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The negative gate bias stress (NBS) reliability of n-type polycrystalline silicon (poly-Si) thin-film transistors (TFTs) with a distinct defective grain boundary (GB) in the channel is investigated. Results show that conventional NBS degradation with negative shift of the transfer curves is absent. The on-state current is decreased, but the subthreshold characteristics are not affected. The gate bias dependence of the drain leakage current at V ds of 5.0 V is suppressed, whereas the drain leakage current at V ds of 0.1 V exhibits obvious gate bias dependence. As confirmed via TCAD simulation, the corresponding mechanisms are proposed to be trap state generation in the GB region, positive-charge local formation in the gate oxide near the source and drain, and trap state introduction in the gate oxide. [ABSTRACT FROM AUTHOR]

Nitrogen doping is introduced in elevated-metal metal-oxide (EMMO) thin-film transistors (TFTs) by sputtering amorphous indium gallium zinc oxide (a-IGZO) channel in Ar and N2 gas mixture. The electrical characteristic and reliability of TFTs under negative/positive bias illumination stress (N/PBIS) are systematically investigated on TFTs of different channel lengths (${L}\text{s}$). Compared with undoped TFTs, the short-channel effect (SCE) of the N-doped TFTs is significantly suppressed, the persistent photoconductivity (PPC) effect is weakened, and N/PBIS reliability is largely improved. Short- and long-channel N-doped TFTs have about the same reliability performance. X-ray photoelectron spectroscopy (XPS) analysis shows that N-doping forms Zn=N bonds in the channel and oxygen vacancies (${V}_{O}$) are reduced. Based on a group of TFTs with different ${L}\text{s}$ , channel mobility ($\mu _{{\mathrm {ch}}}$) and source–drain series resistance (${R}_{{\mathrm {sd}}}$) are correctly extracted. In N-doped TFTs, $\mu _{{\mathrm {ch}}}$ has a limited decrease owing to the increase in ${R}_{{\mathrm {sd}}}$. N-doped TFTs with different Ar/N2 gas-flow ratios show similar electrical and reliability performance, indicating a wide process window. [ABSTRACT FROM AUTHOR]

The active region of organic thin film transistors (OTFTs) is usually sensitive to moisture and O2 in the air ambient, which hinders their practical applications. In this study, the effects of air ambient on characteristics of unpassivated 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene (dif-TES-ADT) OTFTs are clarified. The device is much more sensitive to H2O vapor than O2, although both of which cause TFT instability. To improve the environmental stability, inkjet-printed passivation of the fluoropolymer CYTOP and UV curing adhesive are compared. It is found that the CYTOP passivation is well compatible with the organic channel while the UV curing adhesive has excellent barrier ability to both H2O vapor and O2. Then CYTOP/UV adhesive bilayer passivation combining advantages of both materials is introduced to achieve stable operation of OTFTs, which can resist saturated H2O vapor and O2 for 25 days. [ABSTRACT FROM AUTHOR]

Amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) severely degrade under AC gate bias stress and AC drain bias stress, whose transfer curves respectively undergo positive shifts of 6.3 and 14.2 V after a stress time of 3000 s with a stress amplitude of 20 V. In this study, annealing at 400 °C in O2 atmosphere is performed to effectively reduce the acceptor-like trap states in the a-IGZO channel and the electric field in the etching-stop layer under the extended drain electrode, which participate in dynamic and DC degradation mechanisms, respectively. Thus, a-IGZO TFTs exhibiting excellent stability without any shift of the transfer curve under the same AC gate bias stress and AC drain bias stress are experimentally demonstrated simultaneously. [ABSTRACT FROM AUTHOR]

Dynamic degradation becomes a critical issue for thin-film transistors (TFTs) used in emerging new displays driven by high frequency gate voltage ${V}_{\text {G}}$ pulses. In this study, ${V}_{\text {G}}$ pulse rising edge dependent dynamic hot carrier degradation of poly-Si TFTs is investigated. It is demonstrated that rising edge of ${V}_{\text {G}}$ pulses which swing within the OFF-state of TFTs causes the degradation, while that of normal ${V}_{\text {G}}$ pulses which switch between the ON and OFF states across the flat band voltage ${V}_{\text {FB}}$ of TFTs does not. Based on transient TCAD simulation, the underlying mechanism of rising edge dependent degradation is proposed, which is based on the non-equilibrium PN junction degradation model previously proposed to explain ${V}_{\text {G}}$ pulse falling edge dependent degradation of poly-Si TFTs. Hence, the dynamic degradation model of poly-Si TFTs related to both rising and falling edge of the ${V}_{\text {G}}$ pulses can be unified now, which is applicable to fast gate pulses with steep rising and/or falling edges in sub- $\mu \text{s}$ level. [ABSTRACT FROM AUTHOR]

In this letter, the carrier mobility of amorphous InGaZnO (a-IGZO) thin-film transistor (TFT) was remarkably enhanced by the introduction of nitrogen and the formation of Zn3N2, in which the saturation field-effect mobility ($\mu _{\text {sat}}$) was 61.6 cm2/Vs. Annealing temperature plays a key role on the enhancement of carrier mobility. When the annealing temperature was increased to 400 °C, $\mu _{\text {sat}}$ was reduced to 4.1 cm2/Vs, which was proposed to be due to the formation of defective ZnxNy based on X-ray photoelectron spectroscopy results. In addition, the a-IGZO TFT with enhanced mobility did not exhibit persistent photoconductivity behavior. The high carrier mobility could expand the application of a-IGZO TFTs to functional circuits in active-matrix displays. [ABSTRACT FROM AUTHOR]

The degradation mechanism of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) under pulse drain bias stress is investigated. Although the degradation mechanism is found to be a dc one, the shift of the transfer curve under the pulse drain bias (0–20 V) is unexpectedly larger than that under dc drain bias (20 V) within the same equivalent stress time. The degradation mechanism is proposed to be tunneling and trapping of electrons in the etching stop layer during the pulse peak time and that in the gate dielectric during the pulse base time under the extended drain electrode region of the a-IGZO TFT, where the occurrence of the latter is triggered by the former. A solution for enhancing the stability of the a-IGZO TFT is also suggested. [ABSTRACT FROM AUTHOR]

Roles of gate voltage (VG) and stress power in self-heating (SH) degradation of amorphous InGaZnO thin-film transistors have been clarified. Normally observed positive threshold voltage (Vth) shift is attributed to the electron trapping mechanism, which is enhanced by both the effective VG and stress power. The effective VG plays a dominant role in most cases, while stress power takes control only if it is high enough. Following the positive Vth shift, a second-stage negative Vth backshift is also observed and is attributed to the water-related positive charges generation mechanism. It is enhanced by both stress power and effective VG for which stress power is found to be the dominant factor. [ABSTRACT FROM AUTHOR]

Effects of low-temperature annealing in nitrogen atmosphere on elevated-metal metal-oxide (EMMO) thin-film transistors (TFTs) are investigated and reported in this article. Compared with the short-circuit behavior of EMMO TFTs after annealing in nitrogen atmosphere at 300 °C, it is found that TFTs received annealing in nitrogen atmosphere at 200 °C prior to that at 300 °C not only remain the transfer characteristics, but also exhibit improved on-state current and subthreshold characteristics. Both the results of X-ray photoelectron spectroscopy (XPS) and persistent photoconductivity (PPC) confirmed the reduction of oxygen vacancies in the a-IGZO after annealing in nitrogen atmosphere at 200 °C, indicating that the passivation of oxygen vacancy with nitrogen is dominant rather than the generation of oxygen vacancy. Furthermore, the thermal stability improvement could not be realized when the annealing in nitrogen atmosphere at 200 °C was performed before the source/drain formation annealing at 400 °C, which emphasizes the importance of annealing procedures during device fabrication. [ABSTRACT FROM AUTHOR]

Different from the conventional degradation phenomenon under positive bias stress (PBS), the shift of the transfer characteristic curve of polycrystalline silicon thin-film transistors (TFTs) to the negative gate bias direction after PBS is observed and reported. The PBS degradation is found to be recoverable and the recovery proceeds at a faster rate first and then continues at a much slower rate. The recovery can be further accelerated at a higher temperature or by applying a negative gate bias. After detailed data analysis, the degradation mechanism is proposed to be the generation of protons in the gate oxide and its accumulation at the channel/gate oxide interface. The proposed degradation model could explain both the degradation phenomena and the recovery behaviors of PBS degradation. [ABSTRACT FROM AUTHOR]

Persistent photoconductivity (PPC) effect and its light-intensity dependence of both enhancement and depletion (E-/D-) mode amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) are systematically investigated. Density of oxygen vacancy (VO) defects of E-mode TFTs is relatively small, in which formation of the photo-induced metastable defects is thermally activated, and the activation energy (Ea) decreases continuously with increasing light-intensity. Density of VO defects of D-mode TFTs is much larger, in which the formation of photo-induced metastable defects is found to be spontaneous instead of thermally activated. Furthermore, for the first time it is found that a threshold dose of light-exposure is required to form fully developed photo-induced metastable defects. Under low light-exposure below the threshold, only a low PPC barrier is formed and the PPC recovery is fast. With increasing the light-exposure to the threshold, the lattice relaxation of metal cations adjacent to the doubly ionized oxygen vacancies () is fully developed, and the PPC barrier increases to ∼ 0.25 eV, which remains basically unchanged under higher light-exposure. Based on the density of VO defects in the channel and the condition of light illumination, a unified model of formation of photo-induced metastable defects in a-IGZO TFTs is proposed to explain the experimental observations. [ABSTRACT FROM AUTHOR]

The origin of an abnormal spontaneous degradation of low-temperature polycrystalline Si (LTPS) thin-film transistors (TFTs) is investigated. After being subjected to dynamic bending stress, LTPS TFTs will continuously degrade when stored in a normal condition. Studies have shown that the interaction between the sensitive area of the TFTs and the O2 in ambient environment causes the spontaneous degradation. Defects at the gate oxide/channel interface generated by mechanical stress continuously capturing electrons with the help of oxygen is regarded as the origin of spontaneous degradation. In contrast, moisture in air can effectively suppress spontaneous degradation by reacting with the above defects to form relatively stable atom groups. [ABSTRACT FROM AUTHOR]

Characteristics of amorphous indium–gallium–zinc oxide (a-IGZO) thin-film transistors (TFTs) are highly dependent on the hydrogen (H) content within the device architecture, for example in the etch-stop layer (ESL) of the elevated-metal metal-oxide (EMMO) TFTs. The serious apparent “short-channel effect (SCE)” was caused by the H diffusion from source/drain (S/D) to channel. Such SCE deterioration can be suppressed by thermal dehydrogenation at a cost of long annealing time, especially for a transistor architecture with a-IGZO S/D covered with H-rich silicon nitride (SiNx:H) and further capped with metallic H-diffusion barrier. The dehydrogenation efficiency is found to be significantly enhanced by fluorinating the a-IGZO. By optimizing the device architecture and/or the fluorination process to enhance the dehydrogenation, the SCE can be efficiently eliminated with well-maintained performance even for 2~μm-long a-IGZO TFTs. [ABSTRACT FROM AUTHOR]

A physical-based analytical kink current model for polycrystalline -silicon (poly-Si) thin-film transistors (TFTs) has been proposed. Two important mechanisms for the kink current, namely carrier impact ionization and the parasitic bipolar junction transistor effect, are physically included in the model without introducing any artificial parameters. The proposed kink current model is fully compatible with existing ON-state drain current models of poly-Si TFTs. Model parameter extraction procedure is presented, which is based on a group of output characteristics of poly-Si TFTs with different channel lengths. The model prediction straightforwardly calculated with a set of extracted parameters is verified by excellent agreement with experimental output characteristics measured from TFTs over a range of channel lengths and gate voltages. [ABSTRACT FROM AUTHOR]

Two-stage degradation of p-type polycrystalline silicon thin-film transistors under dynamic positive bias temperature (PBT) stress is reported for the first time. ON-state current ( \(I_\mathrm{{\scriptstyle ON}}\) ) gradually increases in the first degradation stage while dramatically drops in the second degradation stage, which are, respectively, due to the channel length shortening effect, caused by electrons trapping into the gate oxide, and grain boundary potential barrier buildup, caused by dynamic hot-carrier-induced traps generation. The transition from the first to the second degradation stage is clarified, in which the pulse peak duration plays a key role. Longer pulse peak duration is preferred to suppress the dynamic PBT stress-induced degradation. [ABSTRACT FROM AUTHOR]