David Russell Hughart

Effects of Hydrogen on the Radiation Response of Bipolar Transistors: Experiment and Modeling

Reactions of H2 in lateral PNP BJTs are investigated through experiments and simulations. Pre-irradiation hydrogen exposure makes the devices more sensitive to ionizing radiation, which is explained through first-principles calculations and numerical simulations. Mechanisms for the cracking of hydrogen molecules and proton generation are proposed. We also suggest a mechanism of formation of border traps. When protons are trapped by oxygen vacancies right at or very near the interface, they form electrically active defects near the middle of the band gap. Activation energies of the reaction are used to construct rate equations. The rate equations are solved numerically to determine the spatial and temporal concentrations of hydrogen, holes, and protons. The calculated concentrations of interface and border traps agree well with the experimental results and help to explain the role of hydrogen in determining the total-dose response of BJTs.

The Effects of Proton-Defect Interactions on Radiation-Induced Interface-Trap Formation and Annealing

Interface-trap buildup and annealing are examined as a function of temperature, dose rate, and H2 concentration using a physics-based model. The roles of a number of common oxide defects in radiation-induced interface-trap buildup are evaluated under various conditions. Defects near the interface play a significant role in determining interface-trap buildup by trapping protons as proton concentration and temperature increase.

Mechanisms of Interface Trap Buildup and Annealing During Elevated Temperature Irradiation

Both enhanced interface trap buildup and interface trap annealing can occur during elevated temperature irradiation (ETI), depending on the temperature, total dose, and dose rate. In this paper we describe mechanisms that govern the rate-limiting processes of interface trap buildup and annealing during ETI. Hydrogenated oxygen vacancies can facilitate hydrogen dimerization at elevated temperatures. This results in the removal of protons that can create interface traps, so degradation is suppressed. Hydrogen dimerization becomes more competitive with degradation mechanisms as the concentration of protons near the interface increases and/or as temperature increases.

The susceptibility of TaOx-based memristors to high dose rate ionizing radiation and total ionizing dose

This paper investigates the effects of high dose rate ionizing radiation and total ionizing dose (TID) on tantalum oxide ( TaOx) memristors. Transient data were obtained during the pulsed exposures for dose rates ranging from approximately 5.0 ×107 rad(Si)/s to 4.7 ×108 rad(Si)/s and for pulse widths ranging from 50 ns to 50 μs. The cumulative dose in these tests did not appear to impact the observed dose rate response. Static dose rate upset tests were also performed at a dose rate of ~ 3.0 ×108 rad(Si)/s. This is the first dose rate study on any type of memristive memory technology. In addition to assessing the tolerance of TaOx memristors to high dose rate ionizing radiation, we also evaluated their susceptibility to TID. The data indicate that it is possible for the devices to switch from a high resistance off-state to a low resistance on-state in both dose rate and TID environments. The observed radiation-induced switching is dependent on the irradiation conditions and bias configuration. Furthermore, the dose rate or ionizing dose level at which a device switches resistance states varies from device to device; the enhanced susceptibility observed in some devices is still under investigation. Numerical simulations are used to qualitatively capture the observed transient radiation response and provide insight into the physics of the induced current/voltages.

Mapping of Radiation-Induced Resistance Changes and Multiple Conduction Channels in

The locations of conductive regions in TaOx memristors are spatially mapped using a microbeam and Nanoimplanter by rastering an ion beam across each device while monitoring its resistance. Microbeam irradiation with 800 keV Si ions revealed multiple sensitive regions along the edges of the bottom electrode. The rest of the active device area was found to be insensitive to the ion beam. Nanoimplanter irradiation with 200 keV Si ions demonstrated the ability to more accurately map the size of a sensitive area with a beam spot size of 40 nm by 40 nm. Isolated single spot sensitive regions and a larger sensitive region that extends approximately 300 nm were observed.

{\rm TaO}_{\rm x}

Complex interplay between hydrogen-related defect formation and passivation is observed in irradiated bipolar transistors. Hydrogen soaking experiments are performed to evaluate the dependence of defect buildup and annealing in gated lateral bipolar transistors on hydrogen exposure. Comparisons of the radiation responses of transistors tested in 2009 to identical devices from the same wafer tested in 2003 show that aging has reduced the amount of radiation-induced interface trap and oxide trapped charge formation in most cases. These results demonstrate that the way in which the radiation response of a hydrogen-sensitive device evolves with age depends on whether hydrogen is diffusing into or out of the device, and whether the initial defect concentration favors passivation or depassivation reactions. These results strongly suggest that hydrogen exposure cannot replace low-dose-rate irradiation in ELDRS tests for bipolar devices and ICs without extensive characterization testing.

Memristors

Analog resistive memories promise to reduce the energy of neural networks by orders of magnitude. However, the write variability and write nonlinearity of current devices prevent neural networks from training to high accuracy. We present a novel periodic carry method that uses a positional number system to overcome this while maintaining the benefit of parallel analog matrix operations. We demonstrate how noisy, nonlinear TaOx devices that could only train to 80% accuracy on MNIST, can now reach 97% accuracy, only 1% away from an ideal numeric accuracy of 98%. On a file type dataset, the TaOx devices achieve ideal numeric accuracy. In addition, low noise, linear Li1−xCoO2 devices train to ideal numeric accuracies using periodic carry on both datasets.