Excess Base Current Research Articles

Physical mechanisms contributing to enhanced bipolar gain degradation at low dose rates

We have performed capacitance-voltage (C-V) and thermally-stimulated-current (TSC) measurements on non-radiation-hard MOS capacitors simulating screen oxides of modern bipolar technologies. For O-V irradiation at /spl sim/25/spl deg/C, the net trapped-positive-charge density (N/sub ox/) inferred from midgap C-V shifts is /spl sim/25-40% greater for low-dose-rate ( 100 rad(SiO/sub 2/)/s) exposure. Device modeling shows that such a difference in screen-oxide N/sub ox/ is enough to account for the enhanced low-rate gain degradation often observed in bipolar devices, due to the /spl sim/exp(N/sub ox//sup 2/) dependence of the excess base current. At the higher rates, TSC measurements reveal a /spl sim/10% decrease in trapped-hole density over low rates. Also, at high rates, up to /spl sim/2.5-times as many trapped holes are compensated by electrons in border traps than at low rates for these devices and irradiation conditions. Both the reduction in trapped-hole density and increased charge compensation reduce the high-rate midgap shift. A physical model is developed which suggests that both effects are caused by time-dependent space charge in the bulk of these soft oxides associated with slowly transporting and/or metastably trapped holes (e.g. in E/sub /spl delta//' centers). On the basis of this model, bipolar transistors and screen-oxide capacitors were irradiated at 60/spl deg/C at 200 rad(SiO/sub 2/)/s in a successful effort to match low-rate damage. These surprising results provide insight into enhanced low-rate bipolar gain degradation and suggest potentially promising new approaches to bipolar and BiCMOS hardness assurance for space applications. >

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. >

GaAs HBT RELIABILITY

This paper reviews recent studies on the reliability of GaAs heterojunction bipolar transistors (HBTs), mainly carbon-doped-base AlGaAs/GaAs HBTs, and discusses degradation modes in these devices. The carbon-doped-base AlGaAs/GaAs HBTs generally have more stable current-voltage characteristics than beryllium-doped-base ones. However, even carbon-doped-base HBTs show a decrease in the current gain due to an anomalous increase in the base current that occurs under high-current stress. The estimated ohmic contacts of these HBTs show that their stability is high enough under actual operating conditions. Bias stress testing has shown that two factors dominate the reliability of carbon-doped-base AlGaAs/GaAs HBTs: the material and the passivation. HBTs with an AlGaAs surface passivation layer on the extrinsic base region exhibit a longer life than those without one. The base current ideality factor of short-lived HBTs increases to larger than 2 during bias stress testing, suggesting that the origin of the excess base current cannot be simply explained by a space-charge recombination process. The reported activation energies are distributed around 0.5 eV, which is almost the same as those for AlGaAs/GaAs laser diodes. Cross-sectional TEM analysis has revealed that current-induced defects are formed in the base layer. The indium-incorporated-base HBTs show a median life of longer than 15,000 hours at a junction temperature of 215°C with a collector current density of 50 kA/cm 2, implying that GaAs HBTs have sufficient reliability for very-high-speed communication systems.

Charge separation for bipolar transistors

The effect of net positive oxide trapped charge and surface recombination velocity on excess base current in BJTs (bipolar junction transistors) is identified. Although the interaction of these two radiation-induced defects is physically complex, simple approaches for estimating these quantities from measured BJT characteristics are presented. The oxide charge is estimated using a transition voltage in the plot of excess base current vs. emitter bias. Two approaches for quantifying the effects of surface recombination velocity are described. The first measures surface recombination directly using a gated diode. The second estimates its effects using an intercept current that is easily obtained from the BJT itself. The results are compared to two-dimensional simulations and measurements made on test structures. The techniques are simple to implement and provide insight into the mechanisms and magnitudes of radiation-induced damage in BJTs.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Reliability imposed design aspects of submicrometer polysilicon-emitter bipolar transistors

The device reliability of narrow poly-emitter bipolar transistors with very shallow junctions is studied experimentally. The excess base current due to nonuniform poly doping, which is typically seen in such devices, is found not to accelerate device degradation. The lower doping at the emitter edge due to known perimeter depletion and emitter plug effects (PPEs) leads to a reduced increase in base current per perimeter length with stress. The results show that bipolar device scaling can probably be pursued to a point where PPEs start to appear, and that lateral emitter grading is effective in improving the device reliability. >

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. >

Technique for thermal stabilization of transistors

An alternative scheme to transistor thermal stabilization by emitter resistor ballasting is proposed. The technique involves a resistor with a negative temperature coefficient of resistance connected across the emitter-base junction which bypasses excess base current that tends to be drawn due to local transistor heating.