Qualifying silicon-germanium electronics for harsh radiation environments

The objective of this thesis is to investigate the robustness of Silicon-Germanium Heterojunction Bipolar Transistors (SiGe HBTs) to radiation-induced damage. The work described in this document delves into both total ionizing dose (TID) and single-event effect (SEE) mechanisms. Background information is provided for the general operation of SiGe HBTs and basic radiation effects (generic and specifically for SiGe HBTs). Four unique investigations are covered in this work: the first two focus on TID effects for high dose environments and to investigate enhanced-low-dose-rate-sensitivity, and the latter two studies investigate advances in hardening SiGe HBT profiles and methods to conduct SEE experiments using pulsed-lasers in place of highly energetic ionized particles.Ph.D

Heavy Ion Current Transients in SiGe HBTs

Time-resolved ion beam induced charge reveals heavy ion response of IBM 5AM SiGe HBT: a) Position correlation[ b) Unique response for different bias schemes; c) Similarities to TPA pulsed-laser data. Heavy ion broad-beam transients provide more realistic device response: a) Feedback using microbeam data; b) Overcome issues of LET and ion range with microbeam. Both micro- and broad-beam data sets yield valuable input for TCAD simulations. Uncover detailed mechanisms for SiGe HBTs and other devices fabricated on lightly-doped substrates

Silicon-germanium BiCMOS and silicon-on-insulator CMOS analog circuits for extreme environment applications

Extreme environments pose major obstacles for electronics in the form of extremely wide temperature ranges and hazardous radiation. The most common mitigation procedures involve extensive shielding and temperature control or complete displacement from the environment with high costs in weight, power, volume, and performance. There has been a shift away from these solutions and towards distributed, in-environment electronic systems. However, for this methodology to be viable, the requirements of heavy radiation shielding and temperature control have to be lessened or eliminated. This work gained new understanding of the best practices in analog circuit design for extreme environments. Major accomplishments included the over-temperature -180 C to +120 C and radiation validation of the SiGe Remote Electronics Unit, a first of its kind, 16 channel, sensor interface for unshielded operation in the Lunar environment, the design of two wide-temperature (-180 C to +120 C), total-ionizing-dose hardened, wireline transceivers for the Lunar environment, the low-frequency-noise characterization of a second-generation BiCMOS process from 300 K down to 90 K, the explanation of the physical mechanisms behind the single-event transient response of cascode structures in a 45 nm, SOI, radio-frequency, CMOS technology, the analysis of the single-event transient response of differential structures in a 32 nm, SOI, RF, CMOS technology, and the prediction of scaling trends of single-event effects in SOI CMOS technologies.Ph.D

Design and characterization of BiCMOS mixed-signal circuits and devices for extreme environment applications

State-of-the-art SiGe BiCMOS technologies leverage the maturity of deep-submicron silicon CMOS processing with bandgap-engineered SiGe HBTs in a single platform that is suitable for a wide variety of high performance and highly-integrated applications (e.g., system-on-chip (SOC), system-in-package (SiP)). Due to their bandgap-engineered base, SiGe HBTs are also naturally suited for cryogenic electronics and have the potential to replace the costly de facto technologies of choice (e.g., Gallium-Arsenide (GaAs) and Indium-Phosphide (InP)) in many cryogenic applications such as radio astronomy. This work investigates the response of mixed-signal circuits (both RF and analog circuits) when operating in extreme environments, in particular, at cryogenic temperatures and in radiation-rich environments. The ultimate goal of this work is to attempt to fill the existing gap in knowledge on the cryogenic and radiation response (both single event transients (SETs) and total ionization dose (TID)) of specific RF and analog circuit blocks (i.e., RF switches and voltage references). The design approach for different RF switch topologies and voltage references circuits are presented. Standalone Field Effect Transistors (FET) and SiGe HBTs test structures were also characterized and the results are provided to aid in the analysis and understanding of the underlying mechanisms that impact the circuits' response. Radiation mitigation strategies to counterbalance the damaging effects are investigated. A comprehensive study on the impact of cryogenic temperatures on the RF linearity of SiGe HBTs fabricated in a new 4th-generation, 90 nm SiGe BiCMOS technology is also presented.Ph.D

Silicon-Germanium Bipolar Technology for Enabling Cold-Capable, Radiation-Tolerant Electronics for Spacecraft

The objective of this research is to investigate the effect that low temperature has on the radiation effects on advanced silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) for the application of deep-space exploration missions that are specifically classified as extreme low-temperature and highly radiation active environments, such as Jovian exploration missions. We designed a unique experimental testbed that enabled DC and RF measurements to be taken in situ at various temperature and radiation points. The experiment was conducted at the Jet Propulsion Laboratory (JPL) where low-temperature and radiation environments can be mimicked. We showed that while there is some radiation damage in base leakage current on the single transistor level, there is no observed damage due to total-ionizing dose (TID) in noise figure, linearity, or gain for a 2.4 GHz low-noise amplifier (LNA) that was irradiated at an ambient temperature of about 100 K up to 1 Mrad (Si). Furthermore, we confirmed the notion that radiation at lower temperatures yields less damage and showed why it is important to separate temperature-dependent performance with measurable radiation damage at different temperatures. We also took a simulation approach to determine whether single-event transients (SETs) get worse as a result of the device being in low ambient temperatures. For a single standalone device, the results show that the transient gets larger in magnitude but shorter in duration. However, the circuit results show that the effects of an SET get worse in some cases with low temperatures such as in the context of LNAs, but can also get better in other cases such as current-mode logic (CML) D-flip-flops.M.S

Improving the Readout of Semiconducting Qubits

Semiconducting qubits are a promising platform for quantum computers. In particular, silicon spin qubits have made a number of advancements recently including long coherence times, high-fidelity single-qubit gates, two-qubit gates, and high-fidelity readout. However, all operations likely require improvement in fidelity and speed, if possible, to realize a quantum computer. Readout fidelity and speed, in general, are limited by circuit challenges centered on extracting low signal from a device in a dilution refrigerator connected to room temperature amplifiers by long coaxial cables with relatively high capacitance. Readout fidelity specifically is limited by the time it takes to reliably distinguish qubit states relative to the characteristic decay time of the excited state, T1. This dissertation explores the use of heterojunction bipolar transistor (HBT) circuits to amplify the readout signal of silicon spin qubits at cryogenic temperatures. The cryogenic amplification approach has numerous advantages including low implementation overhead, low power relative to the available cooling power, and high signal gain at the mixing chamber stage leading to around a factor of ten speedup in readout time for a similar signal-to-noise ratio. The faster readout time generally increases fidelity, since it is much faster than the T1 time. Two HBT amplification circuits have been designed and characterized. One design is a low-power, base-current biased configuration with non-linear gain (CB-HBT), and the second is a linear-gain, AC-coupled configuration (AC-HBT). They can operate at powers of 1 and 10 μW, respectfully, and not significantly heat electrons. The noise spectral density referred to the input for both circuits is around 15 to 30 fA/√Hz, which is low compared to previous cases such as the dual-stage, AC-coupled HEMT circuit at ~ 70 fA/√Hz. Both circuits achieve charge sensitivity between 300 and 400 μe/√Hz, which approaches the best alternatives (e.g., RF-SET at ~ 140 μe/√Hz) but with much less implementation overhead. For the single-shot latched charge readout performed, both circuits achieve high-fidelity readout in times \u3c 10 μs with bit error rates \u3c 10-3, which is a great improvement over previous work at \u3e 70 μs. The readout speed-up in principle also reduces the production of errors due to excited state relaxation by a factor of ~ 10. All of these results are possible with relatively simple, low-power transistor circuits which can be mounted close to the qubit device at the mixing chamber stage of the dilution refrigerator

Optical properties and carrier transport in Si/ Si1-xGex nanostructures

Defect-free epitaxial growth of Ge on a ~4 % lattice-mismatched single-crystal Si substrate is achieved using reduced dimension nanoscale heterostructures, where efficient structural relaxation might occur due to an enhanced adatom migrations and high surface-to-volume ratio. For development of novel electronic or optical devices based on these novel structures, understanding of their electrical and optical properties is crucial. This study explores the optical properties and carrier transport in two different types of Si/Si 1-xGe xnanostructures: Ge nanowires (NWs) forming nanoscale heterojunctions with Si substrates and multilayers of SiGe clusters embedded into a Si matrix. Micron-long Ge nanowires grown on n+ (100) and p+ (111) Si substrates exhibit distinct current-voltage (I-V) characteristics, which are explained using a model of an abrupt and defect-free Ge NW/Si substrate heterointerfaces. From the measurements on Si/SiGe cluster superlattices (SLs) where SiGe clusters are vertically aligned due to strain-induced self-organization, a series of step-like structures are observed in the I-V characteristics obtained using mapping of electrical characteristics by a scanning-tunneling-microscope (STM). The origin of this step-like I-V behavior and distinct peaks in differential conductivity is suggestively explained by a conduction energy band configuration that considers phonon-assisted carrier transfer at the interfaces between Si separating layers and Ge-rich SiGe clusters. Optical measurements show that the photoluminescence (PL) intensity in Si/SiGe cluster SL exhibits excitation dependent thermal quenching under continuous wave (CW) excitation by an Ar+ laser with the excitation intensities of 0.1-10 W/cm2. A novel mechanism, where nonradiative carrier recombination is controlled by a competition between hole tunneling and hole thermionic emission over the valence band energy barriers at Si/SiGe heterointerfaces is suggested. In addition, the carrier transitions at a high photoexcitation using Q-switched pulsed Nd:YAG laser are found to be mediated by Auger processes with high energy Auger holes repopulating Si barriers in valence energy band. An efficient light emission due to radiative recombination of electron-hole condensates in Si separating layers of the Si/SiGe clusters SL is observed with a lifetime approaching 10 -8 s. These results, attributed to a mechanism similar to Auger fountain in quantum wells with type-II energy band alignment, indicate a new route toward light-emitters monolithically integrated into CMOS environment