods for annealing recovery
• Component rotation: Using redundant channels alternately to allow individual annealing

Component Selection Hierarchy

Space programs typically define component quality grades:

• Radiation-hardened by design (RHBD): Circuits designed using standard processes with radiation-tolerant design techniques
• Radiation-hardened by process (RHBP): Specialized foundry processes with inherent radiation tolerance
• Commercial off-the-shelf (COTS): Standard commercial parts, sometimes acceptable for lower-dose missions with appropriate derating
• Lot-specific qualification: Testing specific production lots to verify radiation tolerance

Qualification Testing Requirements

Comprehensive testing validates radiation tolerance:

• Total dose testing: Exposure to gamma or X-ray sources to mission dose requirements with margin
• Dose rate testing: Low dose rate testing for ELDRS-susceptible technologies
• Single-event testing: Heavy ion and proton exposure to characterize SEE response
• Displacement damage testing: Proton and neutron exposure for susceptible devices
• Combined environment testing: Testing under simultaneous thermal, radiation, and other stress conditions

Design Margin Philosophy

Space-qualified designs incorporate substantial margins:

• Radiation design margin (RDM): Typically 2x or greater margin between tested dose and specification
• Parameter derating: Operating devices well below rated limits to improve reliability
• Worst-case analysis: Circuit must meet requirements at end-of-life with worst-case parameter drift
• Temperature margin: Operation over extended temperature range with margin

Parts Qualification Process

Individual component qualification follows systematic procedures:

• Construction analysis: Physical examination of device construction and materials
• Screening: Burn-in and testing to eliminate infant mortality failures
• Qualification lot testing: Radiation characterization of representative samples
• Lot acceptance testing: Verification testing of flight lots
• Traceability: Complete documentation of part history and test results

Design Review and Analysis

Formal design reviews verify radiation hardness:

• Radiation hardness assurance: Documented analysis showing design meets radiation requirements
• Parts list review: Verification that all parts have appropriate radiation qualification
• SEE rate prediction: Calculation of expected error rates based on particle environment and device cross-sections
• System-level analysis: Evaluation of SEE impact on system functionality and safety

Manufacturing and Process Controls

Production of space-qualified electronics requires strict controls:

• Controlled baseline: Locked design files, mask sets, and process recipes
• Process monitors: In-line testing to verify process consistency
• ESD controls: Electrostatic discharge protection throughout manufacturing
• Cleanliness requirements: Contamination control to prevent reliability degradation
• Documentation: Complete records supporting traceability and quality verification

Shielding Considerations

Physical shielding can reduce radiation exposure:

• Mass shielding: Bulk material attenuates particle flux; effectiveness varies with particle type and energy
• Secondary particle generation: High-energy particles interacting with shielding can create additional secondary particles
• Optimal shielding thickness: For some radiation types, there is an optimal thickness; beyond this, secondary particle production increases dose
• Spot shielding: Local shielding around sensitive components rather than entire system
• Material selection: Low-Z materials (aluminum) for electrons; high-Z materials for gamma; polyethylene for neutrons

Functional Partitioning

System architecture can isolate and manage radiation effects:

• Critical function protection: Highest hardening levels for mission-critical functions
• Graceful degradation: Non-critical functions can accept higher error rates or be disabled
• Isolation: Prevent SEE in one subsystem from propagating to others
• Watchdog monitoring: Independent monitors detect anomalies and trigger recovery

Error Detection and Recovery

System-level error handling complements component hardening:

• Reasonableness checking: Verify analog outputs fall within expected ranges
• Redundancy voting: Use multiple sensors or channels with majority voting
• Automatic reset: Detect anomalous conditions and reset affected circuits
• Power cycling: Capability to power-cycle subsystems to clear latch-up or other conditions
• Safe mode: Defined low-power, known-good state for recovery from anomalies

Power System Design

Power systems require special attention in radiation environments:

• Latch-up protection: Current limiting or crowbar circuits to protect against latch-up damage
• Transient filtering: Prevent SET from propagating through power distribution
• Independent supplies: Separate power domains allow isolated recovery
• Power cycling capability: Remote ability to power-cycle subsystems

Total Dose Testing Facilities

Sources for total ionizing dose testing:

• Cobalt-60 gamma sources: Standard source for TID testing; 1.17 and 1.33 MeV gamma rays
• X-ray sources: Provide controlled, localized exposure; useful for IC-level testing
• Linear accelerators: Electron beams converted to X-rays for high dose rate testing
• Dose rate control: Ability to vary dose rate for ELDRS characterization

Single-Event Testing Facilities

Facilities for SEE characterization:

• Heavy ion accelerators: Cyclotrons and linear accelerators provide various ion species and LET ranges
• Proton facilities: Lower energy protons for direct ionization; higher energy for nuclear reactions
• Pulsed laser testing: Simulates charge deposition for screening and failure analysis
• Neutron sources: For testing atmospheric neutron effects

Test Procedures and Standards

Standardized methods ensure consistent, comparable results:

• MIL-STD-883, Method 1019: Standard total dose test method for ICs
• ESCC 22900: European Space Components Coordination TID test standard
• JEDEC JESD57: Heavy ion test procedures for SEE
• Dose rate requirements: Low dose rate testing per MIL-STD-883, Method 1019.8 for ELDRS-susceptible parts

Data Analysis and Modeling

Interpreting test data and predicting mission performance:

• Cross-section vs. LET curves: Characterize SEE susceptibility as function of particle LET
• Rate prediction tools: CREME96, SPENVIS, and other tools calculate upset rates from test data and mission environment
• TID response modeling: Predict parameter drift over mission lifetime
• Statistical analysis: Account for part-to-part variation in radiation response