Study On Fabrication And Properties Of P-Type Thin-Film Transistors With High Bias-Stress Stability

Thin-film transistors(TFTs),as a key component of electronic devices,have the advantages of low cost,easy to fabricate in large areas,solution processing,and compatibility with flexible substrates,and thereby TFTs are commonly regarded as the basic components of the next generation of electronic devices.With the recent advances in semiconductor material synthesis and thin film preparation technology,the TFTs have been far superior to the traditional silicon-based transistors in operational performances such as carrier mobility,on/off ratio,and so on.These superiorities have made the TFTs meet the commercial needs.However,all TFTs face their intrinsic disadvantages in biasstress instability,which might lead to the inferior stability and reliability of the TFTdriven devices and integrated circuits.The bias-stress instability of TFTs has become the huge stone in the way of the TFT industrial applications.In this thesis,the purpose of the study is to investigate the physical mechanism of bias-stress instability of TFTs,and further to explore ways to improve the bias-stress stability of TFTs.The main works of this thesis are as follows:(1)The mechanisms of bias-stress instability of TFTs were studied.A series of physical models were proposed to analyze the electron injection,the carrier capture and neutralization,and the bias-stress stability related to carrier dynamics in TFTs.The presented results revealed the main reason of the bias-stress instability of TFTs.That is the minor carriers injected from the drain electrode trigger the major carrier capturing,recombination and neutralization in semiconducting active layer,which induces persistent capturing and consumption of major carriers and creates the trap states,and thereby leading to significant decrease in the density of major carriers and the corresponding bias-stress instability of TFTs,such as threshold voltage shift,attenuation of operational current,and so on.(2)A method was proposed to improve the bias-stress stability of P-type organic thin-film transistors by modifying the source-drain electrodes with organic wide-bandgap materials.The effectiveness and the universality of this method was examined.By adopting this method,a large energy gap is formed by the Fermi level of the drain electrode and the lowest unoccupied molecular orbital(LUMO)of the organic widebandgap material.As a result,an electron injection barrier is constructed at the metal/organic interface,which can prevent the electron injection and therefore improve the bias-stress stability of the devices.(3)It is determined that the electron injection from the drain electrode to the organic semiconductor channel is in accordance with the Fowler-Nordheim tunneling theory.Using the wide-bandgap organic semiconductors with higher LUMO level,it can increase the electron injection barrier at the metal/organic interface,which benefits to reduce the electron injection efficiency,and thereby reducing the hole capturing and recombination,and improving the bias-stress stability of the P-type organic thin-film transistors.The presented results provide a general method for improving the bias-stress stability of OTFTs by modifying the source-drain electrodes with wide-bandgap semiconductors.The relative physical models were also proposed to provide guidance for designing OTFTs with better performances.