The basic physics underlying the operation and key performance issues of amorphous-silicon thin-film transistors (TFTs) are discussed. The static transistor characteristics are determined by the localized electronic states that occur in the bandgap of the amorphous silicon. The deep states, mostly consisting of Si dangling bonds, determine the threshold voltage, and the conduction band-tail states determine the field-effect mobility. The finite capture and emission times of the deep localized states lead to a dynamic transistor characteristic that can be described by a time-dependent threshold voltage. The transistors also show longer time threshold voltage shifts due to two other distinct mechanisms: charge trapping in the silicon nitride gate insulator and metastable dangling bond state creation in the amorphous silicon. These two mechanisms show characteristically different bias, temperature, and time dependencies of the threshold voltage shift. Illumination of a TFT causes the generation of electron-hole pairs in the space-charge region leading to a steady-state equal flux of electrons and holes and a reduction in the band-bending. In most applications, the photosensitivity should be minimized. The uniformity of large arrays of transistors for display applications is excellent, with variations in the threshold voltage of 0.5-1.0 V. >

The physics of amorphous-silicon thin-film transistors

In this paper we have analyzed the influence of the mask channel length (LM) on the performance of the 55‐nm‐hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors (TFTs), incorporating nitrogen‐rich hydrogenated amorphous silicon nitride gate dielectric and phosphorus‐doped microcrystalline silicon (n+μc‐Si:H) source/drain (S/D) contacts. In our TFTs the n+μc‐Si:H S/D contacts have a specific contact resistance around or below 0.5 Ω cm2. We have shown that in our TFTs a field‐effect mobility and threshold voltage are dependent on LM, and this dependence is most likely due to the influence of the S/D contact series resistance on TFTs characteristics. Finally, we have demonstrated that if the mask channel length is extended by a ΔL (which is a distance from the S/D via edge at which the electron injection/collection is taking place) the field‐effect mobility and threshold voltage are independent of the channel length. In such a case μFE, VT, and ON/OFF current ratio around 0.76 cm2/V s, 2.5 V, and 1...

Performance of thin hydrogenated amorphous silicon thin‐film transistors

Bias-temperature-stress (BTS) induced electrical instability of the RF sputter amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) was investigated. Both positive and negative BTS were applied and found to primarily cause a positive and negative voltage shift in transfer (I DS-V GS) characteristics, respectively. The time evolution of bulk-state density (N BS) and characteristic temperature of the conduction-band-tail-states (TG) are extracted. Since both values showed only minor changes after BTS, the results imply that observed shift in TFT I DS-V GS curves were primarily due to channel charge injection/trapping rather than defect states creation. We also demonstrated the validity of using stretch-exponential equation to model both positive and negative BTS induced threshold voltage shift (DeltaV th) of the a-IGZO TFTs. Stress voltage and temperature dependence of DeltaV th evolution are described.

Electrical Instability of RF Sputter Amorphous In-Ga-Zn-O Thin-Film Transistors

We studied both the wavelength and intensity dependent photo‐responses (photofield‐effect) in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors (TFTs). During the a‐IGZO TFT illumination with the wavelength range from 460∼660 nm (visible range), the off‐state drain current (IDS_off) only slightly increased while a large increase was observed for the wavelength below 400 nm. The observed results are consistent with the optical gap of ∼3.05eV extracted from the absorption measurement. The a‐IGZO TFT properties under monochromatic illumination (λ=420nm) with different intensity was also investigated and IDS_off was found to increase with the light intensity. Throughout the study, the field‐effect mobility (μeff) is almost unchanged. But due to photo‐generated charge trapping, a negative threshold voltage (Vth) shift is observed. The mathematical analysis of the photofield‐effect suggests that a highly efficient UV photocurrent conversion process in TFT off‐region takes place. Finally, a‐IGZO mid‐g...

https://www.tandfonline.com/doi/pdf/10.1080/15980316.2008.9652066

Photofield‐effect in amorphous In‐Ga‐Zn‐O (a‐IGZO) thin‐film transistors

A thin film transistor is connected to a picture element displaying electrode formed on an insulating base plate so that a voltage is applied to the electrode. The thin film transistor comprises a gate electrode, one or more drain electrodes connected to the picture element displaying electrode and a source bus line which -applies a voltage to one or more source electrodes connected thereto, the source bus line functioning by itself as a source electrode, wherein the one drain electrode is formed between the source bus line and one of the one source electrode connected to the source bus line, or between the source electrodes adjacent to each other.

Thin film transistor, method of repairing the thin film transistor and displaying apparatus having the thin film transistor

Amorphous silicon staggered‐electrode thin‐film transistors (TFT’s) can show current crowding near the origin in the output characteristics. The degree of current crowding is governed by the voltage dependence of the current flowing from the n+ contact to the conducting channel. This current is a space‐charge‐limited current whose magnitude depends on the bulk density of states in the undoped intrinsic layer. For a 0.5‐μm‐thick i layer, calculations predict negligible current crowding for N(E) 3×1016 cm−3 eV−1. Experimental results are consistent with N(E) in the range 1016 cm−3 eV−1–2×1016 cm−3 eV−1. This is lower than the value derived from the transfer characteristic of the TFT (∼1017 cm−3 eV−1), which is evidence for an inhomogeneous distribution of deep gap states through the 0.5‐μm film of α‐Si:H.

J. W. Orton

Papers

Characteristics of amorphous silicon staggered-electrode thin-film transistors