Stability of unhydrogenated polysilicon thin film transistors and structural quality of the channel material

The performance of polysilicon thin film transistors used in large-area ele ctronics applications, directly depends on the structural quality of the ch annel material. Moreover, their stability under electrical stress is shown, in this work, to also depend on the quality of the channel material. TFTs were fabricated using several channel materials, deposited as an amorphous film by low-pressure chemical vapor deposition (LPCVD), and then crystalliz ed using solid-phase annealing, a large-area pulsed excimer laser, or scann ing with a 532-nm beam of a pulsed diode pumped Nd:YVO4 laser. The stabilit y of the TFTs, determined from the increase in the subthreshold slope S, is shown to be related to the importance of the surface roughness and to the structural quality of the crystallized active layer. With similar surface r oughness, the stability is better when the structural quality of the active layer is improved. The increase in S is then explained by the creation of a state in the channel material that is more effective when the structure o f the polysilicon is more disordered. (C) 2001 Elsevier Science B.V. All ri ghts reserved.