Important progress in the research of thin-film transistors

Source: School of Electronics and Information Technology
Written by: School of Electronics and Information Technology
Edited by: Wang Dongmei

Thin film transistors (TFTs) based on metal oxide semiconductors (especially amorphous InGaZnO) have the advantages of smaller off-state current, higher field-effect mobility and transparency in the visible wavelength, as compared with traditional amorphous silicon. Now it has been used in the driving panel of OLED, QLED, and other displays. However, there are still several unsolved problems in fundamental understanding for such semiconductors. Among them, how to obtain excellent transistor performance and accurately characterize carrier mobility has always been the focus of academic research. On the one hand, it has been reported that the field effect mobility of amorphous InGaZnO can be increased to more than 100 cm2/(Vs) by complicated micro- or nano-treatment. On the other hand, it has also been reported that the field effect mobility of single crystal InGaZnO is only about 80 cm2/(Vs). Therefore, to further understand and utilize these semiconductors, we have to solve two urgent problems: (1) Can we use simple technologies to obtain oxide transistors with high apparent mobility? (2) How to analyse and understand transistors with high apparent mobility?

To solve the above problems, Professor Chuan Liu's research team has recently found that stable hydrogen-rich IGZO transistors can be fabricated by simple encapsulating and thermal annealing, and their electrical properties and stability have been greatly improved. The preparation method is simple and repeatable. After encapsulated with silicon nitride film, hydrogens in silicon nitride are diffused to InGaZnO film by thermal annealing. The on-state current and switching ratio of hydrogen-doped transistors have been increased by an order of magnitude (about 40 times), and the threshold voltage remains a reasonable value. The field effect mobility of the corresponding extracted samples was abnormal, which was more than 300 cm2/(Vs), much higher than that of the untreated control samples. Secondary ion mass spectrometry (SIMS) and X-ray photoelectron spectroscopy (XPS) characterization showed that the hydrogen concentration in the films increased by about one order of magnitude. Also, the oxygen vacancy defects were greatly reduced and the free electron concentration increased accordingly.

Unlike previous studies, the off current of hydrogen-doped transistors remains at a very low level, and the switching ratio has not decreased, but increased by about two orders of magnitude. Researchers used four-probe test and Kelvin probe method to measure the channel potentials, and found the carrier concentration in a non-uniform distribution. Therefore, a low-high-low electron concentration model is proposed in the channel and near the electrode. Combined with computer aided semiconductor device simulation and density functional theory calculations, the new working principle is further verified. The reasons for the increased switching ratio and the abnormal distortion of field effect mobility extraction are clarified. It is also pointed out that this mode of operation is particularly effective in long channel transistors, while it is limited in short channel devices. This work reveals a method to stabilize hydrogen in oxide semiconductors and its key role in conductivity. It provides a new device mode for improving the current driving ability of long channel transistors. It also provides a general theoretical basis and experimental method for the verification and analysis of high mobility thin film transistors.

The above research results, titled "Analysis of Ultrahigh Apparent Mobility in Oxide Field Effect Transistors", were recently published in Advanced Science (DOI: 10.1002/advs. 201801189, impact factor 12.441). The School of Electronics and Information Engineering of Sun Yat-sen University is the first unit and corresponding affiliation. The authors include Changdong Chen, Professor Bo-ru Yang, Dr. Qian Wu, Dr. Gongtan Li, Dr. Runze Zhan and Professor Shaozhi Deng. The corresponding author is Professor Chuan Liu. This work has been supported by the Science and Technology Program of Guangdong Province (Grant no. 2015B090924001), State Key Lab of Opto-Electronic Materials & Technologies, and Guangdong Province Key Lab of Display Material and Technology.

The paper is online: http://dx.doi.org/10.1002/advs.201801189