The FET-Type FeRAM

Abstract

The current status of the fabrication and integration of ferroelectric-gate field effect transistors (FETs) is reviewed. Novel applications of ferroelectric-gate FETs are first discussed, which include a single-transistor-cell-type digital memory, reconfigurable LSIs, and an analog memory for an artificial neural network. In the neural network application, the fabrication and basic operation of a pulse-frequency-modulation-type adaptive-learning neuron circuit are described, and also anSOI (silicon-on-insulator)-type FET array for synaptic connection is proposed. Then, the data retention characteristics of ferroelectric-gate FETs with MFIS (M, metal; F, ferroelectric; I, insulator (buffer layer); S, semiconductor) and MFMIS structures are discussed. It is shown that the important factors in improving the data retention characteristics are (1) increasing the buffer layer capacitance, (2) decreasing the leakage current of both the ferroelectric film and the buffer layer, and (3) optimization of the area ratio between the MFM and MIS parts in the MFMIS structure. In the experiment, the properties of novel ferroelectrics such as Sr$_{2}$(Ta,Nb)$_{2}$O$_{7}$ and Pb$_{5}$Ge$_{3}$O$_{11}$ are discussed as well as those for the conventional ferroelectrics. Finally, a novel FET-type FeRAM with a 1T-2C structure is introduced and some experimental results are presented.