The Time Response of the On-Current for the Amorphous In-Ga-Zn-O Thin Film Transistor to the Illumination Pulse

Abstract

In this study, the time response behavior of the amorphous indium gallium zinc oxide (a-IGZO) thin film transistors (TFTs) to the illumination pulse is analyzed. The mechanism is proposed to correlate the oxygen vacancy reacting with the light-induced electron-hole pairs. The temperature effect on the time response to the illumination pulse is also studied. The higher excitation level, either from light or temperature, results in the similar excited and recovering behaviors. The formulas for the time response are proposed to be possibly used in the simulation for the circuit performance in real situation of illumination, which is important in the development of transparent electronics using a-IGZO TFT. Nowadays, amorphous-silicon (a-Si) thin film transistor (TFTs) have been used widely in flat-panel displays (FPD) in production, with the growing need for large area displays for home entertainment and the full adoption of digital broadcasting. Even with the success, there are still some drawbacks for the TFTs. For example, the low mobility leads to low open area in a pixel because of designing the large size to achieve the high current. For the state-of-art process rule with definite gate length, adopting TFTs with higher mobility is one possible solution to achieve high aperture ratio in a small pixel area without sacrificing the display performance. Therefore, new TFTs with high mobility are needed to replace a-Si TFTs. Among those newly proposed TFTs with high mobility, poly-crystalline silicon (poly-Si) TFTs and amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs attract the most attention.Although poly-Si TFT has μ eff close to 100 cm 2 /V s, it requires additional re-crystallization steps such as excimer-laser annealing, metal seeding or solid phase crystallization. These add more complexity and costs to the process. The substrate size used by poly-Si TFT technology is about 4 generations behind what a-Si TFTs can achieve today, while a-IGZO device encounters no special issue when it comes to the glass size. A-IGZO TFTs have many advantages like low temperature process (below 300• C) and high on/off ratio (∼10 6 ). 1 In addition, it is highly transparent in visible light with transmittance over 90% as illustrated due to wide bandgap (∼3 eV).These properties open up to new applications such as transparent electronics, flexible electronics, and photo sensor.2,3 Even though a-IGZO TFTs exhibit good electrical characteristics and stability in the dark state, the significant electrical instability is observed when they are illuminated. Many article reported threshold voltage (V TH ) shift and mobility change after illumination at different light intensities, and those changes can recover in time. 4,5 In the applications of transparent electronics and photo sensor, the a-IGZO TFTs are expected to operate in the transmission of ambient light with frequently varying intensity. In addition, most of the reports studied the change in the devices under for long-time illumination, 6-8 but the response to the illumination ...

The Vacancy Pool Model for Amorphous In-Ga-Zn-O Thin-Film Transistors

“…The word pool implies that V O in a-IGZO are so many and various that they can have different reaction rates distributing over a very wide range. The fitting parameters had been well discussed in detail in our previous reports [20]- [23], [31]. We found the number of reacting V O was dependent on the light intensity.…”

“…In our previous research [20]- [23], a fitting formula had been proposed to describe the change of drain current ( I D ) under illumination in short and long term. More importantly, with more fitting terms, the accuracy of fitting can be better, which reveals the reaction contained by several reaction rates of Vo [23].…”

“…The different models were proposed to depict the degradation behavior of TFTs under various stress conditions involving light intensity, temperature, bias stress [13]- [19]. In our previous studies, we proposed a reaction model to describe the different reaction rate of oxygen vacancy (V O ) reacting with the light pulses [20]- [23]. Even though all the deterioration behaviors under all kinds of stress conditions are similar, the reaction rates of them are too diverse to be pictured in a unified expression.…”

“…To verify the proposed model, the broad stress conditions in light intensity, gate bias, light pulse of period, and light pulse of duty cycle are applied to the a-IGZO TFTs. In this study, the devices are subject to the same fabrication process and measurement method, which are described in our previous papers [20]- [24]. Based on the previous research, oxygen vacancies reacting with the light-induced electron hole pairs play the leading role in Vth [2], [8]- [10], [18].…”

P‐2: The Improved Photosensitivity of Amorphous In‐Ga‐Zn‐O TFTs with Gap‐type Structure

“…Using the same process, the a-IGZO TFTs as switches and light sensors can be monolithically fabricated on the same substrate in the display panel. However, the time response of drain current in conventional a-IGZO TFT under illumination is in the stretched-exponential formula, which limit a-IGZO to be a high sensitivity image sensor due to the slow response time [6][7][8][9]. Therefore, increasing the photosensitivity is a primary issue to solve.…”