In neuroprosthetic applications, long-term electrode viability is necessary for robust recording of the activity of neural populations used for generating communication and control signals. The corrosion of tungsten microwire electrodes used for intracortical recording applications was analyzed in a controlled bench-top study and compared to the corrosion of tungsten microwires used in an in vivo study. Two electrolytes were investigated for the benchtop electrochemical analysis: 0.9% phosphate buffered saline (PBS) and 0.9% PBS containing 30 mM of hydrogen peroxide. The oxidation and reduction reactions responsible for corrosion were found by measurement of the open circuit potential and analysis of Pourbaix diagrams. Dissolution of tungsten to form the tungstic ion was found to be the corrosion mechanism. The corrosion rate was estimated from the polarization resistance, which was extrapolated from the electrochemical impedance spectroscopy data. The results show that tungsten microwires in an electrolyte of PBS have a corrosion rate of 300–700 µm/yr. The corrosion rate for tungsten microwires in an electrolyte containing PBS and 30 mM H2O2 is accelerated to 10,000–20,000 µm/yr. The corrosion rate was found to be controlled by the concentration of the reacting species in the cathodic reaction (e.g. O2 and H2O2). The in vivo corrosion rate, averaged over the duration of implantation, was estimated to be 100 µm/yr. The reduced in vivo corrosion rate as compared to the benchtop rate is attributed to decreased rate of oxygen diffusion caused by the presence of a biological film and a reduced concentration of available oxygen in the brain.

Gallium Nitride based high electron mobility transistors (HEMTs) are attractive for use in high power and high frequency applications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20-30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN-based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain ...

Changes in biotic and abiotic factors can be reflected in the complex impedance spectrum of the microelectrodes chronically implanted into the neural tissue. The recording surface of the tungsten electrode in vivo undergoes abiotic changes due to recording site corrosion and insulation delamination as well as biotic changes due to tissue encapsulation as a result of the foreign body immune response. We reported earlier that large changes in electrode impedance measured at 1 kHz were correlated with poor electrode functional performance, quantified through electrophysiological recordings during the chronic lifetime of the electrode. There is a need to identity the factors that contribute to the chronic impedance variation. In this work, we use numerical simulation and regression to equivalent circuit models to evaluate both the abiotic and biotic contributions to the impedance response over chronic implant duration. COMSOL® simulation of abiotic electrode morphology changes provide a possible explanation for the decrease in the electrode impedance at long implant duration while biotic changes play an important role in the large increase in impedance observed initially.

No abstract

No abstract

AlGaN/GaN high electron mobility transistors (HEMTs) are desirable for space applications because of their relative radiation hardness. Predictive modeling of these devices is therefore desired; however, physics-based models accounting for radiation-induced degradation are incomplete. In this work, we show that a partially ionized impurity scattering mobility model can explain the observed reduction in mobility. Electrostatic changes can be explained by confinement of negative charge near the 2DEG in the GaN buffer layer. Simulation results from FLOODS (a TCAD simulator) demonstrate that partial ionization of donor traps is responsible for this phenomenon. Compensation of the acceptor traps by the ionized donors in the GaN confine the acceptor traps (negative space charge) to a thin layer near the AlGan/GaN interface. The simulation results show that near equal concentrations of acceptor traps and donor traps of 1 × 10 17 cm −3 can account for the performance degradation of HEMTs given 5 MeV proton radiation at a fluence of 2 × 10 14 cm −2 . Our results imply that device performance can be accurately simulated by simultaneously accounting for mobility and electrostatic degradation in TCAD solvers using the presented approach. Over the last ten years, there has been a significant amount of research evaluating the effects of radiation on the performance of GaNbased high electron mobility transistors (HEMTs). In general, protonbased radiation damage to AlGaN/GaN HEMTs results in mobility degradation and an increase in the threshold voltage, both of which lead to reductions in peak transconductance and drain current. 1-17The change in mobility has the potential to be greater in magnitude than changes observed in the other parameters. For example Lu et al. measured 40% reduction in mobility and only a 0.1V shift (3% change) in threshold voltage and 13% reduction in drain saturation current for a specific case of proton radiation.14 Additionally, Gaudreau measured a decrease in carrier concentration by a factor of two and a decrease in mobility by a factor of a thousand in response to proton radiation. 1More insight is needed with respect to how radiation defects affect both electron mobility and device electrostatics. Confirmed physics-based models will allow prediction of device performance that depends on the coupled interplay of both parameters.Changes in device performance are primarily attributed to charged point defects, including vacancies and interstitials created by the radiation damage.18 While both donor-and acceptor-like traps are expected to be created during irradiation, it has been shown that a majority of acceptor-like traps are necessary to explain the positive shifts in threshold voltage. [6][7][8]19 Coulombic interaction with charged, radiationinduced defect states has been suggested as the physical reason for mobility reduction. 3,5,6,20 However preliminary calculations show that the concentration of ionized traps responsible for mobility reduction is incompatible with the reduced change in thresh...

No abstract

The temperature rise and distribution in large area β-Ga 2 O 3 rectifiers is simulated using self-consistent solution of the partial differential equations governing the physics in the electrical and thermal domains with the Florida Object Oriented Device and Process simulator (FLOODS) TCAD simulator. The effect of forward voltage (0−2.5V) and power (0−5.5W) was examined for the different epitaxial layer and bulk substrate thicknesses, as well as edge termination and heat sink geometry. A higher maximum temperature is seen for the devices with thicker bulk substrates, while the effect of Joule heating was more evident in the thinner epilayer structures since the resistance decreases and the power generation increases, resulting in a higher temperature. The maximum temperature rise was ∼170K under high power conditions. The heat sink simulation results show a drop in the maximum temperature, where a Cu fin heat sink reduced the maximum temperature by 26.76%.