Gwang Jun Lee

High performance, transparent a-IGZO TFTs on a flexible thin glass substrate

We investigated electrical properties of transparent amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) with amorphous indium zinc oxide (a-IZO) transparent electrodes on a flexble thin glass substrate. The TFTs show a high field-effect mobility, a good subthreshold slope and a high on/off ratio owing to the high temperature thermal annealing process which cannot be applied to typical transparent polymer-based flexible substrates. Bias stress instability tests applying tensile stress concurrently with the bending radius of up to 40 mm indicated that mechanically and electrically stable a-IGZO TFTs can be fabricated on the transparent thin glass substrate.

Electrical characterization of a-InGaZnO thin-film transistors with Cu source/drain electrodes

We analyzed the effects of Cu source/drain (S/D) electrodes on the performance of a-InGaZnO (a-IGZO) thin-film transistors (TFTs). Owing to the Cu migration, the parasitic resistance was as low as 10 Ω cm with small current transfer length. Based on the transfer characteristics, we found that VDS dependent Cu migration creates donor-like deep and tail states in the sub-bandgap region. The feasibility of Cu S/D electrodes for a-IGZO TFTs using inverter circuits indicates that fabrication of high performance circuits is possible by controlling the Cu electro-migration.

Oxygen Dispersive Diffusion Induced Bias Stress Instability in Thin Active Layer Amorphous In–Ga–Zn–O Thin-Film Transistors

We studied the bias stress instability of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) by varying the active layer thickness (t) from 6 to 100 nm. We found that the stretched exponential relationship between the threshold voltage shift and the stress time can be explained by oxygen dispersive diffusion which is absorbed near the back channel region during an oxygen annealing process in the active layer. For an a-IGZO TFT with t=6 nm, direct exposure of the channel layer to the ambient oxygen greatly increases the bias stress instability and induces hump like characteristics, indicating that the creation of acceptor-like states is the dominant mechanism of the instability of a-IGZO TFTs with a thin active layer.

Scaling behaviour of a-IGZO TFTs with transparent a-IZO source/drain electrodes

We analysed the scaling behaviour of amorphous indium gallium zinc oxide thin-film transistors (a-IGZO TFTs) with amorphous indium zinc oxide (a-IZO) transparent source/drain (S/D) electrodes. Due to the sputtering damage of the back-channel region during the a-IZO deposition process, the output characteristics show early saturation behaviour and the field-effect mobility in the saturation region is severely decreased in comparison with that in the linear region, especially when the channel length is decreased. Based on the transmission line method, we found that a long gate overlap distance is required due to the long current transfer length. Therefore, optimizing the parasitic resistance is required for the scaling down of a-IGZO TFTs with transparent a-IZO S/D electrodes.

Intrinsic parameter extraction of a-InGaZnO thin-film transistors by a gated-four-probe method

We analyzed the intrinsic electrical characteristics of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) using a gated-four-probe method. Based on the back channel potential, the extraction of intrinsic field-effect mobility (μFEi) and parasitic resistance in source (Rs) and drain (Rd) electrodes was performed especially for low VGS and VDS conditions. The resulting μFEi showed typical VGS dependency of amorphous semiconductor TFTs. However, Rs and Rd showed that there can be non-uniformity in source/drain parasitic resistance, which indicates that a separate analysis of the parameters of each electrode is essential for further improvement of the performance of a-IGZO TFTs.

Analysis of temperature-dependent electrical characteristics in amorphous In-Ga-Zn-O thin-film transistors using gated-four-probe measurements

We analyzed the temperature-dependent electrical characteristics of amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) using a gated-four-probe method (GFP) with temperatures ranging from 93 to 373 K. The intrinsic field-effect mobility and source/drain parasitic resistance were separately extracted using the GFP method. We found that temperature-dependent transfer characteristics originated from the temperature-dependent intrinsic field-effect mobility of the a-IGZO TFTs. The parasitic resistance was also correlated with the intrinsic-field effect mobility, which decreases as the intrinsic field-effect mobility increases, indicating that access parasitic resistance originated from bulk regions rather than metal/semiconductor junction barrier is a key factor to determine the parasitic resistance of a-IGZO TFTs.

Wireless thin film transistor based on micro magnetic induction coupling antenna

A wireless thin film transistor (TFT) structure in which a source/drain or a gate is connected directly to a micro antenna to receive or transmit signals or power can be an important building block, acting as an electrical switch, a rectifier or an amplifier, for various electronics as well as microelectronics, since it allows simple connection with other devices, unlike conventional wire connections. An amorphous indium gallium zinc oxide (α-IGZO) TFT with magnetic antenna structure was fabricated and studied for this purpose. To enhance the induction coupling efficiency while maintaining the same small antenna size, a magnetic core structure consisting of Ni and nanowires was formed under the antenna. With the micro-antenna connected to a source/drain or a gate of the TFT, working electrical signals were well controlled. The results demonstrated the device as an alternative solution to existing wire connections which cause a number of problems in various fields such as flexible/wearable devices, body implanted devices, micro/nano robots, and sensors for the ‘internet of things’ (IoT).

Screen-Printed Cu Source/Drain Electrodes for a-InGaZnO Thin-Film Transistors

We report screen-printed copper source/drain electrodes for a-InGaZnO (a-IGZO) thin-film transistors (TFTs). The best electrical characteristics of the a-IGZO TFTs were a field-effect mobility of 2.06 cm2/Vs, a threshold voltage of 3.40 V, an on/off current ratio of 6.0 × 103A/A, and a subthreshold swing of 7.02 V/decade. Resulting TFT performances indicate that blocking the inter-diffusion of Cu and impurities is a key factor to fabricate low leakage current and high performance a-IGZO TFTs with printed Cu S/D electrodes.

External Light Noise-Robust Multi-Touch Screen Using Frame Data Differential Method

Camera based multi-touch systems with an infrared (IR) light source have been widely developed and applied to large-size interactive electrical devices. These multi-touch systems are easy to embed in large-sized screens because of their simple structure; however, touch errors due to external IR noise need to be resolved in various applications. An environment with incandescent lamps or sun light has a higher touch error ratio than that with fluorescent lights or LEDs because the aforementioned contain enough IR rays to induce a signal error. To solve this problem, the frame data differential (FDD) method was employed in a multi-touch system along with the rear side diffuse illumination method. The FDD method removed well the IR noise signal without any additional hardware modification to the touch system, and the touch accuracy and sensitivity were dramatically improved in a high IR noise environment compared with the conventional touch sensing method.