Publications

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We present a comprehensive physics investigation of electrothermal effects in III-V heterojunction bipolar transistors (HBTs) via extensive Technology Computer Aided Design (TCAD) simulation and modeling. We show for the first time that the negative differential resistances of the common-emitter output responses in InGaP/GaAs HBTs are caused not only by the well-known carrier mobility reduction, but more importantly also by the increased base-To-emitter hole back injection, as the device temperature increases from self-heating. Both self-heating and impact ionization can cause fly-backs in the output responses under constant base-emitter voltages. We find that the fly-back behavior is due to competing processes of carrier recombination and self-heating or impact ionization induced carrier generation. These findings will allow us to understand and potentially improve the safe operating areas and circuit compact models of InGaP/GaAs HBTs.

We present a comprehensive physics investigation of electrothermal effects in III-V heterojunction bipolar transistors (HBTs) via extensive Technology Computer Aided Design (TCAD) simulation and modeling. We show for the first time that the negative differential resistances of the common-emitter output responses in InGaP/GaAs HBTs are caused not only by the well-known carrier mobility reduction, but more importantly also by the increased base-To-emitter hole back injection, as the device temperature increases from self-heating. Both self-heating and impact ionization can cause fly-backs in the output responses under constant base-emitter voltages. We find that the fly-back behavior is due to competing processes of carrier recombination and self-heating or impact ionization induced carrier generation. These findings will allow us to understand and potentially improve the safe operating areas and circuit compact models of InGaP/GaAs HBTs.

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We present an analytic band-to-trap tunneling model based on the open boundary scattering approach. The new model has three major advantages: (i) It includes not only the well-known electric field effect, but more importantly, the effect of heterojunction band offset. This feature allows us to simulate both electric field and band offset enhanced carrier recombination near a heterojunction in heterostructures. (ii) Its analytic form enables straightforward implementation into a parallel Technology Computer Aided Design device and circuit simulators. (iii) The developed method can be used for any potentials which can be approximated to a good degree such that the Schrödinger equation with open boundary conditions results in piecewise analytic wave functions. Simulation results of an InGaP/GaAs heterojunction bipolar transistor (HBT) reveal that the proposed model predicts significantly increased base currents, because the tunneling of holes in the base to traps in the emitter is greatly enhanced by the emitter-base band offset. This finding, which is not captured by existing band-to-trap tunneling models, is consistent with the experimental observation for an InGaP/GaAs HBT after neutron irradiation.

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We present an analytic band-to-trap tunneling model developed using the open boundary scattering approach. The new model explicitly includes the effect of heterojunction band offset, in addition to the well known electric field effect. Its analytic form enables straightforward implementation into TCAD device and circuit simulators. The model is capable of simulating both electric field and band offset enhanced carrier recombination due to the band-to-trap tunneling in the depletion region near a heterojunction. Simulation results of an InGaP/GaAs heterojunction bipolar transistor reveal that the proposed model predicts significantly increased base currents, because the hole-to-trap tunneling from the base to the emitter is greatly enhanced by the emitter base heterojunction band offset. The results compare favorably with experimental observations. The developed method can be applied to all one dimensional potentials which can be approximated to a good degree such that the approximated potentials lead to piecewise analytic wave functions with open boundary conditions.

We present an analytic band-to-trap tunneling model developed using the open boundary scattering approach. The new model explicitly includes the effect of heterojunction band offset, in addition to the well known electric field effect. Its analytic form enables straightforward implementation into TCAD device and circuit simulators. The model is capable of simulating both electric field and band offset enhanced carrier recombination due to the band-to-trap tunneling in the depletion region near a heterojunction. Simulation results of an InGaP/GaAs heterojunction bipolar transistor reveal that the proposed model predicts significantly increased base currents, because the hole-to-trap tunneling from the base to the emitter is greatly enhanced by the emitter base heterojunction band offset. The results compare favorably with experimental observations. The developed method can be applied to all one dimensional potentials which can be approximated to a good degree such that the approximated potentials lead to piecewise analytic wave functions with open boundary conditions.

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In this paper, we present an efficient band-to-trap tunneling model based on the Schenk approach, in which an analytic density-of-states (DOS) model is developed based on the open boundary scattering method. The new model explicitly includes the effect of heterojunction band offset, in addition to the well-known field effect. Its analytic form enables straightforward implementation into TCAD device simulators. It is applicable to all one-dimensional potentials, which can be approximated to a good degree such that the approximated potentials lead to piecewise analytic wave functions with open boundary conditions. The model allows for simulating both the electric-field-enhanced and band-offset-enhanced carrier recombination due to the band-to-trap tunneling near the heterojunction in a heterojunction bipolar transistor (HBT). Simulation results of an InGaP/GaAs/GaAs NPN HBT show that the proposed model predicts significantly increased base currents, due to the hole-to-trap tunneling enhanced by the emitter-base junction band offset. Finally, the results compare favorably with experimental observation.

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This paper describes our work over the past few years to use tools from quantum chemistry to describe electronic structure of nanoelectronic devices. These devices, dubbed "artificial atoms", comprise a few electrons, con ned by semiconductor heterostructures, impurities, and patterned electrodes, and are of intense interest due to potential applications in quantum information processing, quantum sensing, and extreme-scale classical logic. We detail two approaches we have employed: nite-element and Gaussian basis sets, exploring the interesting complications that arise when techniques that were intended to apply to atomic systems are instead used for artificial, solid-state devices.

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Transition metal oxide (TMO) memristors have recently attracted special attention from the semiconductor industry and academia. Memristors are one of the strongest candidates to replace flash memory, and possibly DRAM and SRAM in the near future. Moreover, memristors have a high potential to enable beyond-CMOS technology advances in novel architectures for high performance computing (HPC). The utility of memristors has been demonstrated in reprogrammable logic (cross-bar switches), brain-inspired computing and in non-CMOS complementary logic. Indeed, the potential use of memristors as logic devices is especially important considering the inevitable end of CMOS technology scaling that is anticipated by 2025. In order to aid the on-going Sandia memristor fabrication effort with a memristor design tool and establish a clear physical picture of resistance switching in TMO memristors, we have created and validated with experimental data a simulation tool we name the Memristor Charge Transport (MCT) Simulator.

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We present a study of the 'snap-back' regime of resistive switching hysteresis in bipolar TaOx memristors, identifying power signatures in the electronic transport. Using a simple model based on the thermal and electric field acceleration of ionic mobilities, we provide evidence that the 'snap-back' transition represents a crossover from a coupled thermal and electric-field regime to a primarily thermal regime, and is dictated by the reconnection of a ruptured conducting filament. We discuss how these power signatures can be used to limit filament radius growth, which is important for operational properties such as power, speed, and retention.

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As CMOS technology approaches the end of its scaling, oxide-based memristors have become one of the leading candidates for post-CMOS memory and logic devices. To facilitate the understanding of physical switching mechanisms and accelerate experimental development of memristors, we have developed a three-dimensional fully-coupled electrical and thermal transport model, which captures all the important processes that drive memristive switching and is applicable for simulating a wide range of memristors. The model is applied to simulate the RESET and SET switching in a 3D filamentary TaOx memristor. Extensive simulations show that the switching dynamics of the bipolar device is determined by thermally-activated field-dominant processes: with Joule heating, the raised temperature enables the movement of oxygen vacancies, and the field drift dominates the overall motion of vacancies. Simulated current-voltage hysteresis and device resistance profiles as a function of time and voltage during RESET and SET switching show good agreement with experimental measurement.