Study Of A Normally On Driving Under Gate Field Emission Display Panel

Field Emission Display (FED) device has acquired further achievement with the development of vacuum microelectronics. Fabrication process of Carbon Nanotube (CNT) has been improved greatly since 1990s. With the reduction of cost, large FED panel display with CNT became to be a hot development direction. In order to realize high luminance and deep grey level, triode structure FED based on thick film technology shows a destiny direction.Special simulating software designed for thick film based field emission display is developed. The operational property of normal gate (NG) FED and traditional under gate (TUG) FED is calculated by the CAD software.The calculated results show that the modulation range of gate is relatively small in NG FED. However, the current density at the edge of the cathode is higher than that at the center of the cathode,which will reduce the cathode's lifetime. In addition, the cathode field emission is sensitive to gate aperture, thickness of dielectric layer and alignment accuracy. Although small driving voltage could be obtained, NG FED structure is not suitable for thick film process considering of the process and working principal. So it is difficult to cut down the fabrication cost, which is essential for large panel display. Calculated results for TUG FED show that the anode voltage is limited to be rather lower value because the cathode located between the anode and under gate electrode. Over high anode voltage will introduce background emission. Comparing with NG structure, TUG FED shows even worse uniformity of cathode emission under normal operating condition. So the TUG FED faces more serious lifetime problem. Furthermore, this structure brings distinct divergence of the electron beam, which causes large spot size on the screen and cross talk between adjacent pixels. However, the tolerance of the structural parameters influences the emission characteristic little, which makes UG structure adaptable to thick film process.To solve these problems, a novel normally on driving under-gate (NOUG) FED is presented in this paper. In NOUG FED, the ratio of the cathode width to the thickness of the dielectric layer is reduced significantly comparing with the TUG FED. 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. Thus the divergence problem can be solved effectively. The normally on driving under-gate (NOUG) FED structure is simulated by the self-designed CAD software. The calculated results show that higher anode voltage can be adopted in NOUG FED structure.Even though the gate electrodes are buried under the cathodes, which will make the cathode exposed directly to the anode, the negative voltage applied on the under gate can cut off the influence of the high anode voltage. Extracted by the high anode voltage, cathode field emission electrons show excellent uniformity along the cathode surface, which will certainly increase the lifetime greatly. By simulating the influence of the structure tolerance, it is found that the structure tolerances have little influence on cathode emission property. That is why it can be said the NOUG FED is a low cost structure and can be applied to large panel display.The NOUG FED structure is further optimized to improve the possibility of mass production. A double cathode NOUG FED structure, which has two cathodes in one pixel, is presented in this paper. With more narrow width of cathodes, the gate operating voltage is reduced and the anode voltage is increased further. The peak emission current density on the cathode is also reduced to lower the cathode load. Electron spot with uniform density and proper size of is obtained on the anode screen by optimization of anode voltage, which will increase the efficiency and lifetime of phosphor.Traditional under gate and normally on driving under gate structure FED sample panels are fabricated based on thick film process to validate the simulated result. The measurement results reveal the possibility of normally on driving under gate structure.