Small geometry effects in n- and p-channel polysilicon thin film transistors

The authors describe the results of an experimental study of small-geometry effects in n- and p-channel polysilicon TFTs (thin film transistors) fabricated on quartz substrates. Short-channel effects are shown to be severe, with significant threshold shifts observed in devices with gate lengths of less than about 8 μm and degradation in drain breakdown voltages for gate lengths below about 5 μm. The performance of simple digital CMOS circuits fabricated using TFTs with a range of gate lengths is also reported, and the improvement in speed achieved by reducing gate lengths is shown. In particular, the operation of shift registers, designed using 5-μm-long TFTs at clock frequencies in excess of 50 MHz, is demonstrated.