To enhance brightness and realize the high gray level of field emission display device, the triode structure is necessary. Some different triode structures are proposed, such as normal-gate[1], under-gate[2] and planar-gate[3], etc. Due to different reasons, all these structure don't realize the advantages of triode structure FED totally. Aim to the traditional under-gate structure, the normally-on driving under-gate FED was proposed. In this structure, the ratio of the cathode width to the thickness of the dielectric layer is reduced significantly comparing with the traditional under-gate FED. In the new driving scheme, the field emission electrons are extracted from the whole cathode surface by the high anode voltage directly. The effect of the under-gate is to prevent the field emission when the negative voltage is applied on the gate electrode. The electric field inside the emission region is calculated by finite element method. The emission property of the CNT film and spot on the anode are also studied by numerical calculation method. As shown in Fig.1, a uniform emission from cathode is obtained using this driving scheme, which prolong the life of cathode. However there are still some disadvantages for this driving scheme, such as the modulation voltage of gate is large and the spot size is about one third of the pixel area, which brings unideal display effect and decrease the lifetime of phosphor layer.
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