A novel Si/SiGe sandwich polysilicon TFT for SRAM applications

In this work, the performance of polysilicon TFTs has been improved for the first time by introduction of a thin polycrystalline silicon-germanium alloy layer sandwiched between thin polysilicon films in the channel of the TFTs. The bandgap discontinuity in the valence band between polysilicon and SiGe confines holes in the middle of the channel away from the high defect-density polysilicon-oxide interface. From the process point of view, it is better to have a sandwich layer than having SiGe along the entire depth of the channel because oxides on SiGe are known to be of bad quality.