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Evaluation and characterization of Silicon MESFETs in low dropout regulators

Description

The partially-depleted (PD) silicon Metal Semiconductor Field Effect Transistor (MESFET) is becoming more and more attractive for analog and RF applications due to its high breakdown voltage. Compared to conventional CMOS high voltage transistors, the silicon MESFET can be fabricated

The partially-depleted (PD) silicon Metal Semiconductor Field Effect Transistor (MESFET) is becoming more and more attractive for analog and RF applications due to its high breakdown voltage. Compared to conventional CMOS high voltage transistors, the silicon MESFET can be fabricated in commercial standard Silicon-on-Insulator (SOI) CMOS foundries without any change to the process. The transition frequency of the device is demonstrated to be 45GHz, which makes the MESFET suitable for applications in high power RF power amplifier designs. Also, high breakdown voltage and low turn-on resistance make it the ideal choice for switches in the switching regulator designs. One of the anticipated applications of the MESFET is for the pass device for a low dropout linear regulator. Conventional NMOS and PMOS linear regulators suffer from high dropout voltage, low bandwidth and poor stability issues. In contrast, the N-MESFET pass transistor can provide an ultra-low dropout voltage and high bandwidth without the need for an external compensation capacitor to ensure stability. In this thesis, the design theory and problems of the conventional linear regulators are discussed. N-MESFET low dropout regulators are evaluated and characterized. The error amplifier used a folded cascode architecture with gain boosting. The source follower topology is utilized as the buffer to sink the gate leakage current from the MESFET. A shunt-feedback transistor is added to reduce the output impedance and provide the current adaptively. Measurement results show that the dropout voltage is less than 150 mV for a 1A load current at 1.8V output. Radiation measurements were done for discrete MESFET and fully integrated LDO regulators, which demonstrate their radiation tolerance ability for aerospace applications.

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Date Created
2013

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Optimizing the design of partially and fully depleted MESFETs for low dropout regulators

Description

The constant scaling of supply voltages in state-of-the-art CMOS processes has led to severe limitations for many analog circuit applications. Some CMOS processes have addressed this issue by adding high voltage MOSFETs to their process. Although it can be a

The constant scaling of supply voltages in state-of-the-art CMOS processes has led to severe limitations for many analog circuit applications. Some CMOS processes have addressed this issue by adding high voltage MOSFETs to their process. Although it can be a completely viable solution, it usually requires a changing of the process flow or adding additional steps, which in turn, leads to an increase in fabrication costs. Si-MESFETs (silicon-metal-semiconductor-field-effect-transistors) from Arizona State University (ASU) on the other hand, have an inherent high voltage capability and can be added to any silicon-on-insulator (SOI) or silicon-on-sapphire (SOS) CMOS process free of cost. This has been proved at five different commercial foundries on technologies ranging from 0.5 to 0.15 μm. Another critical issue facing CMOS processes on insulated substrates is the scaling of the thin silicon channel. Consequently, the future direction of SOI/SOS CMOS transistors may trend away from partially depleted (PD) transistors and towards fully depleted (FD) devices. FD-CMOS are already being implemented in multiple applications due to their very low power capability. Since the FD-CMOS market only figures to grow, it is appropriate that MESFETs also be developed for these processes. The beginning of this thesis will focus on the device aspects of both PD and FD-MESFETs including their layout structure, DC and RF characteristics, and breakdown voltage. The second half will then shift the focus towards implementing both types of MESFETs in an analog circuit application. Aside from their high breakdown ability, MESFETs also feature depletion mode operation, easy to adjust but well controlled threshold voltages, and fT's up to 45 GHz. Those unique characteristics can allow certain designs that were previously difficult to implement or prohibitively expensive using conventional technologies to now be achieved. One such application which benefits is low dropout regulators (LDO). By utilizing an n-channel MESFET as the pass transistor, a LDO featuring very low dropout voltage, fast transient response, and stable operation can be achieved without an external capacitance. With the focus of this thesis being MESFET based LDOs, the device discussion will be mostly tailored towards optimally designing MESFETs for this particular application.

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Date Created
2010

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Virtual Office Assistant

Description

With the progression of different industries moving away from employing secretaries for business professionals and professors, there exists a void in the area of personal assistance. This problem has existing solutions readily available to replace this service, i.e. secretary or

With the progression of different industries moving away from employing secretaries for business professionals and professors, there exists a void in the area of personal assistance. This problem has existing solutions readily available to replace this service, i.e. secretary or personal assistant, tend to range from expensive and useful to inexpensive and not efficient. This leaves a low cost niche into the market of a virtual office assistant or manager to display messages and to help direct people in obtaining contact information. The development of a low cost solution revolves around the software needed to solve the various problems an accessible and user friendly Virtual Interface in which the owner of the Virtual Office Manager/Assistant can communicate to colleagues who are at standby outside of the owner's office and vice versa. This interface will be allowing the owner to describe the status pertaining to their absence or any other message sent to the interface. For example, the status of the owner's work commute can be described with a simple "Running Late" phrase or a message like "Busy come back in 10 minutes". In addition, any individual with an interest to these entries will have the opportunity to respond back because the device will provide contact information. When idle, the device will show supplemental information such as the owner's calendar and name. The scope of this will be the development and testing of solutions to achieve these goals.

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Date Created
2016-12

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Improved microfabrication technologies for single cell metabolic analysis

Description

Within the last decade there has been remarkable interest in single-cell metabolic analysis as a key technology for understanding cellular heterogeneity, disease initiation, progression, and drug resistance. Technologies have been developed for oxygen consumption rate (OCR) measurements using various configurations

Within the last decade there has been remarkable interest in single-cell metabolic analysis as a key technology for understanding cellular heterogeneity, disease initiation, progression, and drug resistance. Technologies have been developed for oxygen consumption rate (OCR) measurements using various configurations of microfluidic devices. The technical challenges of current approaches include: (1) deposition of multiple sensors for multi-parameter metabolic measurements, e.g. oxygen, pH, etc.; (2) tedious and labor-intensive microwell array fabrication processes; (3) low yield of hermetic sealing between two rigid fused silica parts, even with a compliance layer of PDMS or Parylene-C. In this thesis, several improved microfabrication technologies are developed and demonstrated for analyzing multiple metabolic parameters from single cells, including (1) a modified "lid-on-top" configuration with a multiple sensor trapping (MST) lid which spatially confines multiple sensors to micro-pockets enclosed by lips for hermetic sealing of wells; (2) a multiple step photo-polymerization method for patterning three optical sensors (oxygen, pH and reference) on fused silica and on a polyethylene terephthalate (PET) surface; (3) a photo-polymerization method for patterning tri-color (oxygen, pH and reference) optical sensors on both fused silica and on the PET surface; (4) improved KMPR/SU-8 microfabrication protocols for fabricating microwell arrays that can withstand cell culture conditions. Implementation of these improved microfabrication methods should address the aforementioned challenges and provide a high throughput and multi-parameter single cell metabolic analysis platform.

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Date Created
2014

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Grain boundary passivation of multicrystalline silicon using hydrogen sulfide as a sulfur source

Description

Hydrogen sulfide (H2S) has been identified as a potential ingredient for grain boundary passivation of multicrystalline silicon. Sulfur is already established as a good surface passivation material for crystalline silicon (c-Si). Sulfur can be used both from solution and hydrogen

Hydrogen sulfide (H2S) has been identified as a potential ingredient for grain boundary passivation of multicrystalline silicon. Sulfur is already established as a good surface passivation material for crystalline silicon (c-Si). Sulfur can be used both from solution and hydrogen sulfide gas. For multicrystalline silicon (mc-Si) solar cells, increasing efficiency is a major challenge because passivation of mc-Si wafers is more difficult due to its randomly orientated crystal grains and the principal source of recombination is contributed by the defects in the bulk of the wafer and surface.

In this work, a new technique for grain boundary passivation for multicrystalline silicon using hydrogen sulfide has been developed which is accompanied by a compatible Aluminum oxide (Al2O3) surface passivation. Minority carrier lifetime measurement of the passivated samples has been performed and the analysis shows that success has been achieved in terms of passivation and compared to already existing hydrogen passivation, hydrogen sulfide passivation is actually better. Also the surface passivation by Al2O3 helps to increase the lifetime even more after post-annealing and this helps to attain stability for the bulk passivated samples. Minority carrier lifetime is directly related to the internal quantum efficiency of solar cells. Incorporation of this technique in making mc-Si solar cells is supposed to result in higher efficiency cells. Additional research is required in this field for the use of this technique in commercial solar cells.

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Date Created
2014

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Development of silver-free silicon photovoltaic solar cells with all-aluminum electrodes

Description

To date, the most popular and dominant material for commercial solar cells is

crystalline silicon (or wafer-Si). It has the highest cell efficiency and cell lifetime out

of all commercial solar cells. Although the potential of crystalline-Si solar cells in

supplying energy demands

To date, the most popular and dominant material for commercial solar cells is

crystalline silicon (or wafer-Si). It has the highest cell efficiency and cell lifetime out

of all commercial solar cells. Although the potential of crystalline-Si solar cells in

supplying energy demands is enormous, their future growth will likely be constrained

by two major bottlenecks. The first is the high electricity input to produce

crystalline-Si solar cells and modules, and the second is the limited supply of silver

(Ag) reserves. These bottlenecks prevent crystalline-Si solar cells from reaching

terawatt-scale deployment, which means the electricity produced by crystalline-Si

solar cells would never fulfill a noticeable portion of our energy demands in the future.

In order to solve the issue of Ag limitation for the front metal grid, aluminum (Al)

electroplating has been developed as an alternative metallization technique in the

fabrication of crystalline-Si solar cells. The plating is carried out in a

near-room-temperature ionic liquid by means of galvanostatic electrolysis. It has been

found that dense, adherent Al deposits with resistivity in the high 10^–6 ohm-cm range

can be reproducibly obtained directly on Si substrates and nickel seed layers. An

all-Al Si solar cell, with an electroplated Al front electrode and a screen-printed Al

back electrode, has been successfully demonstrated based on commercial p-type

monocrystalline-Si solar cells, and its efficiency is approaching 15%. Further

optimization of the cell fabrication process, in particular a suitable patterning

technique for the front silicon nitride layer, is expected to increase the efficiency of

the cell to ~18%. This shows the potential of Al electroplating in cell metallization is

promising and replacing Ag with Al as the front finger electrode is feasible.

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Date Created
2016

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Surface Plasmon Resonance (SPR) bio-sensors to detect target molecules in undiluted human serum

Description

Biosensors aiming at detection of target analytes, such as proteins, microbes, virus, and toxins, are widely needed for various applications including detection of chemical and biological warfare (CBW) agents, biomedicine, environmental monitoring, and drug screening. Surface Plasmon Resonance (SPR), as

Biosensors aiming at detection of target analytes, such as proteins, microbes, virus, and toxins, are widely needed for various applications including detection of chemical and biological warfare (CBW) agents, biomedicine, environmental monitoring, and drug screening. Surface Plasmon Resonance (SPR), as a surface-sensitive analytical tool, can very sensitively respond to minute changes of refractive index occurring adjacent to a metal film, offering detection limits up to a few ppt (pg/mL). Through SPR, the process of protein adsorption may be monitored in real-time, and transduced into an SPR angle shift. This unique technique bypasses the time-consuming, labor-intensive labeling processes, such as radioisotope and fluorescence labeling. More importantly, the method avoids the modification of the biomarker’s characteristics and behaviors by labeling that often occurs in traditional biosensors. While many transducers, including SPR, offer high sensitivity, selectivity is determined by the bio-receptors. In traditional biosensors, the selectivity is provided by bio-receptors possessing highly specific binding affinity to capture target analytes, yet their use in biosensors are often limited by their relatively-weak binding affinity with analyte, non-specific adsorption, need for optimization conditions, low reproducibility, and difficulties integrating onto the surface of transducers. In order to circumvent the use of bio-receptors, the competitive adsorption of proteins, termed the Vroman effect, is utilized in this work. The Vroman effect was first reported by Vroman and Adams in 1969. The competitive adsorption targeted here occurs among different proteins competing to adsorb to a surface, when more than one type of protein is present. When lower-affinity proteins are adsorbed on the surface first, they can be displaced by higher-affinity proteins arriving at the surface at a later point in time. Moreover, only low-affinity proteins can be displaced by high-affinity proteins, typically possessing higher molecular weight, yet the reverse sequence does not occur. The SPR biosensor based on competitive adsorption is successfully demonstrated to detect fibrinogen and thyroglobulin (Tg) in undiluted human serum and copper ions in drinking water through the denatured albumin.

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Date Created
2015

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Development of novel sensor devices for total ionization dose detection

Description

Total dose sensing systems (or radiation detection systems) have many applications,

ranging from survey monitors used to supervise the generated radioactive waste at

nuclear power plants to personal dosimeters which measure the radiation dose

accumulated in individuals. This dissertation work will present two

Total dose sensing systems (or radiation detection systems) have many applications,

ranging from survey monitors used to supervise the generated radioactive waste at

nuclear power plants to personal dosimeters which measure the radiation dose

accumulated in individuals. This dissertation work will present two different types of

novel devices developed at Arizona State University for total dose sensing applications.

The first detector technology is a mechanically flexible metal-chalcogenide glass (ChG)

based system which is fabricated on low cost substrates and are intended as disposable

total dose sensors. Compared to existing commercial technologies, these thin film

radiation sensors are simpler in form and function, and cheaper to produce and operate.

The sensors measure dose through resistance change and are suitable for applications

such as reactor dosimetry, radiation chemistry, and clinical dosimetry. They are ideal for

wearable devices due to the lightweight construction, inherent robustness to resist

breaking when mechanically stressed, and ability to attach to non-flat objects. Moreover,

their performance can be easily controlled by tuning design variables and changing

incorporated materials. The second detector technology is a wireless dosimeter intended

for remote total dose sensing. They are based on a capacitively loaded folded patch

antenna resonating in the range of 3 GHz to 8 GHz for which the load capacitance varies

as a function of total dose. The dosimeter does not need power to operate thus enabling

its use and implementation in the field without requiring a battery for its read-out. As a

result, the dosimeter is suitable for applications such as unattended detection systems

destined for covert monitoring of merchandise crossing borders, where nuclear material

tracking is a concern. The sensitive element can be any device exhibiting a known

variation of capacitance with total ionizing dose. The sensitivity of the dosimeter is

related to the capacitance variation of the radiation sensitive device as well as the high

frequency system used for reading. Both technologies come with the advantage that they

are easy to manufacture with reasonably low cost and sensing can be readily read-out.

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Date Created
2017

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The Study of the Fabrication Process for Surface Nanotexturing With Modification of Al2O3 Passivation

Description

In this dissertation, the nanofabrication process is characterized for fabrication of nanostructure on surface of silicon and gallium phosphide using silica nanosphere lithography (SNL) and metal assisted chemical etching (MACE) process. The SNL process allows fast process time and well

In this dissertation, the nanofabrication process is characterized for fabrication of nanostructure on surface of silicon and gallium phosphide using silica nanosphere lithography (SNL) and metal assisted chemical etching (MACE) process. The SNL process allows fast process time and well defined silica nanosphere monolayer by spin-coating process after mixing N,N-dimethyl-formamide (DMF) solvent. The MACE process achieves the high aspect ratio structure fabrication using the reaction between metal and wet chemical. The nanostructures are fabricated on Si surface for enhanced light management, but, without proper surface passivation those gains hardly impact the performance of the solar cell. The surface passivation of nanostructures is challenging, not only due to larger surface areas and aspect ratios, but also has a direct result of the nanofabrication processes. In this research, the surface passivation of silicon nanostructures is improved by modifying the silica nanosphere lithography (SNL) and the metal assisted chemical etching (MACE) processes, frequently used to fabricate nanostructures. The implementation of a protective silicon oxide layer is proposed prior to the lithography process to mitigate the impact of the plasma etching during the SNL. Additionally, several adhesion layers are studied, chromium (Cr), nickel (Ni) and titanium (Ti) with gold (Au), used in the MACE process. The metal contamination is one of main damage and Ti makes the mitigation of metal contamination. Finally, a new chemical etching step is introduced, using potassium hydroxide at room temperature, to smooth the surface of the nanostructures after the MACE process. This chemical treatment allows to improve passivation by surface area control and removing surface defects. In this research, I demonstrate the Aluminum Oxide (Al2O3) passivation on nanostructure using atomic layer deposition (ALD) process. 10nm of Al2O3 layer makes effective passivation on nanostructure with optimized post annealing in forming gas (N2/H2) environment. However, 10nm thickness is not suitable for hetero structure because of carrier transportation. For carrier transportation, ultrathin Al2O3 (≤ 1nm) layer is used for passivation, but effective passivation is not achieved because of insufficient hydrogen contents. This issue is solved to use additional ultrathin SiO2 (1nm) below Al2O3 layer and hydrogenation from doped a-Si:H.
Moreover, the nanostructure is creased on gallium phosphide (GaP) by SNL and MACE process. The fabrication process is modified by control of metal layer and MACE solution.

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Date Created
2021