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This investigation examines the physical mechanisms that can increase the variability of both p-n junctions and silicon–germanium heterojunction bipolar transistors (SiGe HBTs) at cryogenic temperatures. The important operative mechanisms responsible for device parameter variability include bandgap narrowing due to heavy doping, mechanical stress, and the Ge profile. The impact of direct tunneling on cryogenic parameter variability in SiGe HBTs is also examined. Measurement results are compared with TCAD simulations to provide additional insights, and possible mitigation methods are discussed.

We investigate high current stress mechanisms and demonstrate how Auger hot carriers can damage both oxide interfaces and polysilicon regions of the emitter and base. A new current gain enhancement (CGE) effect is proposed, which involves hot-carrier damage to the polysilicon emitter and extrinsic base leading to the degradation of the associated minority carrier mobilities. We demonstrate the different CGE mechanisms in SiGe HBTs under forward and inverse modes of operation. The hot-carrier damage responsible for CGE at high injection is explored in depth with the help of TCAD simulations. Evidence for this effect has been gathered with good statistical significance from various stress conditions, various technologies, complimentary (NPN + PNP) devices, and from dc and ac measurements. The new polysilicon degradation mechanism proposed in this paper has been generalized and is important for accurately modeling the changes in base resistance and current gain ( $\beta$ ) at high injection, where circuits are typically biased to extract maximum device performance.

The implementation of a “superjunction” collector design in a silicon–germanium heterojunction bipolar transistor technology is explored for enhancing breakdown performance. The superjunction collector is formed via the placement of a series of alternating the p/xn-doped layers in the collector-base space charge region and is used to reduce avalanche generation leading to breakdown. An overview of the physics underlying superjunction collector operation is presented, together with TCAD simulations, and a parameterization methodology is developed to explore the limits of the superjunction collector performance. Measured data demonstrate the limitations explored in simulation.

The doping profile of silicon-germanium heterojunction bipolar transistors (SiGe HBTs) is modified to enhance inverse-mode (IM) device operation. Device improvements are presented in this paper, along with the impact the alterations have on the radiation effects response. This investigation represents the first published occurrence of a radiation-hardening-by-process approach in a SiGe HBT technology. Results show that improving IM performance can degrade the radiation tolerance of the structure. Total ionizing dose and single-event transient (SET) results are provided along with an analysis that utilizes TCAD simulation. An additional profile modification using an implanted vertical superjunction is included in this paper to expand upon how nonradiation specific device modifications can impact SETs.

Total ionizing dose (TID) effects are evaluated for a high-voltage (>30 V) complementary SiGe on SOI technology. Devices are irradiated with 10-keV X-rays at doses up to 5 Mrad(SiO2). The results depend strongly on collector-to-emitter bias, in both forward- and inverse-mode. An anomalous reduction in current gain at high injection in forward-mode operation is observed at doses >500 krad(SiO2). Calibrated 2-D TCAD simulations suggest that this high injection phenomenon is primarily due to interface traps near the STI/Si interface, which is exhibited as a collector resistance increase in the forward Gummel characteristics. Additionally, a strong collector doping dependence is observed, which indicates that this is primarily driven by the thick and lightly doped collector used in this technology. These results illustrate, that high concentrations of interface traps at the STI can have a strong impact on the forward-mode TID response of SiGe HBTs.

This paper examines the fundamental reliability differences between n-p-n and p-n-p SiGe HBTs. The device profile, hot carrier transport, and oxide interface differences between the two device types are explored in detail as they relate to device reliability. After careful analysis under identical electrical stress conditions for n-p-n and p-n-p, the differences in activation energies for the damage of the oxide interfaces of the two devices is determined to be the primary cause for accelerated degradation seen in p-n-p SiGe HBTs. An analytical model has been adapted for simulating these aging differences between p-n-p and n-p-n devices. This paper has significant implications for predicting the degradation of complementary SiGe HBTs and even engineering future generations with well-matched n-p-n and p-n-p device-level reliability.

We present measurement results of commercially-available 90-nm SiGe HBTs at 300 K, 78 K, and 7 K. The data reveal increased variability of SiGe HBTs at 78 K and 7 K compared to that at 300 K. The variation of collector current, base current, and DC current gain (β) increase from 5% at 300 K to 50% at 7 K around the average measured values. Transconductance (g m ) variation increases from 0.6% at 300 K to 5% at 7 K. Peak f T variation increases from 2.6% to 7.1%, while the f T at low injection increases from 1.8% at 300 K to 24.9% at 7 K. Increased variability is observed at lower injection levels at 78 K and 7 K and DC parameters tend to have more variation than RF parameters. The implications of increased variability for circuit designs supporting emerging cryogenic applications such as quantum computing are discussed.

The capability of inverse-mode (IM) silicon- germanium (SiGe) heterojunction bipolar transistors (HBTs) for the mitigation of single-event transients (SETs) under large-signal operation was investigated in an RF down-conversion single- balanced mixer using a through-wafer, two-photon absorption pulsed-laser beam experiment and TCAD heavy-ion simulations. The IM SiGe HBTs replace conventional forward-mode (FM) SiGe HBTs in the differential pair, which provides full current steering for frequency mixing operation. Under steady-state conditions, the IM SiGe HBT differential pair exhibits smaller transient peaks with shorter durations compared to the FM SiGe HBTs. In addition, under the injection of a local oscillator (LO) signal with large swing, the IM SiGe HBTs show faster recovery (50% reduction in the best case) from the impact of SETs. In the frequency domain, it is observed that IM SiGe HBTs produce less distortion at the output for an intermediate frequency below 1 GHz. Based on the performance comparison between FM and IM SiGe HBT down-conversion mixers, system design guidelines to compensate the noise figure degradation associated with using IM SiGe HBTs are discussed.

The effects of negative feedback, both external and internal, on single event transients (SETs) in SiGe HBT analog circuits are investigated. In order to examine internal negative feedback effects, basic common-emitter NPN current mirrors, with and without emitter degeneration resistors, are utilized. A Wilson current mirror and a Wilson mirror with its intrinsic external feedback removed are used to study external negative feedback effects under the influence of laser-induced single events. The measurement data clearly show notable improvements in SET response that can be made by employing both internal and external negative feedback. The peak transient in the output current is reduced, and the settling time upon a laser strike is shortened significantly by negative feedback. All four investigated current mirrors were fabricated with IBM 8HP 130 nm SiGe BiCMOS technology.

We investigate and compare the hot-carrier degradation of SiGe HBTs under both traditional mixed-mode electrical stress conditions and high-current electrical stress conditions using measured stress data and an in-depth analysis of the underlying degradation mechanisms. While large electric fields are the driving force in mixed-mode hot-carrier degradation, the Auger recombination process is shown to be the hot-carrier source under high-current stress conditions. Auger hot-carrier degradation shows a positive temperature dependence, unlike mixed-mode degradation, due to the temperature dependence of Auger recombination and its energy distribution function. We also use calibrated TCAD simulations to explain an unexpected stress threshold behavior that occurs due to the formation of a potential well in the neutral base region, and to explore a field-compression effect at the collector/subcollector junction that contributes to trap formation at the shallow trench isolation oxide interface.

A SiGe RF low-noise amplifier (LNA) with built-in tolerance to single-event transients is proposed. The LNA utilizes an inverse-mode SiGe HBT for the common-base transistor in a cascode core. This new cascode configuration exhibits reduced transient peaks and shorter transient durations compared to the conventional cascode one. The improved SET response was verified with through-wafer two-photon absorption pulsed-laser experiments and supported via mixed-mode TCAD simulations. In addition, analysis of the RF performance and the reliability issues associated with the inverse-mode operation further suggests this new cascode structure can be a strong contender for space-based applications. The LNA with the inverse-mode-based cascode core was fabricated in a 130 nm SiGe BiCMOS platform and has similar RF performance to the conventional schematic-based LNA, further validating the proposed approach.

High-current pulsed stress measurements are performed on SiGe HBTs to characterize the damage behavior and create a comprehensive physics-based TCAD damage model for Auger-induced hot-carrier damage. The Auger hot-carrier generation is decoupled from classical mixed-mode damage and annealing on the output plane by using pulsed stress conditions to modulate the self-heating within the device under stress. The physics of high-current degradation is analyzed, and a temperature dependent degradation model is presented. This model is the first of its kind in both the CMOS and bipolar communities and solves a significant portion of the puzzle for predictive modeling of SiGe HBT safe-operating-area (SOA) and reliability.

We study mixed-mode stress degradation in SiGe HBTs using a novel physical TCAD model in which the processes of hot carrier generation within the semiconductor, carrier propagation to the oxide interface, and formation of interface traps are directly modeled. Transient degradation simulations using a calibrated 2-D SiGe HBT model correlate well with measured data. With this novel simulation tool, we investigate the bias dependence and location of interface traps and show that secondary holes produced by impact ionization are the dominant carrier to damage the emitter-base (EB) spacer oxide interface, confirming previously reported results. We also compare in detail trap formation at the EB spacer and shallow-trench-isolation (STI) oxide interfaces as a function of time and stress condition. At the STI oxide interfaces, we find that hot electrons and holes each dominate trap formation in different regions, and the hot carriers that reach the STI predominately originate outside of the selectively implanted collector, revealing the important role played by dopant diffusion from the extrinsic base of quasi-self-aligned SiGe HBTs.

Profile optimization techniques are investigated for silicon-germanium heterojunction bipolar transistors (SiGe HBTs) intended for inverse-mode (IM) operation. IM device operation, also known as inverse active, involves electrically swapping the emitter and collector terminals and has been shown to improve the radiation tolerance of SiGe HBTs to single event transients (SETs). Multiple profile design variations are explored and trade-offs are analyzed with support of TCAD simulation. Modest design variations show marked improvement on IM performance while having minor impact on forward-mode (normal active) operation.