Linear Bipolar Total Ionizing Dose Testing

For the past seven years, the radiation test community has been investigating the Enhanced Low Dose Rate Effect on linear bipolar product. The original discovery was made on the new technologies utilizing poly-emitters but now includes all bipolar technologies, both old and new. These effects are observed when the linear bipolar product is operating in a low dose rate environment that is similar to space, or when irradiated at a low dose rate, e.g., less than 1 rad/sec.

At low dose rate, the total dose response for linear bipolar is affected by two radiation induced effects:

(a) Time Dependent Effects (TDE)
(b) True Dose Rate Effect (TDRE)

TDE can be segmented into two mechanisms: positive oxide trapped charges (high dose rate effect) and interface trap charges (low dose rate effect). Positive oxide trapped charges affect standby current, DC gain, slew rate, open-loop gain, etc. As these are generated as a result of high dose rate irradiation, positive oxide trapped charges are not generally an issue in space since they are generated as a result of high dose rate irradiation. However, interface trap charge generation is a major concern since it is a result of low dose rate irradiation. This detractor causes reduction in DC gain, creates parasitic leakage paths, input offset current and input bias current failures as well input offset voltage failures, etc.

At this time, National Semiconductor’s Radiation Engineering Group is investigating the use of a high temperature, biased, one-week anneal to determine if low dose rate Time Dependent Effects (namely, interface trap generation) exist when using a high dose rate irradiation technique. Essentially, the +100C 168-hour biased anneal removes the positive oxide trapped charges leaving only the interface traps as the sole degradation mechanism. If parametric failures occur after this anneal, then users must be concerned about the system design based upon space application, space orbit and shielding. If functional failure occurs at any time, the product must be rejected.

The second cause of linear bipolar failure at low dose rate irradiation is the True Dose Rate Effect. Degradation resulting from this effect only occurs while the device is in the actual low dose rate environment. The low dose rate mechanism is the lack of space charge generation within the particular oxide area of concern. These oxides are formed directly above the base-emitter junctions of the transistors and are relatively radiation soft to low dose rate effects. The more sensitive transistors to low dose rate radiation are the lateral PNP and substrate PNP. Since there is a lesser amount of space charge developed at low dose rate, the net positive trapped oxide charge is greater at low dose rate due to a reduction in electron trapping caused by the small space change. The increased net positive charge at low dose rate results in increase in surface recombination velocity at the emitter-base junction, causing a further increase in Ib and a reduction in the gain of the device.

At this time there is no approved method for simulating the True Dose Rate effect for linear bipolar devices. An approach that is being considered is to perform high dose rate irradiation at an elevated temperature for simulation of the Enhanced Low Dose Rate Effect. Other methodologies are also being investigated but no final conclusions have been decided.

[Info provided Septemeber '97 by: Mike Maher - Engineer/RHA Program Manager]