Dong Myong Kim

High performance amorphous oxide thin film transistors with self-aligned top-gate structure

We have demonstrated self-aligned top-gate amorphous oxide TFTs for large size and high resolution displays. The processes such as source/drain and channel engineering have been developed to realize the self-aligned top gate structure. Ar plasma is exposed on the source/drain region of active layer to minimize the source/drain series resistances. To prevent the conductive channel, N2O plasma is also treated on the channel region of active layer. We obtain a field effect mobility of 5.5 cm2/V·s, a threshold voltage of 1.1 V, and a sub-threshold swing of 0.35 V/decade at sub-micron a-GIZO TFTs with the length of 0.67#x00B5;m. Furthermore, a-IZO TFTs fabricated for gate and data driver circuits on glass substrate exhibit excellent electrical properties such as a field effect mobility of 115 cm2/V·s, a threshold voltage of 0.2 V, a sub-threshold swing of 0.2 V/decade, and low threshold voltage shift less than 1 V under bias temperature stress for 3 hr.

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

An extraction technique for subgap density of states (DOS) in an n-channel amorphous InGaZnO thin-film transistor (TFT) by using multifrequency capacitance-voltage (<i>C</i> -<i>V</i>) characteristics is proposed and verified by comparing the measured <i>I</i>- <i>V</i> 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 <i>N</i> _TA = 1.1 × 10¹⁷ cm^-3 · eV^-1, <i>N</i> _DA = 4 × 10¹⁵ cm^-3 · eV^-1, <i>kT</i> _TA = 0.09 eV, and <i>kT</i> _DA = 0.4 eV. The proposed technique allows obtaining the frequency-independent <i>C</i>-<i>V</i> 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 <i>V</i> _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.

Modeling of amorphous InGaZnO thin-film transistors based on the density of states extracted from the optical response of capacitance-voltage characteristics

In order to model dc characteristics of n-channel amorphous InGaZnO thin-film transistors from experimentally obtained density of states (DOS), the acceptorlike DOS is extracted from the optical response of capacitance-voltage characteristics and confirmed by the technology computer-aided design (TCAD) simulation comparing with the measured data. Extracted DOS is a linear superposition of two exponential functions (tail and deep states), and its incorporation into TCAD model reproduces well the experimental current-voltage characteristics over the wide range of the gate and drain voltages. The discrepancy at higher gate voltage is expected to be improved by incorporating a gate voltage-dependent mobility in the model.

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

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

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.

Extraction of Density of States in Amorphous GaInZnO Thin-Film Transistors by Combining an Optical Charge Pumping and Capacitance–Voltage Characteristics

A technique for extracting the acceptorlike density of states (DOS) of <i>n</i>-channel amorphous GaInZnO (a-GIZO) thin-film transistors based on the combination of subbandgap optical charge pumping and <i>C</i>-<i>V</i> characteristics is proposed. While the energy level is scanned by the photon energy and the gate voltage sweep, its density is extracted from the optical response of <i>C</i>-<i>V</i> characteristics. The extracted DOS shows the superposition of the exponential tail states and the Gaussian deep states (<i>N</i> _TA=2times10¹⁸ eV^-1ldrcm^-3, <i>N</i> _DA=4times10¹⁵ eV^-1ldrcm^-3, <i>kT</i> _TA=0.085 eV, <i>kT</i> _DA=0.5 eV , <i>E</i> _O=1 eV). The TCAD simulation results incorporated by the extracted DOS show good agreements with the measured transfer and output characteristics of a-GIZO thin-film transistors with a single set of process-controlled parameters.

Impact of Oxygen Flow Rate on the Instability Under Positive Bias Stresses in DC-Sputtered Amorphous InGaZnO Thin-Film Transistors

The effect of O2 flow rate (OFR) during channel deposition is investigated on the electrical instability of the amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) under positive gate bias stresses. From the transfer curves measured before and after bias stresses, we can observe that the high OFR degrades the electrical stability and causes the large threshold voltage shift (ΔVT) in a-IGZO TFTs. To elucidate the origin of the observed phenomenon, we extract and compare the subgap density of states (DOS) in devices with various OFRs. The extracted DOS shows that the subgap states become higher with the increase of OFR in a wide range of bandgap, and the enhanced electron trapping due to the increased number of trap states is considered as the cause of larger ΔVT in higher OFR devices.

Electrical stress-induced instability of amorphous indium-gallium-zinc oxide thin-film transistors under bipolar ac stress

Bipolar ac stress-induced instability of amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors is comparatively investigated with that under a positive dc gate bias stress. While the positive dc gate bias stress-induced threshold voltage shift (ΔVT) is caused by the charge trapping into the interface/gate dielectric as reported in previous works, the dominant mechanism of the ac stress-induced ΔVT is observed to be due to the increase in the acceptorlike deep states of the density of states (DOS) in the a-IGZO active layer. Furthermore, it is found that the variation of deep states in the DOS makes a parallel shift in the IDS-VGS curve with an insignificant change in the subthreshold slope, as well as the deformation of the CG-VG curves.

Differential Ideality Factor Technique for Extraction of Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors

We propose a differential ideality factor technique (DIFT) for extraction of subgap density of states (DOS) over the bandgap in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by using the differential ideality factor dη/dV_GS on behalf of the ideality factor itself. Contrary to the sub threshold current method which requires an accurate threshold voltage (V_T), the DIFT is free from V_T itself and consider ably useful to TFTs with a nonuniform distribution of DOS over the bandgap. Through the DIFT applied to an a-IGZO TFT with W/L = 200 μm/30 μm, the subgap DOS is extracted to be a superposition of exponential deep and tail states with NUA = 7.1 × 10¹⁵ cm^-3 · eV^-1, kT_DA = 0.6 eV, N_TA=1.5 × 10¹⁶ cm^-3 · eV^-1, and kT_TA = 0.024 eV.

Analytical Models for Drain Current and Gate Capacitance in Amorphous InGaZnO Thin-Film Transistors With Effective Carrier Density

Analytical drain current and gate capacitance models for amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) over sub- and above-threshold regions are proposed by adopting an effective carrier density for the dominant carrier density. The effective carrier density fully considers the free carriers in the conduction band, the localized subgap deep states, and tail states over the bandgap for analytical I-V and C-V characteristics. The proposed analytical models are verified by comparing the measured I-V and C-V characteristics. The proposed models make a time-efficient simulation of a-IGZO TFT-based circuits possible due to their analytical form.