48.3 WITHDRAWN 48.4: Invited Paper: Driving Circuit Integration Using Depletion‐Mode Oxide TFTs

Abstract

New circuit schemes are needed to integrate display-driving circuits using oxide TFTs because the oxide TFTs operate only as n-channel field-effect transistor and tend to have negative threshold voltages. So we have invented circuits that apply negative gate bias to turn off the TFTs inside themselves. In this paper we report some of these circuits such as scan driver, DC-DC converter, and level shifter. IntroductionThin film transistors (TFT) employing metal-oxide semiconductors such as In-Ga-Zn-O (IGZO) have attracted much attention in recent years because they exhibit high on-current and low process cost. The oxide TFT is expected to be used not only in the active-matrix liquid-crystal display (AMLCD) but also in the active-matrix organic light-emitting diode (AMOLED) display because it is more stable than the a-Si TFT and has better shortrange uniformity than the laser-annealed poly-Si TFT [1,2].However the oxide TFT operates only as n-channel field-effect transistor and often exhibits depletion-mode characteristics due to microscopic process fluctuation and external influences such as illumination and bias stress [3,4]. As a result, a considerable amount of current may flow at zero gate-to-source bias (V GS ). Due to the possible negative threshold voltage (V T ) of the oxide TFTs, it is not desirable to utilize conventional circuit scheme when we integrate the driving circuitry on a display panel. In this paper, we propose new display-driving circuits compatible with the depletion-mode oxide TFTs such as scan driver, DC-to-DC converter, and level shifter. 2.Scan Driver Figure 1 shows the unit circuit diagram of the scan driver compatible with the depletion-mode oxide TFT. The timing diagram of clock signals is shown in Fig. 2. It should be noted that the low voltage levels of CKL and CKLB are lower than those of CK and CKB. The low levels of CKL and CKLB are also lower than V SSL as well as V SS to turn off the TFTs completely. SmartSpice simulation results of the scan driver showed that it worked successfully over the TFT V T range from -5 V to +5 V. The channel widths of M1 ~ M8 were 100 μm, 180 μm, 50 μm, 40 μm, 30 μm, 80 μm, 80 μm, and 50 μm in order and the channel