Uppili S. Raghunathan

Predictive Physics-Based TCAD Modeling of the Mixed-Mode Degradation Mechanism in SiGe HBTs

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.

Design of Radiation-Hardened RF Low-Noise Amplifiers Using Inverse-Mode SiGe HBTs

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.

Large-Signal Reliability Analysis of SiGe HBT Cascode Driver Amplifiers

This paper presents the results of an investigation of the steady-state safe operating conditions for large-signal silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) circuits. By calculating capacitive currents within the intrinsic transistor, avalanche inducing currents through the transistor junctions are isolated and then compared with dc instability points established through simulation and measurement. In addition, calibrated technology computer-aided design simulations are used to provide further insight into the differences between RF and dc operation and stress conditions. The ability to swing the terminals of a SiGe HBT beyond the static I-V conditions coincident with catastrophic breakdown is explained. Furthermore, hot-carrier effects are also compared from multiple perspectives, with supporting data taken from fully realized X-band and C-band cascode driver amplifiers. This analysis provides microwave circuit designers with the framework necessary to better understand the full-voltage-swing potential of a given SiGe HBT technology and the resultant hot carrier damage under RF operation.

Bias- and Temperature-Dependent Accumulated Stress Modeling of Mixed-Mode Damage in SiGe HBTs

This paper uses a physics-based TCAD degradation model to examine the accumulated stress damage of SiGe HBTs under pseudodynamic mixed-mode stress as a function of both electrical stress bias and temperature. The temperature dependence of mixed-mode stress damage is fully explored, beginning with impact-ionization calibration, and then by identifying and calibrating the dependence of scattering length and hydrogen diffusion parameters of the degradation model. After calibrating the model across electrical bias and temperature, the effectiveness and limitations of accumulated stress damage while varying electrical bias and while varying temperature are identified, and the implications of this aging model for circuit designers are discussed.

An Investigation of the Use of Inverse-Mode SiGe HBTs as Switching Pairs for SET-Mitigated RF Mixers

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.

TCAD modeling of accumulated damage during time-dependent mixed-mode stress

We study the accumulated degradation of SiGe HBTs under time-dependent mixed-mode stress using a new physics-based TCAD degradation model that simulates hot carrier generation and propagation to oxide interfaces, resulting in trap formation. We calibrate the avalanche generation and also do a multipoint calibration of damage on the I-V output plane to accurately predict the accumulated stress damage for a single device over multiple bias points. Looking at the region of the output plane dominated by trap formation, we show that accumulation of traps can be path-independent as long as trap availability is not limiting. We demonstrate this with good correlation between simulation and measurement.

Optimization of SiGe HBT RF Switches for Single-Event Transient Mitigation

Single-event transient (SET)-hardened SiGe HBT RF single-pole single-throw (SPST) switches were designed and fabricated for the first time. TCAD-based heavy-ion simulations and two-photon absorption (TPA) laser-induced beam experiments were used to optimize the switch core configuration for SET mitigation. Among different configurations, the reverse-connected series and shunt device core, where both emitter terminals are connected to the output, exhibits the smallest transient peaks and shortest durations at the output terminal of the switch. Based on this finding, the design considerations for maximizing the RF performance of SiGe HBT SPST RF switches are discussed. In addition, a comparison of the SET response and RF performance of CMOS (nFET) SPST and SiGe HBT SPST switches provides additional information on the trade-offs in the SET mitigation strategy and potential RF capabilities.

A Comparison of Field and Current-Driven Hot-Carrier Reliability in NPN SiGe HBTs

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.

Physical Differences in Hot Carrier Degradation of Oxide Interfaces in Complementary (n-p-n+p-n-p) 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.

On the reliability of SiGe HBT cascode driver amplifiers

This paper investigates the RF reliability of SiGe HBT cascode driver amplifiers. By subtracting capacitive currents internal to the common-base device from its collector waveform, a more accurate depiction of electrical stress in the I-V plane is achieved, and from this revised load line, RF stress data is better correlated to DC stress data. This novel analysis technique provides a framework for designers to simulate the effects of RF stress using DC data from both TCAD models and measurements, allowing for optimized performance in high power and high frequency applications where reliability concerns often lead to under-utilization of the transistors capabilities.