Simulation of Double-Gate Silicon Tunnel FETs with a High-k Gate Dielectric

The down-scaling of conventional MOSFETs has led to an impending power crisis, in which static power consumption is becoming too high. In order to improve the energy-efficiency of electronic circuits, small swing switches are interesting candidates to replace or complement the MOSFETs used today. Tunnel FETs, which are gated p-i-n diodes whose on-current arises from band-to-band tunneling, are attractive new devices for low-power applications due to their low off-current and their potential for a small subthreshold swing. The numerical simulations presented in this thesis have been carried out using a non-local band-to-band tunneling model in Silvaco Atlas. Numerical simulations based on correct underlying models are important for emerging devices, since they can provide insights about optimization before fabrication is carried out, can aid the understanding of device physics through 1D and 2D cross sections, and can be the basis for the formation of an accurate compact model. In general, only CMOS-compatible materials and structures have been used in the Tunnel FET designs presented here. One goal of this thesis was to stay within the framework of what is possible in standard industrial nanoelectronics cleanrooms today, without requiring processes whose mastery lies many years in the future. For this reason, the focus of this thesis is on all-silicon devices, and heterostructures that incorporate other materials are only mentioned. In chapter three, the optimization of the static characteristics of a Tunnel FET is carried out, looking at gate structure (single or double), doping levels of each device region, gate dielectric permittivity, and silicon body thickness. A study of the reduction of the band gap at the tunnel junction is also presented, showing the resulting improvement in on-current and subthreshold swing. Chapter four introduces a new method for threshold voltage extraction in Tunnel FETs. This method has one key advantage over the commonly-used constant current threshold voltage extraction technique: it has a physical meaning. The transconductance method, which has already been used for conventional MOSFETs, pinpoints the Tunnel FET voltage at which the transition from strong control to weak control of the tunneling energy barrier width, and therefore the on-current, takes place. This is analogous to the threshold voltage in a conventional MOSFET which marks the transition from weak inversion to strong inversion at φs=2φF. It is found that Tunnel FETs have two threshold voltages, one in relation to the gate voltage, and the second in relation to the drain voltage, and each depends on the voltage applied at the opposite terminal. A length scaling study is carried out in chapter five, demonstrating the scaling limits of Tunnel FETs at gate lengths on the order of 10-20 nm, due to p-i-n diode leakage current that degrades the off-current. Tunnel FETs designed to have better electrostatic control of the tunnel junction by the gate can scale further before they hit this diode leakage limit at some small gate length. Chapter six presents an additive booster strategy for Tunnel FET optimization, and then uses the resulting optimized device as the basis of a parameter variation study. Here, one parameter is varied at a time, and the effects on the important characteristics (subthreshold swing, threshold voltage, and on-current) are evaluated. The parameters requiring the most control during fabrication are identified. Since Tunnel FETs are emerging devices, the most important future work will be to fabricate fully-optimized n- and p-type devices, and to develop accurate compact models for their incorporation into circuits

An in silico approach for modelling T-helper polarizing iNKT cell agonists

Many analogues of the glycolipid alpha-galactosylceramide (α-GalCer) are known to activate iNKT cells through their interaction with CD1d-expressing antigen-presenting cells, inducing the release of Th1 and Th2 cytokines. Because of iNKT cell involvement and associated Th1/Th2 cytokine changes in a broad spectrum of human diseases, the design of iNKT cell ligands with selective Th1 and Th2 properties has been the subject of extensive research. This search for novel iNKT cell ligands requires refined structural insights. Here we will visualize the chemical space of 333 currently known iNKT cell activators, including several newly tested analogues, by more than 3000 chemical descriptors which were calculated for each individual analogue. To evaluate the immunological responses we analyzed five different cytokines in five different test-systems. We linked the chemical space to the immunological space using a system biology computational approach resulting in highly sensitive and specific predictive models. Moreover, these models correspond with the current insights of iNKT cell activation by α-GalCer analogues, explaining the Th1 and Th2 biased responses, downstream of iNKT cell activation. We anticipate that such models will be of great value for the future design of iNKT cell agonists

Impact of Strain on Drain Current and Threshold Voltage of Nanoscale Double Gate Tunnel Field Effect Transistor: Theoretical Investigation and Analysis

Tunnel field effect transistor (TFET) devices are attractive as they show good scalability and have very low leakage current. However they suffer from low on-current and high threshold voltage. In order to employ the TFET for circuit applications, these problems need to be tackled. In this paper, a novel lateral strained double-gate TFET (SDGTFET) is presented. Using device simulation, we show that the SDGTFET has a higher on-current, low leakage, low threshold voltage, excellent subthreshold slope, and good short channel effects and also meets important ITRS guidelines.Comment: http://web.iitd.ac.in/~mamidal

Asymmetrically strained all-silicon multi-gate n-Tunnel FETs

This paper reports all-silicon asymmetrically strained Tunnel FET architectures that feature improved subthreshold swing and Ion/Ioff characteristics. We demonstrate that a lateral strain profile corresponding to at least 0.2 eV band-gap shrinkage at the BTB source junction could act as an optimized performance Tunnel FET enabling the cancellation of the drain threshold voltage. To implement a real device, we demonstrate using GAA Si NW with asymmetric strain profile using two local stressor technologies to have >4–5 GPa peak of lateral uniaxial tensile stress in the Si NW

Face or building superiority in peripheral vision reversed by task requirements

Peripheral vision has been the topic of few studies compared with central vision. Nevertheless, given that visual information covers all the visual field and that relevant information can originate from highly eccentric positions, the understanding of peripheral vision abilities for object perception seems essential. The poorer resolution of peripheral vision would first suggest that objects requiring large-scale feature integration such as buildings would be better processed than objects requiring finer analysis such as faces. Nevertheless, task requirements also determine the information (coarse or fine) necessary for a given object to be processed. We therefore investigated how task and eccentricity modulate object processing in peripheral vision. Three experiments were carried out requiring finer or coarser information processing of faces and buildings presented in central and peripheral vision. Our results showed that buildings were better judged as identical or familiar in periphery whilst faces were better categorised. We conclude that this superiority for a given stimulus in peripheral vision results (a) from the available information, which depends on the decrease of resolution with eccentricity, and (b) from the useful information, which depends on both the task and the semantic category

Autobiographical recall triggers visual exploration

Autobiographical recall is thought to rely on the ability to generate a visual image of the remembered event. Neuropsychological studies suggest a relationship between deterioration in visual mental imagery and autobiographical distortions, while neuroimaging data similarly implicate visual brain areas in autobiographical recall. However, neither whether autobiographical retrieval is associated with visual exploration, or not. Our paper aimed to provide such evidence one way or the other. Using an eye tracking system, we recorded eye movements of 40 participants during autobiographical recall and during a control condition in which participants had to count aloud. In both conditions, the participants had to look at a blank screen while their gaze location was recorded by the eye-tracker. Autobiographical recall triggered a lower number of fixations and reduced their duration. In contrast, the number, duration, and amplitude of saccades increased compared to the control condition. Our data suggest that autobiographical recall is characterized by visual processing