The vertical metal insulator semiconductor tunnel transistor: A proposed Fowler-Nordheim tunneling device

We propose a new field-effect transistor, the vertical metal insulator semiconductor tunnel transistor (VMISTT) which operates using gate modulation of the Fowler-Nordheim tunneling current through a metal insulator semiconductor (M-I-S) diode. The VMISTT has significant advantages over the metal-oxide-semiconductor field-effect transistor in device scaling. In order to allow room-temperature operation of the VMISTT, the tunnel oxide has to be optimized for the metal-to-insulator barrier height and the current-voltage characteristics. We have grown TiO2 layers as the tunnel insulator by oxidizing 7 and 10 nm thick Ti metal films vacuum-evaporated on silicon substrates, and characterized the films by current-voltage and capacitance-voltage techniques. The quality of the oxide films showed variations, depending on the oxidation temperatures in the range of 450-550 degrees C. Fowler-Nordheim tunneling was observed at low temperatures at bias voltage of 2 V and above and a barrier height of approximately 0.4 eV was calculated. Leakage currents present were due Schottky-barrier emission at room-temperature, and hopping at liquid nitrogen temperature

Investigation of TiO2 as a possible tunneling layer in the vertical metal insulator semiconductor tunnel transistor

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Investigation of TiO2 as a possible tunneling layer in the vertical metal insulator semiconductor tunnel transistor

In this thesis, a new field effect transistor, the vertical metal insulator semiconductor tunnel transistor (VMISTT) is proposed. It is a modified version of the metal oxide tunnelling transistor (MOTT). The principle of operation of the device is based on the gate modulated Fowler Nordheim (F-N) tunnelling of carriers through an insulating layer. The VMISTT is different from the MOTT in two important aspects. Firstly, the metallic source in the MOTT is replaced by doped silicon. The body of the transistor is an oxide, which functions as a tunnel barrier. By choosing a suitable tunnel barrier and metal drain, it is possible to make both n-type and p-type devices and hence complementary devices. Secondly, the vertical structure will allow better control of material growth and device processing. The tunnel barrier can be grown by a conventional scalable process such as evaporation of thin metal film followed by thermal oxidation. This will help to reduce the leakage current and hence enhance the performance of the device. The SILVACO device simulator ATLAS is used to study the VMISTT performance. The barrier height between the tunnel barrier and the Si substrate is a critical device parameter. A low barrier is required for a large F-N tunnelling current to occur. However, Schottky emission, which is one of the main sources of leakage current in the VMISTT, will also be huge for a low value of the barrier height. It is thus important to optimise the barrier height to minimize the effect of Schottky emission on the device performance. The simulation results show that with a barrier height of 0.6 V, an on/off current ratio of at least 4 orders of magnitude, and a subthreshold slope of 42 mV/dec can be obtained. Titanium dioxide TiO2 is a promising candidate for the tunnel barrier due to its low barrier height to Si. The fabrication and optimisation of the tunnel barrier of the VMISTT is the focus of the experimental chapters of this thesis. The observation of F-N tunnelling current in the tunnel barrier is essential such that the modulation of the F-N tunnelling current by the gate bias can be realised at room temperature. Electrical and structural analysis are performed on TiO2 films grown from thermal oxidation of electron beam evaporated Ti thin film. TiO2 MOS capacitors with different top metal electrodes (Al, Pt) and different Si substrate (n-type, p-type) were fabricated to analyse the electrical properties of the TiO2 films. It is shown that the reactivity of Al top contact affects the electrical properties of the oxide layers. The current transport mechanism in the TiO2 films is found to be Poole-Frenkel (P-F) emission at room temperature. At 84 K, F-N tunnelling and trap-assisted tunnelling are observed. By comparing the electrical characteristics of thermally grown TiO2 films with the properties of those films grown by other techniques reported in the literature, it is suggested that irrespective of the deposition technique, annealing of as-deposited TiO2 in O2 is a similar process to thermal oxidation of Ti thin films. In conclusion, it is essential to reduce the defects density in the TiO2 films, so that those trap-related mechanisms can be suppressed for the observation of the F-N tunnelling at room temperature.</p

Observation of Fowler-Nordheim tunneling for room temperature operation of the vertical metal-oxide tunnel transistor

We propose a new field effect transistor, the vertical metal-oxide tunnel transistor (VMOTT) which operates using gate modulation of the Fowler-Nordheim tunneling current through the channel oxide. The VMOTT has significant advantages over the metal-oxide-semiconductor field effect transistor (MOSFET) in device scaling. In order to allow room-temperature operation of the VMOTT, the tunnel oxide has to be optimized for the metal-to-oxide barrier height and the current-voltage characteristics. We have grown TiO2 layers as the tunnel oxide by oxidising 5-10 nm thick Ti metal films vacuum-evaporated on silicon substrates, and characterized the films by current-voltage and capacitance-voltage techniques. The quality of the oxide films showed variations, depending on the oxidation temperatures in the range of 300-500 C. Some of the samples were subjected to a post-oxidation anneal in nitrogen ambient at 700 C. Fowler-Nordheim tunneling was observed clearly at room temperature, which was further confirmed at low temperatures in the samples oxidised at 500 C and annealed subsequently

Progress in utilisation of waste cooking oil for sustainable biodiesel and biojet fuel production

The increase in human consumption of plant and animal oils has led to the rise in waste cooking oil (WCO) production. Instead of disposing the used cooking oil as waste, recent technological advance has enabled the use of WCO as a sustainable feedstock for biofuels production, thereby maximising the value of biowastes via energy recovery while concomitantly solving the disposal issue. The current regulatory frameworks for WCO collection and recycling practices imposed by major WCO producing countries are reviewed, followed by the overview of the progress in biodiesel conversion techniques, along with novel methods to improve the feasibility for upscaling. The factors which influence the efficiency of the reactions such as properties of feedstock, heterogenous catalytic processes, cost effectiveness and selectivity of reaction product are discussed. Ultrasonic-assisted transesterification is found to be the least energy intensive method for producing biodiesel. The production of bio-jet fuels from WCO, while scarce, provide diversity in waste utilisation if problems such as carbon chain length, requirements of bio-jet fuel properties, extreme reaction conditions and effectiveness of selected catalyst-support system can be solved. Technoeconomic studies revealed that WCO biofuels is financially viable with benefit of mitigating carbon emissions, provided that the price gap between the produced fuel and commercial fuels, sufficient supply of WCO and variation in the oil properties are addressed. This review shows that WCO is a biowaste with high potential for advanced transportation fuel production for ground and aviation industries. The advancement in fuel production technology and relevant policies would accelerate the application of sustainable WCO biofuels.N/