Oxide semiconductors have been envisaged to find applications in ubiquitous flexible electronics in daily life such as wearable electronic gadgets to offer novel user experiences. However, one of the bottlenecks to realise these applications is a lack of oxide-semiconductor components capable of wireless communications. As Bluetooth and Wi-Fi are the two dominant communication interfaces, fast enough front-end rectifiers must be developed to operate at their gigahertz (GHz) transmission frequencies. Furthermore, despite of significant developments of n-type oxide semiconductors in the last decade, widespread flexible electronics also requires high-performance p-type oxide semiconductors for use in complementary logic circuits. The objectives of this dissertation are to develop high quality Schottky barriers, achieve GHz speed Schottky diodes on rigid and flexible substrates, evaluate the noise properties of the Schottky diodes, develop p-type oxide semiconductor using sputtering technology, elucidate the hole transport mechanism in p type transistors, and demonstrate their potential applications such as radio receivers, complementary inverters and ring oscillators. First, indium gallium zinc oxide (IGZO) Schottky diodes were fabricated by using radio frequency magnetron sputtering. The oxygen content at the metal-IGZO interface was found to have a profound effect on the electrical performance. By introducing 3% O2 during the deposition of Pt or IGZO, the diodes exhibited excellent electrical properties without requiring any annealing treatment, thus allowing for the realisation of flexible IGZO Schottky diodes. The high-frequency properties of Pt-IGZO Schottky diodes on glass substrates were optimised by testing a range of IGZO thicknesses and diode active areas. The achieved highest cut-off frequency was beyond 20 GHz, which is to the best of our knowledge the fastest oxide-semiconductor device to date. On flexible substrates, the diodes also showed cut-off frequencies up to 6.3 GHz, well beyond the critical benchmark speed of 2.45 GHz for typical wireless communications. In order to assess the feasibility of using IGZO Schottky diodes in practical applications, measurements were taken to discern their low-frequency noise properties. In the as-deposited diodes, logarithmic dependence of the noise spectral density on the applied bias was observed, revealing that the dominant noise was generated in the space-charge region at low biases and in the series-resistance region at high biases, respectively. After annealing the diodes, very different noise mechanism was observed and the interface-trap-induced noise dominated the noise spectra. As one of the most promising p-type oxide semiconductors, SnO was also studied at low temperatures in this thesis. The experiment revealed that hole-transport mechanism was governed by either band conduction or variable range hopping in different temperature ranges. Finally, the potential for fully oxide-based electronics was demonstrated by an amplitude-modulation radio receiver comprising of an IGZO Schottky diode as the demodulator and a complementary ring oscillator based on IGZO and SnO transistors. In reference to IEEE copyrighted material which is used with permission in this thesis, the IEEE does not endorse any of the University of Manchester's products or services. Internal or personal use of this material is permitted. If interested in reprinting/republishing IEEE copyrighted material for advertising or promotional purposes or for creating new collective works for resale or redistribution, please go to http://www.ieee.org/publications_standards/publications/rights/rights_link.html to learn how to obtain a License from RightsLink.

In recent years, amorphous oxide semiconductors have been studied due to their superior characteristics, such as transparent property, high electron mobility exceeding 10 cm2/V¡Es, and can be fabricated on plastic substrates at low temperatures. According to these advantages, a-IGZO thin-film transistors are promising as next-generation electronic devices. Although a-IGZO TFTs have such unique properties, the electrical performances are strongly dependent on its environment such as oxygen, water and visible light. In this study, the electrical characteristics of a-IGZO TFTs under positive bias stress with different ambient gases have been discussed. In particular, the total duration of the negative gate bias applied on the switching transistor is larger than that of the positive gate bias in display application. Therefore, the electrical stability under negative bias stress is vital to investigate. Moreover, a-IGZO TFT regarded as a panel switch may be exposed to visible light for the application of liquid crystal display. The electrical stability under illumination of visible light is also important to study. Experiment results show that device characteristics are affected under water-containing oxygen ambience. We indicates that the existence of water molecules can assist more oxygen to adsorb on the a-IGZO surface than the case without water assisting. That cause the variation of transfer curve under positive bias stress. However, the degradations in subthreshold swing and threshold voltage are caused by the state-related adsorption of water molecules under negative bias stress. Furthermore, adsorbed oxygen on the surface of a-IGZO can be desorbed by illumination of visible light, leading to large variation in transfer curve.

Indium-Gallium-Zinc-Oxide (a-IGZO) is an amorphous oxide semiconductor that has been attracting increasing attention for use in flat panel display and optoelectronic applications. This is largely due to IGZO’s high mobility at low processing temperatures. In this thesis, IGZO films were successfully grown on polyethylene naphthalate (PEN) substrates by RF magnetron sputtering at room temperature. These films were flexible, transparent and had a good Hall mobility (5-12 cm2/Vs). High quality metal oxide Schottky contacts were fabricated on these as-grown IGZO/PEN films with on-off rectification ratios of up to 108. These were then used as the gate contacts in transparent metal semiconductor field effect transistors (MESFETs). The performance and device stability of these IGZO/PEN MESFETs were investigated via a series of stress tests in both dark conditions and under illumination at different wavelengths in the visible spectrum. During constant voltage stress testing under illumination, the threshold voltage shifted by -0.54 V and 0.38 V for negative and positive gate biasing, respectively. These shifts proved reversible when devices were left in dark conditions for extended periods of time. The effect of persistent photoconductivity after exposure to different illumination sources was examined, with three potential passivation coatings to reduce this unwanted effect explored. Transparent IGZO/PEN MESFETs with an absolute transmission of up to 75% were achieved with the use of ITO ohmic contacts. These devices survived mechanical bending down to a radius of 7 mm with negligible variation in on-current and threshold voltage. This allows for the possibility of incorporating their use in future applications such as flexible transparent electronics.

Power consumption is one of the most important design factors in modern electronic design. With a large market increase in portable battery-operated devices and push for environmental focus, it is of interest for the industry to decrease the power consumption of modern chips as much as possible. However, as circuits scale down in size the leakage current increases. This increases the static power consumption, and in future technologies the static power is expected to make up most of the overall power consumption. Power gating can decrease static power by isolating a circuit block from the power supply. In large chips, this requires state-retention flip flops and non-volatile memories in order to keep the circuit functioning continuously between power gating sequences. A design concept utilizing this is a Normally Off computer, which is in an off-state with no static power for the majority of the time. This is achieved by using non-volatile logic and memories. This concept has been realized by using a new semiconductor technology developed at Semiconductor Energy Laboratories Corporation Ltd., which is known as crystalline In-Ga-Zn oxide semiconductor material. This technology realizes transistors with an ultra-low off-state current, and enables several novel designs of state-retention circuits suitable for Normally-Off computers. This thesis presents two different architectures of state retention flip flops utilizing In-Ga-Zn oxide semiconductor transistors, which are produced and compared to determine their tradeoffs and effectiveness. These flip flops are then implemented in a 32-bit Normally-Off microprocessor to determine the performance of each implementation. This is evaluated by calculating the energy break-even time, which is the power gating time required to overcome the power overhead introduced by the state-retention flip flops. The resulting circuits and the work in this thesis has been presented at two conferences and submitted for publication in one scientific journal.

abstract: A low temperature amorphous oxide thin film transistor (TFT) backplane technology for flexible organic light emitting diode (OLED) displays has been developed to create 4.1-in. diagonal backplanes. The critical steps in the evolution of the backplane process include the qualification and optimization of the low temperature (200 °C) metal oxide process, the stability of the devices under forward and reverse bias stress, the transfer of the process to flexible plastic substrates, and the fabrication of white organic light emitting diode (OLED) displays. Mixed oxide semiconductor thin film transistors (TFTs) on flexible plastic substrates typically suffer from performance and stability issues related to the maximum processing temperature limitation of the polymer. A novel device architecture based upon a dual active layer enables significant improvements in both the performance and stability. Devices are directly fabricated below 200 ºC on a polyethylene naphthalate (PEN) substrate using mixed metal oxides of either zinc indium oxide (ZIO) or indium gallium zinc oxide (IGZO) as the active semiconductor. The dual active layer architecture allows for adjustment in the saturation mobility and threshold voltage stability without the requirement of high temperature annealing, which is not compatible with flexible colorless plastic substrates like PEN. The device performance and stability is strongly dependent upon the composition of the mixed metal oxide; this dependency provides a simple route to improving the threshold voltage stability and drive performance. By switching from a single to a dual active layer, the saturation mobility increases from 1.2 cm2/V-s to 18.0 cm2/V-s, while the rate of the threshold voltage shift decreases by an order of magnitude. This approach could assist in enabling the production of devices on flexible substrates using amorphous oxide semiconductors. / Dissertation/Thesis / M.S. Chemical Engineering 2011

Mobile devices for the personalized detection of health and environmental hazards are becoming the basis for futuristic sensing technologies. In recent decades, air and environmental pollution levels have risen globally. Therefore, environmental protection must be strengthened by developing sensors that detect pollutants. The monitoring of these pollutants with high spatial coverage requires inexpensive electronic gas sensors and self sustainable sensing systems that can be deployed everywhere. This dissertation reports on technological developments to provide solutions for inexpensive, compact, power efficient, and easily deployable toxic gas sensors and integrated systems using semiconducting metal-oxide thin-film transistors (TFTs). The first part of the dissertation introduces the fabrication and characterization of an amorphous indium gallium zinc oxide (IGZO) TFT as a toxic gas sensor. In contrast to existing metal-oxide gas sensors, which are active either with light activation or at high temperature, the developed IGZO TFT sensors are operable at room temperature and require only visible light activation to revive them after exposure to NO2. IGZO TFT sensors exhibited remarkable selectivity and sensitivity to low concentrations of nitrogen dioxide (NO2). The second part of the dissertation introduces the design and realization of the IGZO-based fully integrated gas detectors. Unlike existing gas-sensing systems, which have discrete hardware for signal conditioning, read-out, and data acquisition, the developed integrated detectors constitute thesemodules integrated using IGZO TFT technology. The integrated detectors detect ambient NO2 gas and generate a digital output that is proportional to the ambient gas concentrations. Two types of integrated gas detectors were developed that differ in their mode of operation and circuitry design. These detectors are scalable and pave the way for portable systems to realize various gas-sensing applications, including smart cities and sustainable ecosystems. The success of personalized monitoring devices relies on the following factors: minimum power consumption, selectivity, and stability under extreme conditions that determine overall performance. One of the best solutions to minimize power consumption in these devices is to have a complementary energy-harvesting feature. Hence, the dissertation concludes with the design of self-powered sensors, which are IGZO sensors with self-powering capabilities. Self-powered sensors are p-n heterojunction sensors, developed using IGZO and hybrid-perovskites.

No description available.

The higher mobility is needed for thin film transistor (TFT) mainly used to be applied in the larger size flat-panel displays (FPDs). The amorphous metal oxide TFT has mobility higher than 10 cm2/V¡Es and can substitute the poor mobility (<1 cm2/V¡Es) of traditional amorphous silicon TFT, which shows a great potential for the next generation. Due to the superior characteristics in amorphous metal oxide TFT, therefore, the amorphous metal oxide TFT has been studied extensively. Usually, the source/drain with island type device has a large overlapped/contact area that we cannot determine the exact electron path. That the sample of inverted stagger £\-IGZO TFTs with via type device has smaller contact area and can be estimated the electron path. In this thesis, the devices with different M1 overlaps etching stop layer (ESL) via distance, M2 £\-IGZO contact size and the fringe field effect are investigated. Although the characteristics of £\-IGZO TFTs have great performance, the electrical stability under illumination and long term bias stress are still a important issue to study before implement them into display. Thus, the devices with different structures that we mentioned previously are investigated the electrical reliability which are the negative bias stress of gate voltage, hot carrier stress effect and negative bias of illumination. The electron path of via type is extracted by contact resistance which is greater than the distance between S/D via. Experiment results show that the increased offset between M1 and ESL via generates the resistance-liked effect in electrical characteristics. The hot carrier stress effect is independent of M2 £\-IGZO contact size in short channel length devices and there are close depletion lengths in drain side. The negative bias stress of illumination is proceeded in the fringe field effect devices, which results a negative shift of threshold voltage due to the hole trapping.

In this dissertation, the electrical characteristics of generally used hydrogenated amorphous silicon (a-Si:H) TFTs in LCD and newly risen amorphous indium-gallium-zinc oxide (a-IGZO) TFTs were studied. For modern mobile display and large-size flat panel display application, the traditional thin-film transistor-liquid crystal display (TFT-LCD) technology confronts with a lot of challenges and problems. In general, flexible displays must exhibit some bending ability; however, bending applies mechanical strain to electronic circuits and affects device characteristics. Therefore, the electrical characteristics of a-Si:H TFTs fabricated on stainless steel foil substrates with uniaxial bending were investigated at different temperatures. Experimental results showed that the on-state current and threshold voltage degraded under outward bending. This is because outward bending will induce the increase of band tail states, affecting the transport mechanism at different temperatures. In addition, for practical operation, the electrical characteristics of a-Si:H TFTs under flat and bending situations after AC/DC stress at different temperatures were studied. It was found that high temperature and mechanical bending played important roles under AC stress. The dependence between the accumulated sum of bias rising and falling time and the threshold voltage shifts under AC stress was also observed. Because a-Si:H is a photosensitive material, the high intensity backlight illumination will degrade the performance of a-Si:H TFTs. Thus, the photo-leakage current of a-Si:H TFTs under illumination was investigated at different temperatures. Experimental results showed that a-Si:H TFTs exhibited a pool performance at lower temperatures. The indirect recombination rate and the parasitic resistance (Rp) are responsible for the different photo-leakage-current trends of a-Si:H TFTs under varied temperature operations. To investigate the photo-leakage current, the a-Si:H TFTs were exposed to ultraviolet (UV) light irradiation. It was found that the photo current of a-Si:H TFTs was reduced after UV light irradiation. The detail mechanisms on reducing/increasing photo-leakage current by UV light irradiation were discussed. Recently, the oxide-based semiconductor TFT, especially a-IGZO TFT, is considered as one of promising candidates for active matrix flat-panel display. However, the a-IGZO TFT exists significant electrical instability issue and manufacturing problems. As a consequence, we investigated the effect of hydrogen incorporation on a-IGZO TFTs to reduce interface states between active layer and insulator. Experimental results showed that the electrical characteristics of hydrogen-incorporated a-IGZO TFTs were improved. The threshold voltage shift (£GVth) in hysteresis loop is suppressed from 4 V to 2 V due to the hydrogen-induced passivation of the interface trap states. Finally, we reported the effect of ambient environment on a-IGZO TFT instability. As a-IGZO TFTs were stored in atmosphere environment for 40 days, the transfer characteristics accompanying strange hump were observed during bias-stress. The hump phenomenon is attributed to the absorption of H2O molecule. Additionally, the sufficient electric field is also necessary to cause this anomalous transfer characteristic.

Amorphous oxide semiconductors (AOSs) are of great current interest for thin-film transistor (TFT) channel layer applications. In particular, indium gallium zinc oxide (IGZO) is under intense development for commercial applications because of its demonstrated high performance at low processing temperatures. The objective of the research presented in this thesis is to provide detailed assessments of device stability, temperature dependence, and related phenomena for IGZO-based TFTs processed at temperatures between 200 °C and 300 °C. TFTs tested exhibit an almost rigid shift in log₁₀(I[subscript D]) – V[subscript GS] transfer curves in which the turn-on voltage, V[subscript ON], moves to a more positive gate voltage with increasing stress time during constant-voltage bias-stress testing of IGZO TFTs. TFT stability is improved as the post-deposition annealing temperature increases over the temperature range of 200 – 300 ºC. The turn-on voltage shift induced by constant-voltage bias-stressing is at least partially reversible; V[subscript ON] tends to recover towards its initial value of V[subscript ON] if the TFT is left unbiased in the dark for a prolonged period of time and better recovery is observed for a longer recovery period. V[subscript ON] for a TFT can be set equal to zero after bias-stress testing if the TFT electrodes are grounded and the TFT is maintained in the dark for a prolonged period of time. Attempts to accelerate the recovery process by application of a negative gate bias at elevated temperature (i.e., 100 ºC) were unsuccessful, resulting in severely degraded subthreshold swing. An almost rigid log₁₀(I[subscript D]) – V[subscript GS] transfer curve shift to a lower (more negative) V[subscript ON] with increasing temperature is observed in the range of –50 °C to +50 °C, except for a TFT with an initial V[subscript ON] equal to zero, in which case the log₁₀(ID) – V[subscript GS] transfer curve is temperature-independent. A more detailed temperature-dependence assessment, however, indicates that the log₁₀(I[subscript D]) – V[subscript GS] transfer curve shift is not exactly rigid since the mobility is found to increase slightly with increasing temperature. A noticeable anomaly is observed in certain log₁₀(I[subscript D]) – VGS transfer curves, especially when obtained at elevated temperature (e.g., 30 and 50 ºC), in which I[subscript D] decreases precipitously near zero volts in the positive gate voltage sweep. This anomaly is attributed to a gate-voltage-step-involved detrapping and subsequent retrapping of electrons in the accumulation channel and/or channel/gate insulator interface. In fact, all IGZO TFT stability and temperature-dependence trends are attributed to channel interface and/or channel bulk trapping/detrapping. / Graduation date: 2009