Seung-Hee Kuk

A Depletion-Mode a-IGZO TFT Shift Register With a Single Low-Voltage-Level Power Signal

We proposed and fabricated a new depletion-mode amorphous indium-gallium-zinc-oxide thin-film-transistor (a-IGZO TFT) shift register. Although only one low-voltage-level power signal (VGL) rather than widely used two ones was employed, the depletion-mode a-IGZO TFTs in the proposed shift register were turned off completely due to a series-connected two-transistor structure. The proposed shift register exhibited successfully a high-voltage output pulse. The power consumption of the shift register with ten stages is 1.67 mW at the high-voltage-level power signal of 20 V, the VGL of -5 V, and the clock frequency of 13.9 kHz.

Highly Transparent and High Haze Bilayer Al-Doped ZnO Thin Film Employing Oxygen-Controlled Seed Layer

Al-doped ZnO (AZO) film was continuously deposited by DC magnetron sputtering using pure Ar on a thin AZO seed layer prepared using an approximately 4% dilution of oxygen with Ar. X-ray diffraction measurements showed that the AZO film grown on the seed layer exhibited a much higher crystallinity and larger grain size than that without the seed layer. The electrical properties such as resistivity and Hall mobility were improved. The average visible transmittance was increased from 81.6 to 86.2%, and near infrared (NIR) transmittance was increased from 76.0 to 84.4% by employing the seed layer. The haze value characterizing the light scattering property was significantly increased from 59.4 to 89.5% in the visible region by the seed layer, and it was increased from 15.1 to 50.8% in the NIR region. Surface topography analysis showed that the bilayer AZO film had larger craters allowing for improvement of the light scattering properties than the conventional AZO film without the seed layer.

Highly Reliable Depletion-Mode a-IGZO TFT Gate Driver Circuits for High-Frequency Display Applications Under Light Illumination

We proposed and fabricated a depletion-mode amorphous indium-gallium-zinc-oxide thin-film transistor gate driver without any additional signals. The proposed gate driver successfully exhibited a high-voltage output pulse without distortion at a clock frequency of 100 kHz, which is enough to drive a high-frequency panel with a frame rate of ~360 Hz and a resolution of full high definition. The experimental and simulation results showed that the gate driver would be highly reliable under light illumination. Also, the output waveform of the gate driver was not distorted after 240-h driving under 450-nm illumination with an intensity of 1 mW/ cm2 at 60°C.

A Novel Level Shifter Employing IGZO TFT

A new level shifter employing indium-gallium-zinc-oxide thin-film transistor (IGZO TFT) for a display panel was proposed and successfully fabricated. Two clock signals with 180° out of phase and a discharging TFT were employed to obtain a full-swing output. The IGZO level shifter has successfully exhibited a wide swing output from VDD to VSS without any additional power sources and input signals. The power consumption is 0.30 mW at a clock frequency of 12.5 kHz. The proposed level shifter with a depletion-mode device would be an important building block for an oxide TFT display.

A Novel Depletion-Mode a-IGZO TFT Shift Register With a Node-Shared Structure

We proposed and fabricated a new depletion-mode amorphous indium-gallium-zinc-oxide thin-film-transistor (TFT) shift register with a node-shared structure. The proposed shift register requires 14 TFTs, 3 clock lines, and 3 power source lines for two output pulses, whereas the previous shift registers consisted of more than 22 TFTs, 4 clock lines, and 6 power source lines. The experimental results showed that the proposed shift register successfully generated two output pulses at one stage without any distortion. The circuit area of the proposed shift register is reduced by about 30%, as compared with that of the previous one.

Active-Matrix Organic Light Emission Diode Pixel Circuit for Suppressing and Compensating for the Threshold Voltage Degradation of Hydrogenated Amorphous Silicon Thin Film Transistors

A new hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT) pixel circuit for active-matrix organic light emission diodes (AM-OLEDs), which significantly compensates the OLED current degradation by memorizing the threshold voltage of driving TFT and suppresses the threshold voltage shift of a-Si:H TFTs by negative bias annealing, is proposed and fabricated. During the first half of each frame, the driving TFT of the proposed pixel circuit supplies current to the OLED, which is determined by modified data voltage in the compensation scheme. The proposed pixel circuit was able to compensate the threshold voltage shift of the driving TFT as well as the OLED. During the remaining half of each frame, the proposed pixel circuit induces the recovery of the threshold voltage degradation of a-Si:H TFTs owing to the negative bias annealing. The experimental results show that the proposed pixel circuit was able to successfully compensate for the OLED current degradation and suppress the threshold voltage degradation of the driving TFT.

A New Thin-Film Transistor Pixel Structure Suppressing the Leakage Current Effects on AMOLED

We propose a new pixel structure employing solid-phase crystallized silicon thin-film transistors which suppresses the leakage current effects on active-matrix organic light-emitting diode (AMOLED) displays. The pixel structure has been fabricated on a glass substrate employing the field-enhanced rapid thermal annealing technology. In the proposed pixel, the charge holding capability is considerably enhanced due to the capacitor located between two series-connected switch transistors. Our experimental results shows that the average variation range of the OLED current is suppressed less than 0.5% while the conventional one exceeded 4%.

P-25: Nanocrystalline Silicon Thin Film Transistor Fabricated without any Substrate Heating for a Flexible Display

We have fabricated nc-Si film and SiO2 film without substrate heating using ICP-CVD. nc-Si film has amorphous incubation layer of 12nm and crystalline volume fraction of 27%. Hydrogen plasma post-treatment was performed SiO2 film and electrical characteristic of SiO2 film was improved due to reduction of charges and annealing effect. And we have fabricated nc-Si TFT without substrate heating using ICP-CVD. Although there was no external heating, mobility of 6.42cm2/V⋅s was obtained. This result indicates that nc-Si TFT fabricated without any substrate heating may be suitable device for a flexible display.

Time-Domain Quaternary-Weighted Pulse Width Modulation Driving Method for Active Matrix Organic Light-Emitting Diode Displays

We proposed a new digital driving method and its pixel structure for active matrix organic light-emitting diode (AMOLED) displays employing time-domain quaternary-weighted pulse width modulation. In the new digital driving method, the luminance of AMOLED displays is accurately determined by averaging photon flux to the desired level over a frame period. The proposed pixel was verified by spice simulation and the output linearity between the grayscale and the OLED current was successfully achieved. In the proposed digital driving pixel, the timing margin was increased and the effect on luminance of AMOLED displays by the troublesome variation of the thin-film transistors (TFTs) was suppressed without additional compensation schemes.

Effect of boron-doping on transparent conducting Al doped ZnO films for thin film solar cells

Al-doped ZnO (AZO) films with different boron (B) content have been prepared on glass substrates by co-sputtering of AZO (2wt. % Al<inf>2</inf>O<inf>3</inf>) and B-doped ZnO (BZO, 3wt. % B<inf>2</inf>O<inf>3</inf>) targets at room temperature. The B content in the B-doped AZO (BAZO) film was controlled by DC power on BZO target. X-ray diffraction analysis indicated that the B-doping improved the crystalline structure of AZO film. Hall measurement analysis showed that resistivity of BAZO films was lowered to 2.06×10⁻³ Ω·cm, whereas that of AZO film was 5.68×10⁻³ Ω·cm. Hall mobility of AZO film was improved with appropriate B-doping, and carrier concentration was increased with B-doping due to B activation as a donor.