Gated-four-probe a-Si:H thin-film transistor structure

In a-Si:H TFTs, the source/drain series resistances not only influence the TFT electrical performance but also cause difficulties in optimization of the channel material. To understand the intrinsic characteristics of a-Si:H TFTs, the source/drain series resistances have to be excluded. Source/drain series resistances in a-Si:H TFTs consist of: (1) contact resistances (between the source/drain metal and the n/sup +/ a-Si:H layer) and n/sup +/ a-Si:H film resistance; (2) bulk resistance (due to the intrinsic a-Si:H layer between the n/sup +/ a-Si:H and the conducting channel); (3) resistances associated with the overlap between the source/drain and gate electrodes. To analyze the influence and the origin of source/drain series resistances, we have developed a new gated-four-probe (GFP) a-Si:H TFT structure to accurately measure the intrinsic characteristics of a-Si:H TFTs. In the GFP a-Si:H TFT structure, two narrow probes are placed between the source and drain electrodes to sense the voltage difference along the conducting channel. By correlating this voltage difference to the source/drain current induced by applied gate bias, the a-Si:H TFT intrinsic characteristics, such as mobility, threshold voltage, and channel conductance activation energy can be accurately determined without any influence from the source/drain series resistances. Another function of the GFP a-Si:H TFT structure is to investigate the electrical instability mechanisms responsible for gate bias-stress induced apparent threshold voltage shifts.