Dallas T. Morisette

Impedance microbiology-on-a-chip: microfluidic bioprocessor for rapid detection of bacterial metabolism

Detection of a few live bacterial cells in many industrial or clinical samples is a very important technological problem. We have developed a microscale technique for concentrating bacterial cells from a dilute sample, by factors on the order of 10/sup 4/ to 10/sup 5/, and detecting their metabolic activity by purely electrical means. The technique was implemented on a silicon-based microfluidic chip where the cells are concentrated and incubated in a chamber with a volume of 400 pL. Concentration and capture are obtained by the use of dielectrophoresis on the bacterial cells, and metabolism detection is achieved by means of impedance measurements of the medium in which the bacteria are incubated. Performing impedance-based detection at the microscale results in drastically reduced detection times for dilute bacterial samples, thanks to the ability to efficiently concentrate and capture the cells in an extremely small volume. Such concentration eliminates the need to amplify the bacterial population by long culture steps. This detection technique can be used for a wide variety of applications.

Integrated nanoscale silicon sensors using top-down fabrication

Semiconductor device-based sensing of chemical and biological entities has been demonstrated through the use of micro- and nanoscale field-effect devices and close variants. Although carbon nanotubes and silicon nanowires have been demonstrated as single molecule biosensors, the fabrication methods that have been used for creating these devices are typically not compatible with modern semiconductor manufacturing techniques and their large scale integration is problematic. These shortcomings are addressed by recent advancements in microelectronic fabrication techniques which resulted in the realization of nanowire-like structures. Here we report a method to fabricate silicon nanowires at precise locations using such techniques. Our method allows for the realization of truly integrated sensors capable of production of dense arrays. Sensitivity of these devices to changes in the ambient gas composition is also shown.

Static and dynamic characterization of large-area high-current-density SiC Schottky diodes

SiC devices offer potential advantages in power switching applications due to their wide band gap and higher breakdown field compared to silicon. It is widely felt that the first commercial application of SiC power devices will be Schottky diodes used as flyback rectifiers for silicon IGBTs driving inductive loads (motors). The simple substitution of SiC Schottky diodes in place of silicon PiN diodes in these circuits can reduce the overall switching loss of the circuit by up to 35%.

PCR-based detection in a micro-fabricated platform

We present a novel, on-chip system for the electrokinetic capture of bacterial cells and their identification using the polymerase chain reaction (PCR). The system comprises a glass-silicon platform with a set of micro-channels, -chambers, and -electrodes. A platinum thin film resistor, placed in the proximity of the chambers, is used for temperature monitoring. The whole chip assembly is mounted on a Printed Circuit Board (PCB) and wire-bonded to it. The PCB has an embedded heater that is utilized for PCR thermal cycle and is controlled by a Lab-View program. Similar to our previous work, one set of electrodes on the chip inside the bigger chamber (0.6 microl volume) is used for diverting bacterial cells from a flowing stream into to a smaller chamber (0.4 nl volume). A second set of interdigitated electrodes (in smaller chamber) is used to actively trap and concentrate the bacterial cells using dielectrophoresis (DEP). In the presence of the DEP force, with the cells still entrapped in the micro-chamber, PCR mix is injected into the chamber. Subsequently, PCR amplification with SYBR Green detection is used for genetic identification of Listeria monocytogenes V7 cells. The increase in fluorescence is recorded with a photomultiplier tube module mounted over an epifluorescence microscope. This integrated micro-system is capable of genetic amplification and identification of as few as 60 cells of L. monocytogenes V7 in less than 90 min, in 600 nl volume collected from a sample of 10(4) cfu ml(-1). Specificity trials using various concentrations of L. monocytogenes V7, Listeria innocua F4248, and Escherichia coli O157:H7 were carried out successfully using two different primer sets specific for a regulatory gene of L. monocytogenes, prfA and 16S rRNA primer specific for the Listeria spp., and no cross-reactivity was observed.

Theoretical comparison of SiC PiN and Schottky diodes based on power dissipation considerations

In order to select the optimal device for a particular application, designers must carefully analyze the tradeoffs between competing devices. Recent progress in SiC power rectifiers has resulted in the demonstration of high-voltage PiN and Schottky barrier diodes (SBDs). With both technologies maturing, power electronics engineers will soon face the task of selecting between these two devices. Until recently, the choice was simple, since silicon SBDs are only available for relatively low voltage applications. The choice is not as clear when considering SiC diodes, and guidelines for determining the proper application of each are needed. The purpose of this paper is to provide such guidelines, based on an analysis of the most significant tradeoffs involved.

Electrical detection of dsDNA and polymerase chain reaction amplification

Food-borne pathogens and food safety-related outbreaks have come to the forefront over recent years. Estimates on the annual cost of sicknesses, hospitalizations, and deaths run into the billions of dollars. There is a large body of research on detection of food-borne pathogens; however, the widely accepted current systems are limited by costly reagents, lengthy time to completion, and expensive equipment. Our aim is to develop a label-free method for determining a change in DNA concentration after a PCR assay. We first used impedance spectroscopy to characterize the change in concentration of purified DNA in deionized water within a microfluidic biochip. To adequately measure the change in DNA concentration in PCR solution, it was necessary to go through a purification and precipitation step to minimize the effects of primers, PCR reagents, and excess salts. It was then shown that the purification and precipitation of the fully amplified PCR reaction showed results similar to the control tests performed with DNA in deionized water. We believe that this work has brought label free electrical biosensors for PCR amplification one step closer to reality.

4 kV silicon carbide Schottky diodes for high-frequency switching applications

Schottky rectifiers exhibit minimal reverse recovery current, and are thus preferred over PiN diodes for high-frequency switching applications such as industrial motor controls and electric vehicle inverters. Implementation of Schottky devices in SiC material has the advantage of producing very high blocking voltages with a moderate epi thickness compared to silicon devices produced in industry today. In this report we describe the first 4 kV Schottky diodes in 4H-SiC. The development of these devices follows earlier work in which breakdown voltages of 1720 V were obtained on 13 /spl mu/m 4H-SiC epilayers.

Phospho-silicate glass gated 4H-SiC metal-oxide-semiconductor devices: Phosphorus concentration dependence

The correlation between phosphorus concentration in phospho-silicate glass (PSG) gate dielectrics and electrical properties of 4H-SiC MOS devices has been investigated. Varying P uptake in PSG is achieved by changing the POCl3 post-oxidation annealing temperature. The density of interface traps (Dit) at the PSG/4H-SiC interface decreases as the amount of interfacial P increases. Most significantly, the MOSFET channel mobility does not correlate with Dit for all samples, which is highly unusual for SiC MOSFETs. Further analysis reveals two types of field-effect mobility (μfe) behavior, depending on the annealing temperature. Annealing at 1000 °C improves the channel mobility most effectively, with a peak value ∼105 cm2 V−1 s−1, and results in a surface phonon scattering limited mobility at high oxide field. On the other hand, PSG annealed at other temperatures results in a surface roughness scattering limited mobility at similar field.

Interface Trap Profiles in 4H- and 6H-SiC MOS Capacitors with Nitrogen- and Phosphorus-Doped Gate Oxides

We report results on the interface trap density (Dit) of 4H- and 6H-SiC metal–oxide–semiconductor (MOS) capacitors with different interface chemistries. In addition to pure dry oxidation, we studied interfaces formed by annealing thermal oxides in NO or POCl3. The Dit profiles, determined by the C–ψs method, show that, although the as-oxidized 4H-SiC/SiO2 interface has a much higher Dit profile than 6H-SiC/SiO2, after postoxidation annealing (POA), both polytypes maintain comparable Dit near the conduction band edge for the gate oxides incorporated with nitrogen or phosphorus. Unlike most conventional C–V- or G–ω-based methods, the C–ψs method is not limited by the maximum probe frequency, therefore taking into account the “fast traps” detected in previous work on 4H-SiC. The results indicate that such fast traps exist near the band edge of 6H-SiC also. For both polytypes, we show that the total interface trap density (Nit) integrated from the C–ψs method is several times that obtained from the high–low method. The results suggest that the detected fast traps have a detrimental effect on electron transport in metal–oxide–semiconductor field-effect transistor (MOSFET) channels.

Comparison of Single- and Double-Trench UMOSFETs in 4H-SiC

Silicon carbide (SiC) trench MOSFETs, or UMOSFETs, generally exhibit lower specific on-resistance than planar DMOSFETs due to a more compact unit cell, higher electron mobility on the a-face surface, and the absence of a JFET region. In this paper we compare the performance of two types of trench UMOSFETs based on 2-D SentaurusTM Device simulations, and show that the single-trench oxide-protected structure exhibits ~40% lower specific on-resistance and half the peak oxide field of the double-trench design when both are optimized for maximum figure of merit.