Recently, oxide-based thin-film transistors (TFTs) have attracted considerable attention for use in next-generation flat panel displays due to high mobility (µ_eff), large-area uniformity, and large band gap semiconductor material [1,2]. Amorphous IGZO (a-IGZO) is widely accepted as a promising channel material for TFT applications [3]. Many researchers reported that the wavelength of the illumination light affected characteristics of a-IGZO TFTs [4,5]. Recently, it was observed that characteristics of the a-IGZO TFT change under light illumination and at the raised temperature [4]. The sub-threshold swing (S) increased and the threshold voltage (V_th) shifted to the negative direction when the wavelength was decreased [5]. However, the influence of the light wavelength on top- and double-gate of a-IGZO TFTs have not been clarified in detail. In this study, characteristics of top- and double-gate a-IGZO TFTs illuminated with lights of different wavelengths are reported.

Bottom-gate a-IGZO TFTs with an inverted staggered structure were fabricated on a glass substrate. First, a 150 nm-thick molybdenum (Mo) film was used as the gate electrode material. Then, a gate insulator of SiO_x (300 nm) was deposited using the plasma-enhanced chemical vapor deposition (PE-CVD) method. A 50 nm-thick a-IGZO layer was then sputter deposited from an In₂O₃: Ga₂O₃:ZnO (1:1:1 mole ratio) target at room temperature. Next, a SiO_x (200 nm)/AlO_x (20 nm) stack was deposited as the etch stopper layer. The Ti/Al/Ti source and drain electrodes were deposited in contact with the a-IGZO layer through via holes in the etch stop layer. After the device was passivated with a PECVD SiO_x (100 nm)/SiN_x (100 nm) stack, an ITO film was deposited and etched into the top gate electrode. The top electrode is located at the center position of the bottom-gate electrode with a gap space of 20 μm from each of the source and drain electrodes. The channel length (L) and width (W) of the bottom-gate were 80 and 50 μm, respectively. All electrical measurements were carried out in ambient air using an Agilent 4155C precision semiconductor parameter analyzer. During the illumination measurement, the TFT was illuminated with a LED light from the top electrode direction. The wavelengths are: 630 nm (red), 560 nm (green), 470 nm (blue) and 395 nm (UV. The light intensity was 0.3 mW cm⁻².

Figures 1 and 2 show the transfer characteristics of the a-IGZO TFTs measured at room temperature in dark and under red, green, blue, and UV illuminations separately. The µ_eff was extracted from the transconductance in the saturation region. The V_th was the gate voltage (V_GS) at the drain current (I_DS) of 1 nA. The S was the required VGS when I_DS changed from 1 to 10 nA. Electrical properties of top- and double-gate TFTs are summarized in Tables 1 and 2. Those parameters were estimated from transfer characteristics at V_DS= 20 V. With the decrease of the wavelength, the top-gate TFT’s mobility is increased but both V_th and S deteriorate. For the double-gate TFT, similar but less obvious tends are observed except for the V_th. When the wavelength changed from 2 eV (red) to 3.19 eV (UV), much more charge carries are generated in the a-IGZO layer, which induced higher conductivity in the channel. Therefore, µ_eff, V_th and S in both top- and double-gate IGZO TFTs are changed.

Jingxin Jiang would like to thank the Doctoral Scientific Research Foundation of Liaoning Province, No. 201601156 and China Scholarship Council for proving financial support for her visit to Thin Film Nano and Microelectronics Laboratory, Texas A&M University through the Postdoctoral Research Program.

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