Research On The Retention Characteristics Of Ferroelectric Field-Effect Transistor

Ferroelectric memories have good application prospects in computer, aerospace and military industry because of many advantages such as low power consumption, high speed, high endurance, high density, antiradiation, and non-volatility. As one of ferroelectric memories, ferroelectric field-effect transistor (FeFET) has attracted considerable interests due to its simple unit structure, higher storage density, and obeying the scaling rule of ultralarge-scale-integration circuits. However, FeFET has not been put into practical use until now because of its poor retention characteristics. In this thesis, a retention model for metal-ferroelectric-insulator-semiconductor (MFIS) FeFET is developed based on the depolarization field of the ferroelectric. Effects of the electrical and material parameters and ferroelectric-electrode interfaces on the retention characteristics of FeFET are also studied by using the developed retention model. It is supposed to give some constructive guidelines for the improvement of the retention characteristics. The main research contents and the obtained results are summarized as follows:1. By combining the basic device equations of metal-oxide-semiconductor field-effect transistors with the polarization retention characteristics of ferroelectric thin films, a retention model for FeFET is developed based on the depolarization field of the ferroelectric. This model can give the retention characteristics of FeFET both in short and long time scales, so it provides a way to predict the retention characteristics in long time scale experimentally. Moreover, this model facilitates the direct theoretic study on the retention characteristics of FeFET.2. Effects of the electrical and material parameters on the retention characteristics of FeFET are simulated and discussed based on the developed retention model. The results show that the holding voltage has an important influence on the retention characteristics, and an appropriate holding voltage can significantly improve the retention characteristics. The workfunction of gate electrode, the type and doping concentration of the semiconductor substrate, and the thickness and activation field of the ferroelectric thin films have great impacts on the retention characteristics, and good retention characteristics can be achieved through optimizing the workfunction of gate electrode and the substrate doping concentration. The thickness and the relative dielectric constant of the insulating layer, and the relative dielectric constant and the initial polarization of the ferroelectric thin films have no abviou effects on the retention characteristics.3. Effects of ferroelectric-electrode interfaces on the retention characteristics of FeFET are briefly discussed according to the developed retention model. When the ferroelectric-electrode interface is considered as a dead layer, the effect of ferroelectric-electrode interface on the retention characteristics is the same as that of the insulation later in MFIS structure. When the ferroelectric-electrode interface is considered as a threshold conducting insulator, the remaining space charge is beneficial for the the retention characteristics both in on-state and off-state, which can improve the retention characteristics. The ferroelectric-electrode contact potential has the same effect on the retention characteristics with the holding voltage, and its specific impact is determined by its value and polarity.