Contact Resistance in Nanocrystalline Silicon Thin-Film Transistors

Thin-film transistors (TFTs) of nanocrystalline silicon (nc-Si:H) made by plasma-enhanced chemical vapor deposition have higher electron and hole field-effect mobilities than their amorphous counterparts. However, as the intrinsic carrier mobilities are raised, the effective carrier mobilities easily can become limited by the source/drain contact resistance. To evaluate the contact resistance, the nc-Si:H TFTs are made with a range of channel lengths. The TFTs are fabricated in a staggered top-gate bottom source/drain geometry. Both the intrinsic and the - or -doped nc-Si:H source/drain layers are deposited at 80-MHz excitation frequency at a substrate temperature of 150 . Transmission electron microscopy of the TFT cross section indicates that crystallites of doped nc-Si:H nucleate on top of the Cr source/drain contacts. As the film thickness increases, the crystallites coalesce, and the leaf-shaped crystal grains extend through the doped layer to the channel i layer. The contact resistance is estimated by measuring IDSfor several channel lengths at fixed gate and drain voltages. The results show that the contact resistance depends on the gate voltage and that the source/drain current of these TFTs at VDS=10 V becomes limited by the contact resistance when the channel length is less than 10 mum for n-channel and less than 25 mum for p-channel.