(ECS Gordon E. Moore Medal for Outstanding Achievement in Solid State Science & Technology) Progress in Oxide Semiconductors and Oxide TFTs
At a Glance
Section titled āAt a Glanceā| Metadata | Details |
|---|---|
| Publication Date | 2025-07-11 |
| Journal | ECS Meeting Abstracts |
| Authors | Hideo Hosono |
Abstract
Section titled āAbstractāOxide semiconductors have characteristics rather different from conventional semiconductors based on diamond structure like Si. In this talk I introduce the progress of oxide semiconductors and their TFTs in comparison with Si and show a comprehensive view to understand the doping ability and p/n orientation from band alignment. Figure 1 summarizes brief history of oxide-TFTs and their relevant technology. TFT devise structure was proposed in 1926 by Lilienfeld as a patent [1]. In 1960ās, field effect on current modulation in thin films SnO 2 , In 2 O 3 , and ZnO were reported but the operation was restricted to depression mode which is energy consuming. As a result, oxide TFT research almost disappeared from open domains until ~2000. During this period, a breakthrough amorphous semiconductor was reported in 1975, i.e., amorphous hydrogenated Si (a-Si:H). This material is a first amorphous semiconductor in which Fermi level is controllable by impurity doping, and excellent operation of TFT using a-Si and simple liquid panel display (LCDs) using a-Si-H TFT were reported in 1979 [2]. While the mobility of a-Si:H TFT is ~0.5cm 2 (Vs) -1 , which is lower by 2-4 orders of magnitude than crystalline Si, this value was insufficient to drive organic light emitting diodes (OLEDs), which operates by current, but enough to operate voltage-driven LCDs. LCDs with a-Si:H TFTs were first applied to displays in calculators and subsequently to displays of PCs and TVs, leading to opening of active matrix, flat-panel displays (FPDs) in place of a cathode-ray tube monitor. Figure 2 summarizes the research on IGZO-TFTs and flat panel products driven with IGZO-TFT backplane. We proposed n-type transparent amorphous oxide semiconductor (TAOS) through a consideration on chemical bonding in 1996 [3] and reported high mobility ~10cm 2 (Vs) -1 TFTs using amorphous IGZO, a member of TAOS as the channel in 2004 [4](crystalline IGZO-TFTs is 2003 [5]). This paper triggered extensive research on TAOS-TFTs for displays. Practical application for LCDs and OLEDs started from ~2012 and ~2016, respectively, and IGZO-TFT is becoming a defact standard of the switching transistor. Nowadays memory application of IGZO-TFTs is seriously examining utilizing the very low off-current leading to low power consumption. Conduction in wide gap oxide semiconductors is n-type in general, and p-type oxides are rather limited as will be described later. A wide gap p-type oxide material, CuAlO 2 , was reported in 1997 along with a design concept [6], but this material did not work well as p-channel TFTs like Cu 2 O. First p-channel oxide TFT was realized by use of SnO, the first oxide C-MOS was reported by use of SnO with ambipolarity [7]. Carrier dupability and p/n orientation are the major concerns in semiconductor science and technology. Here these issues are considered on the basis of the band energy lineup of semiconductor materials. The levels of the CBM and VBM for various materials are lined up with respect to the vacuum level (Evac). Figure 3 shows a band lineup of oxide semiconductors and relevant semiconductors. The following findings were obtained from the band lineup [8]: (1) n-type doping: EA>3.5eV, (2)p-type doping: Ionization potential < ~-6 eV. Two monographs on this subject are listed in reference[10,11]. References Lilienfeld , J.E. (1926 ). US Patent 1,745,175. Le Comber, P. G., Spear, W. E., & Ghaith, A. (1979) Elect. Lett. 6, 179. Hosono, H., Kikuchi, N., Ueda, N., & Kawazoe, H. (1996) J.Non-Cryst. Solids, 198, 165. Nomura, K., Ohta, H., Takagi, A., Kamiya, T., Hirano, M., & Hosono, H. (2004). Nature, 432,488. Nomura, K., Ohta, H., Ueda, K., Kamiya, T., Hirano, M., & Hosono, H. (2003) Science, 300, 1269. Kawazoe, H., Yasukawa, M., Hyodo, H., Kurita, M., Yanagi, H., & Hosono, H. (1997). Nature, 389, 939. Ogo, Y., Hiramatsu, H., Nomura, K., Yanagi, H., Kamiya, T., Hirano, M., & Hosono, H. (2008). Appl. Phys. Lett, 93(3) 032113. Nomura, K., Kamiya, T., & Hosono, H. (2011) Advanced Materials, 23(30), 3431. Hosono, H. (2013) Jpn. J.App. Phys., 52(9R), 090001. Hosono, H., & Kumomi, H. (Eds.). (2022). Amorphous Oxide Semiconductors: IGZO and Related Materials for Display and Memory. Kuo,Y, Hosono,H, Shur,M. & Jang,J. (2025) Oxide Thin Film Transistors, Wiley Figure 1