IEEE ELECTRON DEVICE LETTERS, VOL. 38, NO. 7, JULY 2017 887 Analysis of the Short-Term Response in the Drain Current of a-IGZO TFT to Light Pulses H.-W. Liu, P.-C. Chan, J.-H. Lin, C.-Y. Chang,and Y.-H. Tai Abstract — In this letter, the response in the drain current (I D ) of the amorphous indium–gallium–zinc oxide thin-film transistors under positive-bias illumination stress is measured with respect to time in less than 5 s under light pulses with altering frequencies and duty ratios. The curves of I D under the light pulses are affected by the different defect-reacting rates under illumination and in the dark. By taking the derivative of I D , the trend of the change in the number of defects becomes clear. The total behavior of I D in response to light pulses can be fairly predicted by the integral of the derivative terms with a correction factor of charge trapping. Index Terms— Amorphous indium gallium zinc oxide (a-IGZO), thin-film transistor (TFT), illumination effect. I. I NTRODUCTION N OWADAYS, amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) have become a potential candidate for the next generation of transparent displays because of many advantages [1]. Although a-IGZO TFTs have shown good performance in the dark state, there are still some drawbacks such as the electrical instability under light illumination and gate bias stress, like threshold voltage (V TH ) shift and mobility change [2], which limit its applications in transparent sensing circuits operating in practical light ambiance [3], [4]. In our previous studies, we analyzed the drain current change (I D ) with respect to the response time of different frequencies light illumination [5] and also proposed a modified fitting model by changing the fitting parameters with time dependent [6], [7]. The fact that the drain current behaves in accordance with the short-term illumination history makes it difficult to establish the model. We therefore apply light pulses of different frequencies and duty ratios in the experiments Manuscript received May 2, 2017; revised May 15, 2017; accepted May 15, 2017. Date of publication May 18, 2017; date of current version June 23, 2017. The review of this letter was arranged by Editor Shoou-Jinn Chang. (Corresponding author: P.-C. Chan.) H.-W. Liu is with the Department of Electrical Engineering and Grad- uate Institute of Optoelectronic Engineering, National Chung Hsing University, Taichung 402, Taiwan. P.-C. Chan, J.-H. Lin, and C.-Y. Chang are with the Department of Pho- tonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: maksim75610@gmail.com). Y.-H. Tai is with the Department of Photonics and College of Electrical and Computer Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2017.2705701 Fig. 1. The time responses of I D under positive DC bias stress at V G = 10 V, V D = 10V with or without illumination of light pulses with different frequency. to give an insight into the mechanism of I D in response to illumination. II. EXPERIMENTAL PROCEDURE The a-IGZO TFTs tested in this study are the same as the ones used before [8]. In all the measurements, the gate and drain voltages (V G and V D ) are both biased at 10 V. The blue light LED is used for illumination. Before each measurement, the TFT was placed in a dark environment for at least 1 hour for better recovery with a relatively stable performance. Light pulses with alternating frequencies and duty ratios are driven by a 4225-Pulse Measure Unit (PMU) in a Keithley 4200 semiconductor parametric analyzer, and I D is measured in sampling mode with another synchronized PMU. III. RESULTS AND DISCUSSION Fig. 1 shows the time responses of I D under DC positive bias stress (PBS) and positive bias illumination stress (PBIS), where the light pulses are with a duty ratio of 50% and different frequencies. The curves are aligned by subtracting their corresponding initial values. Obviously, the response differs with the frequency, even though the average light intensity for all cases is 10,000 Lux. It exhibits the com- plexity of I D in response to the different waveforms of light pulse. As for PBS, the slow decrease of the drain current under is observed and attributed to the effect of charge trapping [6], [7]. 0741-3106 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.