Abstract:
The dissertation presents a comprehensive study of modeling a Topological FerroelectricField-Effect Transistor (FET)that can provide energy-efficient transport by anovel switching mechanism at a low operating voltage region. It is theoretically well known that there is a fundamental lower limit of subthreshold slope (SS) that defines the minimum voltage, which is required to change the channel current by a decade. At room temperature, this voltage is 60mV. The SS is an important parameter that determines the transistor’s operation in low-power applications, such as a switch. In conventional FETs, a significant fraction of energy is dissipated during switching on/off the conduction through the transistor channel. This transition between on/off events will rapidly occur if the SS is reduced and thus the reduction of heat dissipation. To reduce this dissipation, also it can be said that the transistor has to be operated at lower supply voltages. So this is the SS which limits the operation at lower voltages.
In this work, we have modeled a dual-gate Topological FerroelectricFET, where a ferroelectric material is used as a gate insulator, and a nanoribbon (NR) of topological insulator (TI) material (Stanene) is used as a channel. The channel material gives us the advantage in transistor on/off switching operation, which is obtained by electric field-induced topological phase transition that replaces the conventional carrier inversion process in FET and the topological property ensures carrier transport without backscattering through the edges of the channel.The ferroelectric material’s negative capacitance (NC) property helps to boost the channel voltage. Both the NC property of ferroelectric material and the topological property improve the performance of the topological ferroelectric FET in low-power energy-efficient operation.
To investigate the TI NR-channel material's electronic properties like the electronic bandstructure, the density of states (DOS), the local carrier density, and transport properties like the transmittance, and current through the channel by considering spin, spin orbit coupling (SOC) in non-trivial (i.e. quantum spin hall phase) and trivial insulating (i.e. conventional insulator) phases the Non-Equilibrium Green’s Function (NEGF) formalism has been used.Finally, to illustrate the dynamics of ferroelectric material for finding out the interplay of Gibb's free energies between ferroelectric and TI NR-channel material, and to obtain the stability criterion of the whole system, the Landau-Khalatnikov equation has been used. By maintaining an appropriate ferroelectric material thickness, it hasbeen shown that the channel voltage has been amplified. In this way, at lower gate voltage the desired operation in low voltage region can be attained.
At the on-state, it is found that the channel current density is 1141μA/μm(total channel current is, 6.15μA) at 0.08 Volt bias, which is higher than the IRDS 2021 HD requirement (〖 I〗_on value of 861μA/μm). The calculated maximum ‘effective channel capacitance’ value, is 0.252pF/〖μm〗^2, and the delay timeis 1.15ps. Moreover, the dynamic energy dissipation (DED) is 0.5634 aJ/switching event, much lower than IRDS 2021 HD requirement (0.33fJ/switching event).The current on/off ratio is 1060 (above 1000) and an average SS value of 130 mV/decade has been found for total 14 nm of ferroelectric thickness. Overall, all of these aforementioned results support an energy-efficient FET that enables low-power nanoscale operation.