Woojoon Kim

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.

Amorphous InGaZnO Thin-Film Transistors—Part I: Complete Extraction of Density of States Over the Full Subband-Gap Energy Range

A combination of the multifrequency C- V and the generation-recombination current spectroscopy is proposed for a complete extraction of density of states (DOS) in amorphous InGaZnO thin-film transistors (a-IGZO TFTs) over the full subband-gap energy range (EV ≤ E ≤ EC) including the interface trap density between the gate oxide and the a-IGZO active layer. In particular, our result on the separate extraction of acceptor- and donor-like DOS is noticeable for a systematic design of amorphous oxide semiconductor TFTs because the former determines their dc characteristics and the latter does their threshold voltage (VT) instability under practical operation conditions. The proposed approach can be used to optimize the fabrication process of thin-film materials with high mobility and stability for mass-production-level amorphous oxide semiconductor TFTs.

Amorphous InGaZnO Thin-Film Transistors—Part II: Modeling and Simulation of Negative Bias Illumination Stress-Induced Instability

Based on the physical model of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) and the extracted density of states described in Part I, a quantitative investigation of mechanisms on the negative bias illumination stress (NBIS)-induced threshold voltage VT instability of a-IGZO TFTs is presented. It is found that the shallow donor state-creation model explains the NBIS time evolution of the electrical characteristics very well. Furthermore, the semi-empirical rule of the NBIS-induced ΔVT is proposed and demonstrated based on the shallow donor state-creation model. The proposed approach can be used to optimize the fabrication process and to explore high-performance thin-film materials for mass-production-level amorphous oxide semiconductor TFTs to be innovatively used in the near future.

Modified Conductance Method for Extraction of Subgap Density of States in a-IGZO Thin-Film Transistors

We propose a modified conductance method for extraction of the subgap density of states (DOS) in amorphous indium-gallium-zinc oxide thin-film transistors by using the measured capacitance and conductance through the capacitance-voltage (C-V) measurement. In the proposed method, the subgap DOS [g_A(E)] is extracted from the frequency-dispersive C-V characteristics by localized traps in the active channel region. The extracted g_A(E) shows a superposition of the exponential tail states and the exponential deep states over the bandgap (N_TA = 3 × 10¹⁸ cm^-3 · eV_-1, N_DA = 2.8 × 10¹⁷ cm^-3 · eV-1, kT_TA = 0.04 eV, and kT_DA = 0.77 eV). We note that the gate-bias-dependent Cfree by free electron charges can be separated from C_loc by localized trap charges through the proposed method.

Characterization of density-of-states and parasitic resistance in a-InGaZnO thin-film transistors after negative bias stress

Instability mechanism of amorphous InGaZnO thin-film transistors under negative bias stress (NBS) was investigated. After strong NBS stress, we observed a negligible change in the subthreshold swing which is strongly dependent on the subgap density-of-states (DOS). On the other hand, there was substantial increase in the drain current at above-threshold operation. Therefore, the dominant mechanism of the NBS-induced instability is investigated not to be a change in the subgap DOS but a change in the parasitic resistance caused by the reduced Schottky barrier of the metal contacts. This was verified by the extracted source/drain resistance and Technology Computer-Aided Design simulation.

Characterization of Intrinsic Field-Effect Mobility in TFTs by De-Embedding the Effect of Parasitic Source and Drain Resistances

Due to voltage drops across parasitic resistances in semiconductor devices, extracted performance parameters can be strongly dependent on the geometrical structure. In this letter, we report a characterization technique for the intrinsic field-effect mobility μ_FEo in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by de-embedding the parasitic source and drain resistances <i>R</i>_S and <i>R</i>_D, respectively. We obtained the channel-length (<i>L</i>) -independent intrinsic field-effect mobility μ_FEo from TFTs with various channel lengths on the same wafer. We expect that this characterization technique for <i>L</i>-independent intrinsic field-effect mobility is useful for accurate characterization, consistent modeling, and robust simulation of a-IGZO TFT circuits.

Characterization of Density-of-States in Polymer-based Organic Thin Film Transistors and Implementation into TCAD Simulator

In this work, we report extraction of the density-of-states (DOS) in polymer-based organic thin film transistors through the multi-frequency C-V spectroscopy. Extracted DOS is implemented into a TCAD simulator and obtained a consistent output curves with non-linear characteristics considering the contact resistance effect. We employed a Schottky contact model for the source and drain to fully reproduce a strong nonlinearity with proper physical mechanisms in the output characteristics even under a very small drain biases. For experimental verification of the model and extracted DOS, 2 different OTFTs (P3HT and PQT-12) are employed. By controlling the Schottky contact model parameters in the TCAD simulator, we accurately reproduced the nonlinearity in the output characteristics of OTFT.

Physics-Based SPICE Model of a-InGaZnO Thin-Film Transistor Using Verilog-A

In this work, we report the physics-based SPICE model of amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) and demonstrate the SPICE simulation of amorphous InGaZnO (a- IGZO) TFT inverter by using Verilog-A. As key physical parameter, subgap density-of-states (DOS) is extracted and used for calculating the electric potential, carrier density, and mobility along the depth direction of active thin-film. It is confirmed that the proposed DOS-based SPICE model can successfully reproduce the voltage transfer characteristic of a-IGZO inverter as well as the measured I-V characteristics of a-IGZO TFTs within the average error of 6% at VDD=20 V.

P-202L: Late-News Poster: Density-of-States Based Analysis on the Effect of Active Thin-film Thickness on Current Stress-induced Instability in Amorphous InGaZnO AMOLED Driver TFTs

The effect of the active layer thickness (TIGZO) on the constant current stress (CCS)-induced threshold voltage shift (ΔVT) in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) for AMOLED drivers is investigated by using a subgap density-of-states (DOS) model and simulation. The CCS-induced positive ΔVT in a-IGZO TFT with a thicker TIGZO is larger than that in a-IGZO TFTs with a thinner TIGO. It is originated from more activated charge trapping (60∼80 % in total ΔVT) and DOS creation due to a higher surface electric field with a thicker TIGZO. Our results show that using thinner active layer in a-IGZO TFTs is desirable for high performance and highly stable AMOLEDs if the quality of thin-film bulk is guranteed irrespective of TIGZO.