B.G. Rax

Total dose effects in conventional bipolar transistors and linear integrated circuits

Total dose damage is investigated for discrete bipolar transistors and linear integrated circuits that are fabricated with older processing technologies, but are frequently used in space applications. The Kirk effect limits the current density of discrete transistors with high collector breakdown voltage, increasing their sensitivity to ionizing radiation because they must operate low injection levels. Bias conditions during irradiation had different effects on discrete and integrated circuit transistors: discrete devices were strongly dependent on bias conditions, whereas damage in the linear ICs was nearly the same with or without bias. There were also large differences in the response of these devices at low dose rates. None of the discrete transistors exhibited enhanced damage at low dose rates, whereas substantially more damage occurred in the linear devices under low dose rate conditions, particularly for parameters that rely directly on p-n-p transistors. The threshold for dose rate effects in p-n-p transistors was about 0.01 rad(Si)/s, which is approximately two orders of magnitude lower than the corresponding threshold for n-p-n transistors in integrated circuits. >

Enhanced damage in linear bipolar integrated circuits at low dose rate

Enhanced damage at low dose rates was investigated for several different types of linear integrated circuits that were fabricated with conventional junction isolation. Although both npn and pnp transistors exhibit increased damage at low dose rate, the effect is far greater for substrate and lateral pnp transistors from these technologies. The saturation level of damage at high doses was also found to be far greater under low dose rate conditions than at high dose rates. A model for this behavior was developed that is consistent with earlier studies of MOS field oxides under low-field conditions, and accounts for the increased enhanced damage in pnp transistors.

Enhanced damage in bipolar devices at low dose rates: effects at very low dose rates

The effect of very low dose rate irradiation is investigated for several linear bipolar devices that are sensitive to enhanced low dose-rate damage, including one device with super-/spl beta/ input transistors. New results are included at 0.001 and 0.002 rad(Si)/s. Irradiations at elevated temperature at high dose rate are compared with room temperature irradiation at very low dose rate. Possible mechanisms for enhanced damage are discussed.

Total dose and proton damage in optocouplers

Radiation damage from gamma rays and protons is investigated for two types of optocouplers with different physical configurations. Far more damage occurs from protons because of displacement damage, which reduces the photoresponse of the phototransistor and causes severe degradation in LED light output for one of the two device types. The other device type was far more resistant to radiation, primarily because it used a shorter wavelength LED that was relatively unaffected by protons.

Proton degradation of light-emitting diodes

Proton degradation is investigated for several types of light-emitting diodes with wavelengths in the near infrared region. Several basic light-emitting diode (LED) technologies are compared, including homojunction and double-heterojunction devices. Homojunction LEDs fabricated with amphoteric dopants are far more sensitive to displacement damage than double-heterojunction LEDs, and are strongly affected by injection-enhanced annealing. Unit-to-unit variability remains an important issue for all LED technologies. For technologies, degradation of the forward voltage characteristics appears to be more significant than degradation of light output.

Proton damage in advanced laser diodes

Proton radiation damage in laser diodes is investigated for several types of laser diodes with wavelengths from 650 to 1550 nm. Key parameters include slope efficiency, threshold current, and the transition characteristics between laser-emitting diode (LED) and laser operation. Some of the devices exhibited nonlinear relationships between threshold current and proton fluence. All of the lasers, including vertical-cavity surface-emitting lasers, were strongly affected by recombination-enhanced annealing, in contrast to double-heterojunction LEDs, which are only slightly affected by annealing. Analysis of laser characteristics after irradiation showed that the main effect of radiation damage is an increase in bulk recombination that increases loss within the laser cavity.

Proton damage effects in linear integrated circuits

Proton tests of linear integrated circuits have identified devices where significantly more damage occurs at equivalent total dose levels with protons than with gamma rays. The difference is attributed to displacement damage, and it can be important for hardened devices as well as for unhardened technologies. Proton testing may be required for applications of circuits that use substrate and lateral pnp transistors in critical circuit functions where protons comprise a significant fraction of the space environment.

Degradation of precision reference devices in space environments

The degradation of precision reference devices is investigated to determine the relative importance of ionization and displacement damage. The results are compared with theoretical calculations of a basic bandgap reference circuit. Several of the device types were degraded severely at 20 krad(Si), with about the same degradation as that predicted for the basic bandgap reference circuit. One very high precision device with an internal heater performed far better than any of the other devices in the study.

Proton damage in linear and digital optocouplers

Fundamental differences in design influence the way that linear and digital optocouplers are degraded by radiation. Linear optocouplers are more affected by current drive conditions because the detector operates in a high-injection region when the LED produces normal light output, and do not have the extra operating margin that is inherent in digital optocouplers. Although LED degradation is often the dominant degradation mechanism in space environments, degradation of optocouplers with improved LEDs is limited by photoresponse degradation. Phototransistor gain has a relatively minor effect except at very high radiation levels.

Continuing evaluation of bipolar linear devices for total dose dependency and ELDRS effects

We present results of continuing efforts to evaluate total dose bias dependency and ELDRS effects in bipolar linear microcircuits. Several devices were evaluated, each exhibiting moderate to significant bias and/or dose rate dependency.