1. Field of the Invention
The disclosure relates to a field effect transistor, a memory element and a manufacturing method of a charge storage structure; in particular, to field effect transistor, a memory element and a manufacturing method of a charge storage structure applying a paraelectric material and a ferroelectric material.
2. Description of Related Art
Currently existing ferroelectric field effect transistors (“FeFET”) can be applied in a nonvolatile memory array to store data. Specifically, each of the FeFETs includes a semiconductor substrate having a drain region and a source region, a gate disposed over the semiconductor substrate and a ferroelectric layer disposed between the semiconductor substrate and the gate.
The operating principle of the FeFET is that a polarization direction of the ferroelectric layer can be switched by applying a positive or negative bias to the gate, and the conductance of a channel defined between the source region and the drain region is determined by the polarization direction of the ferroelectric layer. Accordingly, by applying a threshold voltage to the gate for switching the polarization direction of the ferroelectric layer, the resistivity of the FeFET can be changed, and then a state (such as “1” or “0”) which has been written into the FeFET can be read by measuring the resistivity of the FeFET.
The conventional ferroelectric layer is usually made of a material having a perovskite structure, such as Pb(Zr,Ti)O3 (PZT) or SrBi2Ta2O9 (SBT). However, both the difficulty of the process and the cost for forming a perovskite structure material on a semiconductor substrate are much higher, and the process is not compatible with current silicon fabrication. Accordingly, the cost of the memory device in which the FeFETs are applied is still less competitive than conventional DRAM and NAND memories. Furthermore, only when a thickness of the ferroelectric layer made of PZT or SBT is greater than 200 nm, the ferroelectric layer may exhibit ferroelectric characteristic. It turns out that downsizing of the FeFET is more difficult.
Accordingly, it is highly desirable for one of ordinary skill in the art to develop a material, which is thinner, chipper and easily fabricated, to replace PZT or SBT for being applied to field effect transistors in a memory array.