Research On Models And Characteristics Of Amorphous InGaZnO Thin-film Transistors

Amorphous InGaZnO(a-IGZO)Thin-Film Transistors(TFTs)are regarded as the most representative in amorphous oxide semiconductor(AOS)TFTs.They usually possess more comprehensive advantages compared with amorphous silicon(a-Si)and polysilicon(poly-Si)TFTs,such as the room-temperature fabrication process,mobility that could exceed 10 cm~2/Vs,good device uniformity and low fabrication cost.Therefore,a-IGZO TFTs are not merely desirable choices for flat-panel display technology,but can also enable transparent and large-area flexible electronics,and various electronic applications in many emerging fields,for instance,the embedded biomedical applications,wearable electronics,radio-frequency identification tags for the internet of things,virtual reality and augmented reality technologies.Circuit design and simulations based on a-IGZO TFTs require physical models with good accuracy and better interpretation on device characteristics.Effects of the device structure and material properties of the channel layer on electrical characteristics of a-IGZO TFTs are investigated and modeled physically in this dissertation,with the major contents summarized as follows.1.Electrical characteristics of the single-gate and dual-gate(DG)a-IGZO TFTs have been investigated based on the two-dimensional numerical model for devices and characteristic distributions of subgap density of states(DOS)in a-IGZO.Influences of the channel layer thickness and the interface coupling effects on DC performances of the device have been analyzed.2.By calculating the normalized noise spectral density of drain current caused by carrier number fluctuations,a physical model for low frequency noise of a-IGZO TFTs in the subthreshold region has been derived.In the low gate voltage region,we consider that the drain current noise of the device originates from both the multiple trapping and release of free carriers in the band tail states and the exchanging of carriers between the channel and border traps in the gate oxides.Distribution parameters of the subgap DOS are determined according to the DC characteristics of the device in the subthreshold region,and the caused drain current noise can be obtained,then the total low frequency noise is modeled by adding the noise contributed from the border traps.Modeling results and analysis indicate that the low frequency noise behavior of a-IGZO TFTs is significantly affected by the subgap DOS,with the normalized current noise spectrum index depending on the characteristic temperature of tail states.3.Considering the percolation transport of carriers in the Gaussian-like distribution of potential barriers above the conduction band edge,the carrier density-and temperature-dependent carrier mobility of a-IGZO has been derived based on the Boltzmann transport equation.The physical drain current model is then developed covering the non-degenerate and degenerate conductions.This model reflects the variations of carrier mobility and current-voltage characteristics of a-IGZO TFTs with the temperature.Modeling results show that the temperature effects on DC performances of the device originate from the change of carrier mobility with the temperature.And the carrier mobility under a certain carrier density and temperature is closely related to the mean free path of carriers,the mean and variance of the potential barriers.4.Circuits and systems based on DG a-IGZO TFTs can realize faster,more stable and less power consuming operations compared with the single-gate-based ones.By solving electrostatic potentials at the surface and middle of the a-IGZO channel layer,a DC model for synchronized symmetric DG a-IGZO TFTs is developed based on the drift and diffusion components of drain current.The threshold voltage of the device is defined as the gate voltage where the drift current equals the diffusion one.A concise drain current expression is then presented,giving good reproduction of current-voltage characteristics of the device.5.Based on an effective characteristic temperature,a channel potential model and non-charge-sheet analytic drain current expression for the asymmetric DG a-IGZO TFTs are derived.This gives solutions for potential distributions in the channel layer and current-voltage characteristics of the device under arbitrary DG biases with reasonable accuracy.Considering the second-gate effects on the threshold voltage in asymmetric DG devices,a threshold voltage definition is given based on the characteristic distribution of subgap DOS in a-IGZO,which explains the variations of threshold voltage with the second-gate bias and distribution parameters of the subgap states.A compact DC model for the asymmetric DG a-IGZO TFTs covering all operation regions is further developed based on the defined threshold voltage,with detailed parameter extraction and determination strategies.