Showing papers by "Chang Jung Kim published in 2010"

Extraction of Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors by Using Multifrequency Capacitance–Voltage Characteristics

Abstract: An extraction technique for subgap density of states (DOS) in an n-channel amorphous InGaZnO thin-film transistor (TFT) by using multifrequency capacitance-voltage (C -V) characteristics is proposed and verified by comparing the measured I- V characteristics with the technology computer-aided design simulation results incorporating the extracted DOS as parameters. It takes on the superposition of exponential tail states and exponential deep states with characteristic parameters for N TA = 1.1 × 1017 cm-3 · eV-1, N DA = 4 × 1015 cm-3 · eV-1, kT TA = 0.09 eV, and kT DA = 0.4 eV. The proposed technique allows obtaining the frequency-independent C-V curve, which is very useful for oxide semiconductor TFT modeling and characterization, and considers the nonlinear relation between the energy level of DOS and the gate voltage V GS. In addition, it is a simple, fast, and accurate extraction method for DOS in amorphous InGaZnO TFTs without optical illumination, temperature dependence, and numerical iteration.

Subgap Density-of-States-Based Amorphous Oxide Thin Film Transistor Simulator (DeAOTS)

Abstract: The amorphous oxide thin-film transistor (TFT)-oriented simulator [subgap Density of states (DOS)-based Amorphous Oxide TFT Simulator (DeAOTS)] is proposed, implemented, and demonstrated for amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs. It only consists of parameters having their physical meanings and is supplied with concrete techniques for parameter extraction. Among the physical parameters, the acceptor-like DOS gA(E) was experimentally extracted using the multifrequency C-V technique, whereas the donor-like DOS gD(E) and the doping concentration ND were extracted using numerical iterations. The simulation result reproduces the DOS and thin-film-thickness-dependence of dc I-V characteristics very well. Compared with the previously reported a-Si TFT models, the proposed DeAOTS model not only reflects the strong VGS dependence of the effective mobility (μeff) but also clarifies the relations between process-controlled DOS parameters and dc I- V characteristics based on experimentally extracted DOS parameters. Also, it sufficiently takes into account the peculiar situation of amorphous oxide TFTs where the free-carrier charge can be larger than the localized one out of the total induced charge. Moreover, it reproduces the measured electrical characteristics within the wide range of VGS/VDS with a single equation, not distinguishing the operation regions such as the subthreshold, linear, and saturation regimes.

Modeling and characterization of metal-semiconductor-metal-based source-drain contacts in amorphous InGaZnO thin film transistors

Abstract: Due to the inherent property of large contact and parasitic resistances in amorphous InGaZnO (a-IGZO) thin film transistors (TFTs), a metal-semiconductor-metal (MSM) structure is a key element in a-IGZO TFTs. Therefore, voltage drops across resistances and MSM structure should be fully considered in the modeling and characterization of a-IGZO TFTs. A physics-based semiempirical model for the current-voltage characteristics of the MSM structure for the source-channel-drain contact in a-IGZO TFTs is proposed and verified with experimental results. The proposed model for the current in a-IGZO MSM structures includes a thermionic field emission [JTFE∝exp(VR,Schottky/Vo)] and trap-assisted generation (Jgen∝VR,Schottky) in addition to the thermionic emission current (JS: Independent of the bias) under reverse bias. Experimental result suggests that electrical characteristics of the MSM structure depend not only on the Schottky barrier but also on the bulk property of the a-IGZO active layer.

Low-frequency noise in amorphous indium-gallium-zinc oxide thin-film transistors from subthreshold to saturation

Abstract: We investigate the low-frequency noise (LFN) behaviors of amorphous indium-gallium-zinc oxide thin-film transistors in the subthreshold, Ohmic, and saturation regimes. Measured LFNs are proportional to 1/fγ, with γ=0.8–0.9 in all operation regimes. It is found that the LFN behavior follows the carrier number fluctuation model in the subthreshold regime, whereas in the Ohmic and saturation regimes, it agrees well with the bulk mobility fluctuation model. We also observe that the origin of 1/f noise in the Ohmic regime changes from the bulk mobility fluctuation to the carrier number fluctuation as the channel length decreases.

Self-Consistent Technique for Extracting Density of States in Amorphous InGaZnO Thin Film Transistors

Abstract: The self-consistent technique for extracting density of states [DOS: g(E)] in an amorphous indium gallium zinc oxide (a-IGZO) thin film transistor is proposed and demonstrated. The key parameters are the g(E) of the a-IGZO active layer and the intrinsic channel mobility (μ ch ). While the energy level (E) is scanned by the photon energy and gate-to-source voltage (V GS ) sweep, its density is extracted from an optical response of capacitance-voltage characteristics. Using the V GS -dependent μ ch as another boundary condition, a linearly mapped DOS assuming a linear relation between V GS and E is translated into a final DOS by fully considering a nonlinear relation between V GS and E. The final DOS is finally extracted and verified by finding the self-consistent solution satisfying both the linearly mapped DOS and the measured V GS dependence of μ ch with the numerical iteration of a DOS-based μ ch model. The extracted final DOS parameters are N TA = 1.73 × 10 17 cm -3 eV -1 , N DA = 3.5 × 10 15 cm -3 eV -1 , kT TA = 0.023 eV, KT DGA = 1.2 eV, and E O = 1.7 eV with the formula of exponential tail states and Gaussian deep states.

Impact of High-k HfO2 Dielectric on the Low-Frequency Noise Behaviors in Amorphous InGaZnO Thin Film Transistors

Abstract: We have investigated the impact of high-k HfO2 gate dielectric on the low-frequency noise (LFN) behaviors of amorphous indium–gallium–zinc oxide thin-film transistors by comparing the LFNs of devices with SiO2 and HfO2 dielectrics. Measured LFNs are nearly 1/ f type for both devices, but the normalized noise for the HfO2 device is around one order of magnitude higher than that for the SiO2 device. The bulk mobility fluctuation is considered as the dominant LFN mechanism in both devices, and the increased LFN in the HfO2 device is attributed to the enhanced mobility fluctuation by the remote phonon scattering from the HfO2.

Charge injection from gate electrode by simultaneous stress of optical and electrical biases in HfInZnO amorphous oxide thin film transistor

Abstract: A comprehensive study is done regarding stabilities under simultaneous stress of light and dc-bias in amorphous hafnium-indium-zinc-oxide thin film transistors. The positive threshold voltage (Vth) shift is observed after negative gate bias and light stress, and it is completely different from widely accepted phenomenon which explains that negative-bias stress results in Vth shift in the left direction by bias-induced hole-trapping. Gate current measurement is performed to explain the unusual positive Vth shift under simultaneous application of light and negative gate bias. As a result, it is clearly found that the positive Vth shift is derived from electron injection from gate electrode to gate insulator.