TWO-TERMINAL VERTICAL THYRISTOR BASED CAPACITORLESS MEMORY CELL USING LATCH-UP CHARACTERISTICS

摘要

Conventional dynamic random access memory (DRAM) has been facing a severe challenge to scale down the capacitor because the cell capacitor should be able to store sufficient charges of 25-30 fF/cell. Many kinds of emerging devices have been proposed to overcome this challenge coming from capacitor; i.e., ReRAM, CBRAM, PRAM, and STT-MRAM. However, the vertical thyristor based two-terminal capacitorless memory is a candidate to replace current DRAM, which consists of only one p~(++)n~+p~+n~(++) vertical structure diode using conventional Si technology. The two-terminal vertical thyristor based capacitorless memory cell having p~(++)(anode)n~+(n-base)p~+(p-base)n~(++)(cathode) structure is easily able to construct cross-point memory array-cells, as shown in Figure 1. It utilizes latch-up characteristics showing bi-stable current-voltage (I-V) characteristics, which are generated by n~+p~+ base region under an applied bias. However, the demonstration of the device has not been successfully performed yet because of its difficult device fabrication process (i.e., dopant fluctuation at the p junction interface, presence of dislocations & stacking faults depending on dopant concentration). In our study, the dependency of the latch-up characteristics (i.e., bi-stability, l_(on)/l_(off) ratio) on dopant concentration and thickness of n~+p~+ base were investigated by performing simulations, as shown in Figure 2. I-V characteristics in Fig. 2 exhibited that latch-up voltage increased with the dopant concentration of n~+p~+ base, while the latch-up characteristic was not established at dopant concentration of 1 x 10~(17) cm~(-3). It was confirmed that the latch-up voltage increasing with dopant concentration of n~+p~+ base region was associated with the hole and electron diffusion barrier in n+p+ base region. In addition, the latch-up voltage was in detail investigated as a function of the n~+(n-base)p~+(p-base) concentration independently. Furthermore, we will review the device physics of this memory cell and demonstrate memory cell operation.