In-Tak Cho

Bias-stress-induced stretched-exponential time dependence of threshold voltage shift in InGaZnO thin film transistors

The experimental and modeling study of bias-stress-induced threshold voltage instabilities in amorphous indium-gallium-zinc oxide thin film transistors is reported. Positive stress results in a positive shift in the threshold voltage, while the transfer curve hardly moves when negative stress is induced. The time evolution of threshold voltage is described by the stretched-exponential equation, and the shift is attributed to the electron injection from the channel into interface/dielectric traps. The stress amplitudes and stress temperatures are considered as important factors in threshold voltage instabilities, and the stretched-exponential equation is well fitted in various bias temperature stress conditions.

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.

Charge trapping and detrapping characteristics in amorphous InGaZnO TFTs under static and dynamic stresses

We have investigated the static and dynamic bias stress-induced charge trapping and detrapping phenomenon in amorphous indium–gallium–zinc oxide thin film transistors. It is observed that the charges trapped after electron injection in the interface and bulk traps are unstable and slowly decay over time. The stretched-exponential equation, which can be derived based on the trapping/detrapping of charges to/from existing traps and continuous redistribution of charges in bulk dielectrics, is successfully applied in fitting the time dependence of the threshold voltage shift during the stress and recovery phases under dynamic stresses. The characteristic time constants decrease with increasing temperature and drain bias during the recovery phase. Under dynamic stresses with various frequencies, the threshold voltage shift strongly depends on the frequency of dynamic stresses, i.e., a high frequency stress results in a small threshold voltage shift and a long lifetime. The stress-induced threshold shift phenomenon is observed to be relieved after a long-time low temperature post thermal annealing process and device passivation with an aluminum oxide layer.

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.

Full-Swing InGaZnO Thin Film Transistor Inverter with Depletion Load

A high performance amorphous indium–gallium–zinc oxide (a-IGZO) thin film transistor (TFT) inverter is implemented using the enhancement mode driver and the depletion mode load. The threshold voltage of the TFT is easily controlled by adjusting the active layer thickness in a-IGZO TFTs. 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.

Investigation of the charge transport mechanism and subgap density of states in p-type Cu2O thin-film transistors

We investigate the charge transport mechanism and subgap density of states (DOS) in p-type Cu2O thin-film transistors (TFTs) using the bias and temperature dependence of the drain currents. Among several charge transport mechanisms, the experimental data are well matched with a multiple trapping and release model, which suggests that the charge transport in the Cu2O TFT is mainly limited by trap states at grain boundaries or dielectric/semiconductor interface. The subgap DOS is extracted based on the Meyer-Neldel rule. Large density of subgap states is extracted, which is considered to be the reason of low mobility in fabricated Cu2O 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}

We have investigated the effects of vacuum annealing on the optical and electrical properties of the p-type copper-oxide thin-film transistors (TFTs). The vacuum annealing of the copper-oxide thin-film was performed using the RF magnetron sputter at various temperatures. From the x-ray diffraction and UV-vis spectroscopy, it is demonstrated that the high-temperature vacuum annealing reduces the copper-oxide phase from CuO to Cu2O, and increases the optical transmittance in the visible part of the spectrum. The fabricated copper-oxide TFT does not exhibit the switching behavior under low-temperature vacuum annealing conditions. However, as the annealing temperature increases, the drain current begins to be modulated by a gate voltage, and the TFT exhibits a high current on?off ratio over 104?as the vacuum annealing temperature increases over 450??C. These results show that the vacuum annealing process can be an effective method of simultaneously improving the optical and electrical performances in p-type copper-oxide TFTs.

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We investigate the effects of ambient atmosphere on the electrical performance of p-type tin monoxide (SnO) thin-film transistors (TFTs), and present the effective method for the passivation of SnO TFTs using a SU-8 organic layer. The experimental data shows that the SnO TFTs without a passivation layer suffer from the electrical performance degradation under humid environments, which implies that the formation of the passivation layer is necessary in p-type SnO TFTs for the stable operation of the devices. The SU-8 organic layer was successfully incorporated as a passivation layer of SnO TFTs. The SnO TFTs with a SU-8 passivation layer exhibit very similar transfer characteristics with those without a passivation layer, and show much improved long-term durability and bias stress stability compared with the SnO TFTs without a passivation layer under air environments.

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

We investigate the low-frequency noise (LFN) behaviors of amorphous indium-gallium-zinc oxide thin-film transistors in the subthreshold, Ohmic, and saturation regimes. Measured LFNs are proportional to 1/fγ, with γ=0.8–0.9 in all operation regimes. It is found that the LFN behavior follows the carrier number fluctuation model in the subthreshold regime, whereas in the Ohmic and saturation regimes, it agrees well with the bulk mobility fluctuation model. We also observe that the origin of 1/f noise in the Ohmic regime changes from the bulk mobility fluctuation to the carrier number fluctuation as the channel length decreases.