S.L. Kosier

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. >

Dose‐rate effects on radiation‐induced bipolar junction transistor gain degradation

Analysis of radiation damage in modern NPN bipolar transistors at various dose rates is performed with a recently introduced charge separation method and pisces simulations. The charge separation method is verified with measurements on metal‐oxide‐semiconductor capacitors. Gain degradation is more pronounced at lower dose rates. The charge separation technique reveals that depletion‐region spreading and effective recombination velocity are both greater for devices irradiated at lower dose rates.

Excess collector current due to an oxide-trapped-charge-induced emitter in irradiated NPN BJT's

Excess collector current in irradiated NPN BJTs is linked to an oxide-trapped-charge-induced inversion layer acting as an additional emitter. Excess collector current is modeled by interpreting the inversion layer as an extension of the emitter. >

Minimizing gain degradation in lateral PNP bipolar junction transistors using gate control

Gain degradation in lateral PNP bipolar junction transistors is minimized by controlling the potential of a gate terminal deposited above the active base region. Gate biases that deplete the base during radiation exposure establish electric fields in the base oxide that limit the generation of oxide defects. Conversely, gate biases that accumulate the base during device operation suppress gain degradation by decreasing the probability of carrier recombination with interface states. The results presented in this paper suggest that, for gate controlled LPNP transistors designed for operation in radiation environments, a dynamic control of the gate potential improves the transistors radiation hardness and extend its operating life.

Synergetic effects of radiation stress and hot-carrier stress on the current gain of npn bipolar junction transistors

The combined effects of ionizing radiation and hot-carrier stress on the current gain of npn bipolar junction transistors were investigated. The analysis was carried out experimentally by examining the consequences of interchanging the order in which the two stress types were applied to identical transistors which were stressed to various levels of damage. The results indicate that the hot-carrier response of the transistor is improved by radiation damage, whereas hot-carrier damage has little effect on subsequent radiation stress. Characterization of the temporal progression of hot-carrier effects revealed that hot-carrier stress acts initially to reduce excess base current and improve current gain in irradiated transistors. PISCES simulations show that the magnitude of the peak electric-field within the emitter-base depletion region is reduced significantly by net positive oxide charges induced by radiation. The interaction of the two stress types is explained in a qualitative model based on the probability of hot-carrier injection determined by radiation damage and on the neutralization and compensation of radiation-induced positive oxide charges by injected electrons. The results imply that a bound on damage due to the combined stress types is achieved when hot-carrier stress precedes any irradiation. >

The effects of ionizing radiation on the breakdown voltage of p-channel power MOSFETs

The effects of ionizing radiation on the breakdown voltage of p-channel power MOSFETs were examined through two-dimension simulation. The breakdown-voltage performance of p-channel power MOSFETs was found to be very different from that of corresponding n-channel power MOSFETs. In p-channel devices, simulation showed breakdown-voltage enhancement for low values of positive oxide-trapped charge, N/sub ot/, whereas for high values of N/sub ot/, the breakdown voltage may or may not continue to increase, and may actually decrease in some topologies. For comparison, in n-channel devices, increases in N/sub ot/ always cause breakdown-voltage degradation. The uncertainties stem from the interaction of the depletion region of the device (which is a function of its termination method) with its isolation technology, making it difficult to predict breakdown voltage for large N/sub ot/. However, insights gained through analysis of depletion-region spreading in p-channel devices suggest a termination/isolation scheme, the VLD-FRR, that will enhance p-channel device reliability in radiation environments. >

Modeling BJT radiation response with non-uniform energy distributions of interface traps

Radiation-induced oxide defects that degrade electrical characteristics of BJTs can be measured with the use of gated diodes. The buildup of defects and their effect on device radiation response are modeled with computer simulation.

Excess collector current due to an oxide-trapped-charge-induced emitter in irradiated NPN BJTs

Excess collector current in irradiated NPN BJTs is linked to an oxide-trapped-charge-induced inversion layer acting as an additional emitter. Excess collector current is modeled by interpreting the inversion layer as an extension of the emitter.