A narrow bandgap SiGe channel superlattice bandgap engineered 1T DRAM cell for low voltage operation and extended hole retention time

Abstract

We propose a SiGe channel superlattice bandgap engineered (SiGe SBE) 1-transistor dynamic random access memory (1T DRAM) cell structure for improved generation and extended retention of hot holes adopting a narrow bandgap Si0.8Ge0.2 channel even with an extremely short gate length of 30 nm. The proposed SiGe channel SBE 1T DRAM shows longer retention time than the Si channel SBE 1T DRAM. It also provides improved design flexibility by optimizing the structural and process parameters, so the retention characteristics get better. Especially, it should be noted that the retention time can be further improved if the doping concentration of the Si buffer layer decreases. The narrow bandgap SiGe channel SBE structure also allows the 1T DRAM cell to generate more electron–hole pairs during the write ‘1’ operation through the impact ionization in the channel under a high electric field. In addition to the long retention time with the SBE structure, the narrow bandgap SiGe channel SBE 1T DRAM cell enables the 1T DRAM cell to have a fast write speed and to operate at lower voltage, thanks to the narrow bandgap Si0.8Ge0.2 channel.