A.H. Johnston

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. >

Radiation effects on advanced flash memories

Radiation tests of advanced flash memories including, multi-level flash technology, are compared with results from previous generations. Total dose failure levels are comparable to or lower than those of older technologies, but are likely still caused by degradation of the internal charge pump. Small numbers of read errors were observed during single event tests of the multi-level devices that appear to be caused by shifts in the sense amplifier detection levels or cell threshold shifts rather than loss of electrons off the floating gate.

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.

The influence of VLSI technology evolution on radiation-induced latchup in space systems

Changes in technology and device scaling have generally increased the sensitivity of VLSI devices to latchup from single interactions of heavy particles in space. This paper discusses latchup mechanisms, comparing latchup from heavy particles in space with electrically induced latchup, which has been more widely studied. The effects of technology changes and device scaling on latchup susceptibility are discussed as well. Test methods and the interpretation of latchup results are also included, along with predictions of the effects of device evolution and scaling on latchup susceptibility in space.

Single-event upset in flash memories

Single-event upset was investigated in high-density flash memories from two different manufacturers. Many types of functional abnormalities can be introduced in these devices by heavy-ions because of their complex internal architecture. Changes in the stored memory contents sometimes occurred, even when devices were irradiated in a read mode with the internal charge pump inactive. For one device technology, unusually high currents were observed during post-irradiation cycling that were high enough to cause catastrophic failure.

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.

Charge generation and collection in p-n junctions excited with pulsed infrared lasers

The author examines optical absorption processes for applications of infrared lasers to the simulation of single-particle effects in silicon and GaAs, which require that the laser is focused to a small area on the device surface. The resulting charge generation is compared with charge generation from heavy ions. Charge funneling is reduced in silicon structures because of the lower charge density unless the LET (linear energy transfer) is above a threshold value. In both materials, the effective LET of a laser is inherently nonlinear because of nonlinear absorption at high intensities. These factors limit quantitative comparisons between lasers and heavy ions, and are increasingly important as devices are scaled to smaller dimensions. >

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.

Radiation effects in micro-electromechanical systems (MEMS): RF relays

GaAs micro-electromechanical RF relays fabricated by surface micromachining techniques were characterized for their response to total ionizing dose. Microrelays with two different geometries were studied. For one geometry, changes in switch actuation voltage at moderate dose levels were observed. For an alternative geometry, no change in actuation voltage was observed. A mechanism for dielectric charge trapping and its effect on the electrostatic force is proposed.

Breakdown of gate oxides during irradiation with heavy ions

Breakdown of thin gate oxides from heavy ions is investigated using capacitor test structures. Soft breakdown was observed for 45 /spl Aring/ oxides, but not for 75 /spl Aring/ oxides. Lower critical fields were observed when experiments were done with high fluences during each successive step. This implies that oxide defects play an important role in breakdown from heavy ions and that breakdown occurs more readily when an ion strike occurs close to a defect site. Critical fields for 75 /spl Aring/ oxides are low enough to allow gate rupture to occur at normal supply voltages for ions with high LET.