Woo-Seok Cheong

Transparent Al–Zn–Sn–O thin film transistors prepared at low temperature

We have fabricated transparent bottom gate thin film transistors (TFTs) using Al-doped zinc tin oxide (AZTO) as active layers. The AZTO active layer was deposited by rf magnetron sputtering at room temperature. The AZTO TFT showed good TFT performance without postannealing. The field effect mobility and the subthreshold swing were improved by postannealing below 180 °C. The AZTO TFT exhibited a field effect mobility (μFET) of 10.1 cm2/V s, a turn-on voltage (Von) of 0.4 V, a subthreshold swing (S/S) of 0.6 V/decade, and an on/off ratio (Ion/Ioff) of 109.

Comparative study of electrical instabilities in top-gate InGaZnO thin film transistors with Al2O3 and Al2O3/SiNx gate dielectrics

A comparative study was made of the performance and electrical instabilities in amorphous indium-gallium-zinc oxide (a-IGZO) thin film transistors with Al2O3 and Al2O3/SiNx gate dielectrics. Steeper subthreshold slope is observed in Al2O3 devices, which shows that the density of trap states at the interface of a-IGZO/Al2O3 is lower than that of a-IGZO/SiNx. Under high bias-stresses, a larger degradation is observed in Al2O3/SiNx devices. The device degradation for both devices are mainly attributed to the charge trapping phenomenon, but the different time dependence of threshold voltage shift shows that trapped electrons are more easily redistributed inside the Al2O3 dielectrics.

Low-Frequency Noise in Amorphous Indium–Gallium–Zinc-Oxide Thin-Film Transistors

We have investigated the low-frequency noise (LFN) properties of amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) as a function of frequency, bias, and channel length of devices. The measured noise power spectral density of drain current (<i>S</i> _iD) shows that the low-frequency noise in a-IGZO TFTs obeys the classical 1/<i>f</i> noise theory, i.e., it fits well to a 1/<i>f</i> ^gamma power law with gamma ~ 1 in the frequency range of 10 Hz to 1 kHz. From the dependence of normalized noise power spectral density (<i>S</i> _iD/<i>ID</i> ²) on the gate voltage, mobility fluctuation is considered as a dominant LFN mechanism in a-IGZO TFTs. The magnitude of <i>S</i> _iD/<i>ID</i> ² is inversely proportional to the channel length of devices, which indicates that contact noise is insignificant in a-IGZO TFTs.

Comparative Study of the Low-Frequency-Noise Behaviors in a-IGZO Thin-Film Transistors With

A comparative study is made of the low-frequency noise (LFN) in amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) with Al₂O₃ and Al₂O₃/SiN_x gate dielectrics. The LFN is proportional to 1/f^gamma, with gamma ~ 1 for both devices, but the normalized noise for the Al₂O₃/SiN_x device is two to three orders of magnitude lower than that for the Al₂O₃ device. The mobility fluctuation is the dominant LFN mechanism in both devices, but the noise from the source/drain contacts becomes comparable to the intrinsic channel noise as the gate overdrive voltage increases in Al₂O₃/SiN_x devices. The SiN_x interfacial layer is considered to be very effective in reducing LFN by suppressing the remote phonon scattering from the Al₂O₃ dielectric. Hooges parameter is extracted to ~6.0 times 10^-3 in Al₂O₃/SiN_x devices.

\hbox{Al}_{2}\hbox{O}_{3}

A high-performance amorphous indium-gallium-zinc-oxide thin-film transistor (TFT) inverter, which is composed of an enhancement mode driver and a depletion mode load, is implemented by selectively inducing the negative bias illumination temperature stress (NBITS) to the load TFT. Under NBITS, the transfer curve of the load TFT shows a parallel shift into the negative bias direction without a significant change in the subthreshold slope and recovers very slowly after terminating the NBITS even under harsh bias and temperature stress conditions. The proposed inverter shows much improved switching characteristics including higher voltage gain, wider swing range, and higher noise margins compared to the conventional inverter with an enhancement load.

and

In this work, we present the results concerning the use of amorphous indium‐ gallium‐zinc‐oxide (a-IGZO) thin-film transistor (TFT) as a driving transistor of the flexible thermal and pressure sensors which are applicable to artificial skin systems. Although the a-IGZO TFT has been attracting much attention as a driving transistor of the next-generation flat panel displays, no study has been performed about the application of this new device to the driving transistor of the flexible sensors yet. The proposed thermal sensor pixel is composed of the series-connected a-IGZO TFT and ZnO-based thermistor fabricated on a polished metal foil, and the ZnO-based thermistor is replaced by the pressure sensitive rubber in the pressure sensor pixel. In both sensor pixels, the a-IGZO TFT acts as the driving transistor and the temperature/pressure-dependent resistance of the ZnO-based thermistor/pressure-sensitive rubber mainly determines the magnitude of the output currents. The fabricated a-IGZO TFT-driven flexible thermal sensor shows around a seven times increase in the output current as the temperature increases from 20 ◦ C to 100 ◦ C, and the a-IGZO TFT-driven flexible pressure sensors also exhibit high sensitivity under various pressure environments. (Some figures may appear in colour only in the online journal)

\hbox{Al}_{2}\hbox{O}_{3}/\hbox{SiN}_{x}

Since the first demonstration of oxide TFT driving AM-OLED, oxide TFT technology has attracted explosive interesting and has been developed for the mass production. The performance of oxide TFT has been verified and the remained issue is bias temperature stability. In this paper, we report the effect of interface process including channel and back channel on the oxide TFT performance in a top gate and a bottom gate structure. We also demonstrate transparent AM-OLED driven by highly stable Al doped ZTO TFT.

Gate Dielectrics

We fabricated environmentally stable and transparent organic/oxide hybrid transistor on a glass substrate using the conventional photolithography. The obtained device, which was composed of an In-Ga-Zn-O active layer/soluble polyimide (KSPI) organic insulator, showed a mobility of 6.65 cm2/Vs, a subthreshold swing slope of 350 mV/decade, a threshold voltage (VT) of 3.10 V, and an on-off ratio of 3.9 × 109. The transistor also showed good uniformity characteristics and was found to be environmentally stable for 90 days under ambient conditions.

Full-Swing a-IGZO Inverter With a Depletion Load Using Negative Bias Instability Under Light Illumination

Abstract Sn-doped ZnO/Ag/Sn-doped ZnO (ZAZ) multilayers prepared using sputtering process was employed in GaN-based blue light-emitting diodes(LEDs) as transparent contact layers (TCLs). The ZAZ layer had better optical and electrical properties without any thermal annealing. The ZAZ TCLs improved the current spreading on p-GaN layer and increased the light output power in the large area devices. The efficiency droop was also quite small in the case of adopting a ZAZ TCL in GaN-based LEDs. These results are promising for the development of a ZAZ TCL using sputtering process for GaN LED applications.

Fabrication of amorphous InGaZnO thin-film transistor-driven flexible thermal and pressure sensors

Transparent flexible displays can be realized using active matrix organic light emitting device (AMOLED) with transparent electrodes on transparent plastic substrates. In this study, we developed low-temperature, high-performance [ZITO, ZnO:In2O3:SnO2=3:1:1 molar ratio] thin-film transistors (TFTs) on polyarylate films. After optimizing the sputtering condition, the ZITO TFT with an ITO electrode had a high mobility of 16.93 cm2 V-1 s-1, and an SS of 0.39, while the ZITO TFT with a ZTO:B electrode showed no hysteresis on sweeping, a mobility of 2.29 cm2 V-1 s-1 and an SS of 0.18.