(Invited) Amorphous Indium-Gallium-Tin Oxide Thin-Film Transistor with High Mobility of 40 cm2 V−1 s−1 and Excellent Stability

Metal oxide semiconductor (MOS) has been studied extensively for use in the backplane of the display due to its extremely low leakage current, low temperature and large area process. Among them, InGaZnO (IGZO) thin-film transistor (TFT) is the most popular due to its relatively high mobility and good stability. MOS TFT has a challenge to replace Low-Temperature Poly-Si (LTPS) TFT which is high cost TFT process. However, the electron mobility of IGZO is typically ~10 cm 2 V −1 s −1 , which is very low compared to LTPS, making it unsuitable for use in high resolution backplane of active-matrix organic light-emitting diode (AM-OLED) display. There are limitations to increasing the mobility of a-IGZO. Therefore, research on high mobility MOS such as In 2 O 3 and InGaO (IGO) to replace LTPS is attracting attention recently. Note that Ga effectively suppresses carrier concentration and lowers the Fermi level (E F ), resulting in stable on/off characteristics with excellent stability. The tin cation (Sn 4+ ) increases the carrier concentration by having an electron orbital similar to In (In 3+ ) and increase the overlap of the 5s orbitals. Therefore, a-InGaSnO (IGTO) TFT can have high mobility and thus a good candidate for low-cost AM-OLED TFT backplane. In our previous study, we developed back channel etched (BCE) structured dual-gate a-IGTO TFT exhibiting high mobility (39.1 cm 2 V −1 s −1 ). When compared with a-IGZO TFT, a-IGTO TFT exhibits high mobility, excellent stability, and steep subthreshold swing (SS). We confirmed low density of state in band gap of a-IGTO. But BCE structure has an unintended parasitic capacitor between gate and source/drain electrode that can deteriorate operating characteristics. This can be improved by introducing coplanar structure TFT. It is important to secure thermal stability characteristics at high temperatures. Tin has a higher bond dissociation energy with oxygen (548 kJ/mol) than indium (360 kJ/mol). Therefore, the thermal stability can be improved in a-IGTO TFT. In this work, we present the high-performance, coplanar structured a-IGTO TFTs fabricated on the glass substrate. The a-IGTO TFT exhibits the field-effect mobility (µ FE ) of 40.0 cm 2 V −1 s −1 , SS of 0.16 V dec −1 , and on/off current ratio of > 10 7 at low drain voltage (V DS ) of 0.1 V. Even with the short channel length of 2 μm, it shows a threshold voltage (V TH ) close to 0 V (−0.3 V) and hysteresis-free characteristics. We confirmed good ohmic contact between Mo electrode with a-IGTO. At gate voltage (V GS ) of +20 V, the channel resistance and contact resistance of the a-IGTO TFT are found to be 27.47 kΩ/μm and 16.77 kΩ, respectively. Additionally, we confirmed excellent thermal stability of the a-IGTO TFT, demonstrating unaffected electrical characteristics even after air annealing. The coplanar structured a-IGTO TFT can be a promising candidate for low-cost AMOLED backplane. Figure 1