Physics and technology of advanced polysilicon transistors and memory devices

Polysilicon TFTs (thin-film transistors) have wide applications in SRAM (static random access memory) and AMLCD (active matrix liquid crystal displays). The major issues are to increase the ON-state drive current and to decrease the OFF-state leakage current. Low drive current is caused by the low effective mobility in the polysilicon channel due to grain boundary scattering. High leakage current is mainly caused by field emission via grain boundaries due to the high electric field near the drain.; To increase the drive current, a vertical polysilicon TFT has been developed to achieve a submicron channel length beyond lithographic limitations. The vertical TFT is compact, yet offers high current drive. Short channel effects in polysilicon TFTs are investigated.; To decrease the leakage current, a novel floating gate spacer (FGS) polysilicon TFT has been designed and fabricated. The FGS TFT effectively reduces the leakage current by reducing the drain field, while maintaining a high drive current. It also suppresses the kink effect in short channel devices. Compared with conventional TFTs and LDD (lightly doped drain) TFTs with oxide spacers, the FGS TFTs have demonstrated an improved ON/OFF current ratio.; To extend the polysilicon TFT technology to even wider applications, a planar polysilicon Twin TFT EEPROM (electrically erasable programmable read only memory) cell has been designed and fabricated. The cell can be both programmed and erased by the Fowler-Nordheim tunneling mechanism. It is suitable for low cost and low thermal budget memory applications.; Some novel concepts for vertical polysilicon EEPROMs are introduced, which could be useful in FPGA (field programmable gate arrays) and analog neural network applications.