Ronald L. Pease

Response of advanced bipolar processes to ionizing radiation

Ionizing radiation induced gain degradation in microcircuit bipolar polysilicon and crystalline emitter transistors is investigated. In this work, /sup 60/Co irradiation testing was performed on bipolar test structures. The effects of collector bias, dose rate, and anneal temperature are discussed. Major differences in the radiation response of polysilicon emitter transistors are demonstrated as a function of dose rate. The worst-case gain degradation occurs at the lowest dose rate complicating hardness assurance testing procedures. The dose rate and anneal data suggest that MIL-STD-883B Test Method 1019.4 is non-conservative for polysilicon emitter transistors, which show enhanced radiation hardness over the crystalline emitter transistors. >

Subbandgap laser-induced single event effects: carrier generation via two-photon absorption

Carrier generation based on subbandgap two-photon absorption is demonstrated and shown to be a viable alternative to the conventional single-photon excitation approach in laser-induced single event effects. The two-photon approach exhibits characteristics distinct from those of single-photon excitation, and may be advantageous for a range of single-event effect investigations. The charge track produced by two-photon absorption more closely resembles that of heavy-ion irradiation and, because the photon energy is subbandgap, backside injection through bulk silicon wafers is straightforward and three-dimensional mapping is possible.

Trends in the total-dose response of modern bipolar transistors

The excess base current in an irradiated BJT increases superlinearly with total dose at low-total-dose levels. In this regime, the excess base current depends on the particular charge-trapping properties of the oxide that covers the emitter-base junction. The device response is dose-rate-, irradiation-bias-, and technology-dependent in this regime. However, once a critical amount of charge has accumulated in the oxide, the excess base current saturates at a value that is independent of how the charge accumulated. This saturated excess base current depends on the device layout, bulk lifetime in the base region, and the measurement bias. In addition to providing important insight into the physics of bipolar-transistor total-dose response, these results have significant circuit-level implications. For example, in some circuits, the transistor gain that corresponds to the saturated excess base current is sufficient to allow reliable circuit operation. For cases in which the saturated value of current gain is acceptable, and where other circuit elements permit such over-testing, this can greatly simplify hardness assurance for space applications. >

Charge separation for bipolar transistors

The effects of the midgap-level interface trap density and net oxide charge on the total-dose gain degradation of a bipolar transistor are separately identified. The superlinear dose dependence of the excess base current is explained.

Physically based comparison of hot-carrier-induced and ionizing-radiation-induced degradation in BJTs

A physically based comparison between hot-carrier and ionizing radiation stress in BJTs is presented. Although both types of stress lead to qualitatively similar changes in the current gain of the device, the physical mechanisms responsible for the degradation are quite different. In the case of hot-carrier stress the damage is localized near the emitter-base junction, which causes the excess base current to have an ideality factor of two. For ionizing radiation stress, the damage occurs along all oxide-silicon interfaces, which causes the excess base current to have an ideality factor between one and two for low total doses of ionizing radiation, but an ideality factor of two for large total doses. The different physical mechanisms that apply for each type of stress imply that improvement in resistance to one type of stress does not necessarily imply improvement in resistance to the other type of stress. Based on the physical model, implications for correlating and comparing hot-carrier-induced and ionizing-radiation-induced damage are discussed. >

Three-dimensional mapping of single-event effects using two photon absorption

Carrier generation based on subbandgap two-photon absorption is used to perform three-dimensional mapping of the single-event transient response of the LM124 operational amplifier. Three classes of single-event-induced transients are observed for the input transistor Q20. A combination of experiment and transistor level modeling is used to assign the different classes of measured transients to charge collection across specific junctions. The large-amplitude, positive-going transients cannot be assigned to a single junction, and are identified with a collector-substrate photocurrent.

Hardness-assurance and testing issues for bipolar/BiCMOS devices

The dose-rate dependence of bipolar current-gain degradation is mapped over a wide range of dose rates. This dependence is very different from analogous MOSFET curves. Annealing experiments following irradiation show negligible change in base current at room temperature, but significant recovery at temperatures of 100 degrees C and above. In contrast to what is observed in MOSFETs, irradiation and annealing tests cannot be used to predict the low-dose-rate response of bipolar devices. A comparison of X-ray-induced and /sup 60/Co gamma-ray-induced gain degradation for bipolar transistors is reported. The role of the emitter bias during irradiation is also examined. Preliminary field-oxide capacitor studies suggest that the mechanism for the dose-rate effect may be related to charge yield in the basic surface oxides. Recommendations for hardness-assurance testing of bipolar devices include testing at dose rates below 10 rad(SiO/sub 2/)/s and applying safety factors to estimate the space-environment response. >

Comparison of ionizing-radiation-induced gain degradation in lateral, substrate, and vertical PNP BJTs

A comparison is presented of ionizing-radiation-induced gain degradation in lateral, substrate, and vertical PNPs. The dose-rate dependence of current gain degradation in lateral PNP BJTs is even stronger than the dependence previously reported for NPN BJTs. Various mechanisms are presented and their relative significance for gain degradation in the lateral, substrate, and vertical PNPs is discussed. A detailed comparison of the lateral and substrate PNP devices is given. The specific lateral and substrate devices considered here are fabricated in the same process and possess identical emitters. Even though these devices have identical emitters and undergo the same processing steps, the lateral devices degrade significantly more than the substrate devices.

Modeling ionizing radiation induced gain degradation of the lateral PNP bipolar junction transistor

Ionizing-radiation-induced gain degradation in lateral PNP bipolar junction transistors is due to an increase in base current as a result of recombination at the surface of the device. A qualitative model is presented which identifies the physical mechanism responsible for excess base current. The increase in surface recombination velocity due to interface traps results in an increase in excess base current and the positive oxide charge moderates the increase in excess base current and changes the slope of the current-voltage characteristics. Analytical and empirical models have been developed to quantitatively describe the excess base current response to ionizing radiation. It is shown that the surface recombination velocity dominates the excess base current response to total dose.

An improved standard total dose test for CMOS space electronics

The postirradiation response of hardened and commercial CMOS devices is investigated as a function of total dose, dose rate, and annealing time and temperature. Cobalt-60 irradiation at approximately=200 rad(SiO/sub 2/)/s followed by a one-week 100 degrees C biased anneal and testing is shown to be an effective screen of hardened devices for space use. However, a similar screen and single-point test performed after Co-60 irradiation and elevated-temperature anneal cannot be generally defined for commercial devices. In the absence of detailed knowledge about device and circuit radiation response, a two-point standard test is proposed to ensure space survivability of CMOS circuits; a Co-60 irradiation and test to screen against oxide-trapped-charge-related failures, and an additional rebound test to screen against interface-trap-related failures. Testing implications for bipolar technologies are also discussed. >