Michael C. Hamilton

Thin-film organic polymer phototransistors

We have studied the electrical performance of organic polymer thin-film transistors (OP-TFTs) under steady-state white-light illumination, as well as the performance of these devices as photodetectors. The off-state drain current of the OP-TFT is significantly increased due to the illumination, while a smaller relative effect is observed on the drain current in the strong-accumulation regime. The illumination effectively decreases the threshold voltage of the device and increases the apparent subthreshold swing, while the field-effect mobility of the charge carriers in the polymer channel is unchanged. We have observed full recovery of our devices after the illumination is removed at room temperature. These observations are explained in terms of the photogeneration of excitons due to the absorbed photons. The photogenerated excitons subsequently diffuse and dissociate into free charge carriers, thereby enhancing the carrier density in the channel of the device. We have found broadband responsivities of approximately 0.7 mA/W for devices biased in the strong-accumulation regime and gate-to-source voltage-independent photosensitivities of approximately 10/sup 3/ for devices in the off-state. We also determine, for the first time, the flatband voltage of these devices to be about -2.3 V.

Organic polymer thin-film transistor photosensors

We present our study of the effects of monochromatic illumination on the electrical performance of organic polymer thin-film transistors (OP-TFTs) and the use of these devices as photosensors. In the case of monochromatic illumination that is strongly absorbed by the polymer, the drain current of a device biased in the OFF-state is significantly increased and the threshold voltage is reduced. Light that is not strongly absorbed by the polymer has little effect on the electrical performance of the OP-TFTs. We explain these effects in terms of the photogeneration of excitons in the polymer channel region of the device. The density of excitons generated in the polymer depends on the energy of the incident photons, as well as on the irradiance level of the incident illumination. The photogenerated excitons subsequently dissociate into electrons and holes. The electrons can be trapped by positively charged states, thereby reducing the threshold voltage, while the photogenerated holes contribute to the excess photocurrent measured at the drain. To demonstrate the possible use of OP-TFTs as photosensors, we also present the responsivity, photosensitivity (signal-to-noise ratio), external quantum efficiency, noise-equivalent power, and specific detectivity of these devices. The dependence of these parameters on the incident photon energy and irradiance level is described.

Proton radiation response of SiGe HBT analog and RF circuits and passives

Presents the first experimental results of the effects of 63 MeV proton irradiation on SiGe heterojunction bipolar transistor (HBT) analog and radio-frequency (RF) circuits and passive elements. A SiGe HBT bandgap, reference circuit, commonly used to generate stable on-chip voltages in analog ICs, a SiGe HBT voltage-controlled oscillator, a key building block for RF transceivers, and an LC bandpass filter routinely used in RF circuit design were each irradiated to proton fluences as high as 5/spl times/10/sup 13/ p/cm/sup 2/. The degradation associated with these extreme proton fluences was found to be minimal, suggesting that SiGe HBT technology is robust for these types of circuit applications.

The effects of proton irradiation on the lateral and vertical scaling of UHV/CVD SiGe HBT BiCMOS technology

We present the first experimental results of the effects of 63 MeV proton irradiation on both the lateral and vertical scaling properties of SiGe HBT BiCMOS technology. Three distinct generations of (unhardened) SiGe technology are examined. The first generation SiGe HBTs experience very minor degradation in current gain at proton fluence as high as 2/spl times/10/sup 13/ p/cm/sup 2/. The second and third generations SiGe HBTs, however, show 60-70% degradation in current gain under similar conditions, suggesting that emitter-base spacer optimization may be required as the technology is scaled. Si nFETs from the first generation SiGe BiCMOS technology are only hard to about 40 krad equivalent gamma dose, and are limited by drain-to-source leakage along the shallow trench edge. The second generation Si nFETs, however, improve with scaling since the shallow trench is thinned, and can withstand up to 150 krad of equivalent dose.

Conductive interpenetrating networks of polypyrrole and polycaprolactone encourage electrophysiological development of cardiac cells.

UNLABELLED Conductive and electroactive polymers have the potential to enhance engineered cardiac tissue function. In this study, an interpenetrating network of the electrically-conductive polymer polypyrrole (PPy) was grown within a matrix of flexible polycaprolactone (PCL) and evaluated as a platform for directing the formation of functional cardiac cell sheets. PCL films were either treated with sodium hydroxide to render them more hydrophilic and enhance cell adhesion or rendered electroactive with PPy grown via chemical polymerization yielding PPy-PCL that had a resistivity of 1.0 ± 0.4 kΩ cm, which is similar to native cardiac tissue. Both PCL and PPy-PCL films supported cardiomyocyte attachment; increasing the duration of PCL pre-treatment with NaOH resulted in higher numbers of adherent cardiomyocytes per unit area, generating cell densities which were more similar to those on PPy-PCL films (1568 ± 126 cells mm(-2), 2880 ± 439 cells mm(-2), 3623 ± 456 cells mm(-2) for PCL with 0, 24, 48 h of NaOH pretreatment, respectively; 2434 ± 166 cells mm(-2) for PPy-PCL). When cardiomyocytes were cultured on the electrically-conductive PPy-PCL, more cells were observed to have peripheral localization of the gap junction protein connexin-43 (Cx43) as compared to cells on NaOH-treated PCL (60.3 ± 4.3% vs. 46.6 ± 5.7%). Cx43 gene expression remained unchanged between materials. Importantly, the velocity of calcium wave propagation was faster and calcium transient duration was shorter for cardiomyocyte monolayers on PPy-PCL (1612 ± 143 μm/s, 910 ± 63 ms) relative to cells on PCL (1129 ± 247 μm/s, 1130 ± 20 ms). In summary, PPy-PCL has demonstrated suitability as an electrically-conductive substrate for culture of cardiomyocytes, yielding enhanced functional properties; results encourage further development of conductive substrates for use in differentiation of stem cell-derived cardiomyocytes and cardiac tissue engineering applications. STATEMENT OF SIGNIFICANCE Current conductive materials for use in cardiac regeneration are limited by cytotoxicity or cost in implementation. In this manuscript, we demonstrate for the first time the application of a biocompatible, conductive polypyrrole-polycaprolactone film as a platform for culturing cardiomyocytes for cardiac regeneration. This study shows that the novel conductive film is capable of enhancing cell-cell communication through the formation of connexin-43, leading to higher velocities for calcium wave propagation and reduced calcium transient durations among cultured cardiomyocyte monolayers. Furthermore, it was demonstrated that chemical modification of polycaprolactone through alkaline-mediated hydrolysis increased overall cardiomyocyte adhesion. The results of this study provide insight into how cardiomyocytes interact with conductive substrates and will inform future research efforts to enhance the functional properties of cardiomyocytes, which is critical for their use in pharmaceutical testing and cell therapy.

High Channel Mobility 4H-SiC MOSFETs by Antimony Counter-Doping

Channel mobility of >100 cm2V-1s-1 has been obtained on enhancement mode 4H-SiC MOSFETs using an antimony (Sb) doped surface channel in conjunction with nitric oxide (NO) postoxidation annealing. Temperature dependence of the channel mobility indicates that Sb, being an n-type dopant, reduces the surface electric field while the NO anneal reduces the interface trap density, thereby improving the channel mobility. This letter highlights the importance of semiconductor/dielectric materials processes that reduce the transverse surface electric field for improved channel mobility in 4H-SiC MOSFETs.

Flexible superconducting Nb transmission lines on thin film polyimide for quantum computing applications

We describe progress and initial results achieved towards the goal of developing integrated multi-conductor arrays of shielded controlled-impedance flexible superconducting transmission lines with ultra-miniature cross sections and wide bandwidths (dc to >10 GHz) over meter-scale lengths. Intended primarily for use in future scaled-up quantum computing systems, such flexible thin-film Nb/polyimide ribbon cables provide a physically compact and ultra-low thermal conductance alternative to the rapidly increasing number of discrete coaxial cables that are currently used by quantum computing experimentalists to transmit signals between the low-temperature stages (from ~ 4 K down to ~ 20 mK) of a dilution refrigerator. S-parameters are presented for 2-metal layer Nb microstrip structures with lengths ranging up to 550 mm. Weakly coupled open-circuit microstrip resonators provided a sensitive measure of the overall transmission line loss as a function of frequency, temperature, and power. Two common polyimide dielectrics, one conventional and the other photo-definable (PI-2611 and HD-4100, respectively) were compared. Our most striking result, not previously reported to our knowledge, was that the dielectric loss tangents of both polyimides are remarkably low at deep cryogenic temperatures, typically 100

Pressureless Sintering of Microscale Silver Paste for 300 °C Applications

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Recent advances in energy harvesting technology and techniques

smaller than corresponding room temperature values. This enables fairly long-distance transmission of microwave signals without excessive attenuation and permits usefully high rf power levels to be transmitted without creating excessive dielectric heating. We observed loss tangents as low as 2.2

Three-Dimensional FDTD Simulation of Nonlinear Ferroelectric Materials in Rectangular Waveguide

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