Scaling of Coplanar Homojunction Amorphous In–Ga–Zn–O Thin-Film Transistors Gwanghyeon Baek 1 , Katsumi Abe 2y , Hideya Kumomi 2y , and Jerzy Kanicki 1 1 Displays and Detectors Laboratory, Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109, U.S.A. 2 Canon Research Center, Canon Inc., Ota, Tokyo 146-8501, Japan E-mail: kanicki@eecs.umich.edu Received July 25, 2012; revised September 11, 2012; accepted October 20, 2012; published online March 21, 2013 Channel length (L) and width (W ) scaling of amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) have been investigated by coplanar homojunction a-IGZO TFTs. The fabricated TFTs have a mobility around 12 cm 2 V 1 s 1 , sub-threshold slope (S) of 110 mV/decade, threshold voltage around 0.3 V and off-current below 10 13 A. The TFTs with L> 5 m have the reduced transconducance (g m ) at lower V GS , however, the short L< 5 m TFTs have the g m reduction at higher V GS . Even though the TFTs with smaller channel length (L  5 m) show proper switching characteristics, threshold voltage lowering and sub-threshold slope degradation are clearly observed. # 2013 The Japan Society of Applied Physics 1. Introduction High resolution (beyond 1920  1080), high pixel density (300 pixels per inch, PPI), large panel size (80-in.) and a fast frame rate (240 Hz) have become key features of future technology in the area of active matrix flat panel displays (AM-FPD). The essential requisite for achieving these features is a high field-effect mobility  of thin-film transistors (TFTs). This is because a higher  induces a higher drain current, so enabling the TFTs to switch faster and/or occupy a smaller pixel area. It is estimated that the ultrahigh-definition (7680  4320) television (UHD TV or 8K TV) with 120 Hz of refresh rate requires  over 3 cm 2 V 1 s 1 . 1) The hydrogenated amorphous silicon (a-Si:H) TFTs are not adequate for such a high resolution application because  of a-Si:H TFTs has the range of 0.5– 1 cm 2 V 1 s 1 . 2,3) In this regard, amorphous indium–gal- lium–zinc–oxide (a-IGZO) TFTs have received considerable attention as a possible replacement for a-Si:H TFTs. 4–7) Thanks to previous intensive research studies on a-IGZO TFTs, it has been proved that a-IGZO TFTs with  over 5 cm 2 V 1 s 1 can be easily fabricated. 8–10) Besides the mobility requirement, TFT miniaturization is another important factor for high pixel density applications. Since an allowed pixel size decreases as the PPI increases — the pixel size for 500 PPI is only 50:8  16:9 m 2 , for instance — the TFT needs to be scaled down along with the pixel size to be fitted inside of the pixel area. This situation is even worse in active-matrix organic light emitting display (AM-OLED) because at least two TFTs must be squeezed in each pixel area. 11) Furthermore, it is known that the miniaturization of TFTs improves the display performance by reducing the parasitic capacitance. The high mobility and miniaturization requirements are also critical for the high resolution active matrix flat panel imager (AM-FPI), especially for the application of digital radiology. 12,13) Even though a-IGZO TFTs with an L smaller than 5 m were previously reported by other groups, 14) a systematic study of the scaling of a-IGZO TFTs has been lacking. Therefore, it is worthwhile to investigate the scaling characteristics of a-IGZO TFTs. In this paper, we present an in-depth study of the scaling dependency of coplanar homojunction a-IGZO TFTs. The coplanar homojunction a-IGZO TFTs investigated in this paper utilize a hydrogen doped a-IGZO source/drain (S/D) region, n þ a-IGZO, to achieve a low resistance S/D contacts. It was previously reported that this structure has the advantages of small S/D contact resistance and the capability of achieving a small L. 15) Therefore, the scaling characteristics for this type of a-IGZO TFT is worthwhile to be investigated. 2. Methods A cross-sectional schematic of the fabricated TFT is shown in Fig. 1(a). The TFTs were fabricated on a Corning 1737 glass (a) (b) Fig. 1. (Color online) (a) Schematic cross-sectional structure of the coplanar homojunction a-IGZO TFT, (b) macroscopic top view of the fabricated TFT. CPL defines channel length (L) and width (W) of the TFT. y Present address: Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan. Japanese Journal of Applied Physics 52 (2013) 03BB05 03BB05-1 # 2013 The Japan Society of Applied Physics REGULAR PAPER http://dx.doi.org/10.7567/JJAP.52.03BB05