Browsing by Author "Hu, Walter"
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Item Extended-gate MOSFET for High Sensitivity Photodetectors and pH Sensors(2021-12-01T06:00:00.000Z) Liu, Jinbo; Young, Chadwin D.; Hu, Walter; Anderson, William; Frensley, William R.; Zakhidov, Anvar A.; Gu, QingOver the past years, semiconductors have been greatly used in sensors. With the development semiconductor technology, the semiconductor sensors showed high sensitivity, large integration and reliable stability. Ion-Sensitive Field Effect Transistor (ISFET) changed the gate electrode of Metal-oxide-semiconductor Field Effect Transistor (MOSFET) from metals to electrolyte. In this dissertation, the perovskite, which is a kind of material with large light absorption coefficient, is used to replace the electrolyte in ISFET based on the structure of ISFET to create high sensitivity photodetector. The perovskite is deposited on a silicon wafer and physically separated with MOSFET. Besides taking both advantages of perovskite with excellent optoelectrical property and silicon as a single crystal with good electrical property to get high responsivity, this extended-gate structure provides convenience for changing the capacitance of perovskite and removing the influence of light on MOSFET. The frequency of electrical signal on perovskite can modulate the capacitance of perovskite, which can be used when the capacitance of perovskite is too high compared with MOSFET. The ionic movement influence, which degrees the performance of this photodetector, can be reduced by adding another MOSFET served as current source at the gate of original MOSFET. Inspired by the ionic movement of perovskite, this dissertation also proves ionic movement in pH electrolyte deteriorates the sensitivity of ISFET by electrical measurement. The extended gate structure is utilized to separate the MOSFET and pH capacitance so the MOSFET is free from changing of temperature. Low temperature can decrease the mobility of ions in pH electrolyte especially after the phase change from liquid to solid. The ions in electrolyte can’t follow the high frequency bias voltage so the ionic movement is less at high frequency. Our results show that the ISFETs have larger sensitivity in low temperature and high frequency since the ionic movement can be suppressed by low temperature and high frequency.Item Nanoelectronic Biosensors and Nanoimprinted Hybrid Perovskite Optoelectronic Devices(2018-12) Wang, Honglei; Hu, WalterOver the past decades, the rapid development of semiconductor technology has triggered a boom in many science and technology fields and brought enormous change to our daily life. Recently, the emergence of nanomaterials, advancement of nanofabrication techniques, and continuous demand of transistor down-scaling have populated research with nanoscale features. With the spirit of application-driven research, my doctoral study has been devoted to enhancing the potential of nanoscale semiconductor devices in electronics, biomedical and optoelectronic applications. In this dissertation, nanoscale field-effect transistors with both quasi one-dimensional structure using Si nanowire and two-dimensional structure using MoS2 are investigated for biosensing, aiming at high-sensitivity, high-stability and high-reliability. The Si nanowire transistors were fabricated using photolithography and electron-beam lithography. The effect of electrical bias with alternating current at both source/drain and gate on sensing sensitivity and stability were studied. The few-layer MoS2 transistors were fabricated using photolithographic patterning, and different gate dielectric structures were investigated for sensing sensitivity and stability. In the next portion of the dissertation, nanoscale organic-inorganic hybrid perovskites, patterned by nanoimprint lithography for the first time, were investigated for optoelectronics, aiming at low-cost and high- efficiency. The physical properties of nanoimprinted perovskite thin-film and the sensitivity of asfabricated photodetectors were investigated. Perovskite nanostructures were further studied for emission enhancement via photonic crystal cavity resonance both numerically and experimentally. This research also extends to investigation of bio-inspired nanoimprint, which utilized pre-existing functional nanostructure from biological surface, for application on optoelectronics.Item One-Step Combined-Nanolithography-And-Photolithography for a 2d Photonic Crystal TM Polarizer(MDPI AG) Choi, Kyung-Hak; Huh, J.; Cui, Yonghao; Trivedi, Krutarth; Hu, Walter; Ju, B. -K; Lee, Jeong-BongPhotonic crystals have been widely investigated since they have great potential to manipulate the flow of light in an ultra-compact-scale and enable numerous innovative applications. 2D slab photonic crystals for the telecommunication C band at around 1550 nm have multi-scale structures that are typically micron-scale waveguides and deep sub-micron-scale air hole arrays. Several steps of nanolithography and photolithography are usually used for the fabrication of multi-scale photonic crystals. In this work, we report a one-step lithography process to pattern both micron and deep sub-micron features simultaneously for the 2D slab photonic crystal using combined-nanoimprint-andphotolithography. As a demonstrator, a 2D silicon photonic crystal transverse magnetic (TM) polarizer was fabricated, and the operation was successfully demonstrated.Item Study of Silicon Nanowire Field Effect Transistor for Analog and Digital Biosensing(2017-05) Zang, Pengyuan; 0000-0001-5604-5118 (Zang, P); Hu, WalterThe advancement of semiconductor technology has popularized the low power, economical and small form-factor solid state devices, such as those highly integrated and interconnected as the fundamental infrastructure for the internet of things (IoT). Due to its CMOS-compatibility and electrical interface, the biosensor utilizing field effect transistor (FET) as transducer has become the perfect candidate to interface directly with the chemical and biological properties of the physical world. Especially, nanowire (NW) FET biosensor has received great attention as a highly sensitive biosensing platform, benefiting from its increased surface-to-volume ratio. In this work, several challenges and key aspects of existing NW FET biosensor were studied, and solutions were proposed to address these problems. For example, the hydrolytic stability of the surface sensing element was evaluated and improved by a hydrolysis process, which led to a significant increase in the overall biosensor performance. Another challenge is the noise in the electric potential of the sensing solutions. A secondary reference electrode was introduced in the biosensing system, and its potential was used to subtract the noise from the measured sensor output. Compared to a reference FET, this approach greatly reduced the system complexity and requirement, yet still improved the limit of detection (LOD) by 50 – 70%. This work also involved careful investigation into the analyte sensitivity, which can be considerably affected by the charge buffering effect from the surface hydroxyl groups. Analytical studies and numerical simulations were carried out, revealing that both low pH sensitivity and large analyte buffer capacity are required to achieve a reasonable analyte sensitivity. The most significant portion of this work was the experimental demonstration of the digital biosensing concept with single serpentine NW FET biosensor. The majority of existing FET biosensors utilized the device as an analog transducer, which measures the captured analyte density to generate an output, and suffers from various noise factors, especially the nonspecific changes of the sensing solutions than cannot be reduced by averaging. Digital biosensor no longer depends on the amplitude of the sensor output and is therefore better immune from these noise factors. Instead, the individual binding event of single analyte is counted and analyzed statistically to determine the analyte concentration. The single serpentine NW FET is the ideal device design to achieve digital biosensing. It maintains the low noise level with the equivalently long channel, yet achieves a small footprint enough for binding of only a single analyte. The binding of analyte to multiple segments of the NW results in both higher sensitivity and binding avidity. The small footprint also enables high integration density of the individual digital biosensors into an array format, which is a responsive, highly sensitive, and cost-effective future biosensing platform.