tailieunhanh - Features of Liquid Crystal Display Materials and Processes Part 7

Tham khảo tài liệu 'features of liquid crystal display materials and processes part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Active Matrix Driving and Circuit Simulation 109 The second factor is the capacitance between the gate and source terminal Cgs which depends on the state on or off of the TFT. Fig. 4. shows a cross-sectional view of a general a-Si TFT. While TFT is the on-state a a conducting channel forms at the bottom of the a-Si layer and a capacitance between the channel and gate electrode Con has been generated. Half of Con can be regarded as Cgs in the on-state of TFT. A conducting channel does not form while TFT is the off-state b . Therefore Cgs during the off-state is the same as the capacitance between the source and gate electrode Coff . Fig. 5 shows the dependence of Cgs on Vgs. Fig. 4. Gate-source capacitance On state Off state Fig. 5. Dependence of Cgs on the applied voltage The cut-off voltage which TFT changes form on-state to off-state is Vgs Vth. The equation for AVfd is updated by taking this behavior into account. In Eq. 6 AVfd in a plus frame is smaller than it is in a minus frame since Con 2 is bigger than Coff. V Con 2 Vgon VPIX Vth Coff VplX Vth Vgoff 6 fd Con 2 Cic Csc Coff Clc Csc 1 110 Features of Liquid Crystal Display Materials and Processes The third factor is the influence of a scan pulse distortion on AVfd. The scan pulse significantly decays at pixels farther away from the scan driver. Such a decay could cause problems especially in larger displays Watanabe 1996 . Fig. 6 a shows the feed through voltage AVfd under the condition that the scan pulse does not decay. b shows the feed through voltage AVfd under the condition that the scan pulse decays. Fig. 6. Influence of a scan pulse distortion on the feed through voltage When the scan pulse decays it takes a finite amount of time At for the TFT to switch off. During At current Ids t continues to flow from a data line to the pixel electrode. Therefore AVfd is modified to Eq. 7 . ÁVfd_d - Vfd - Cgs c c CSC dS t dt 7 According to Eq. 7 the effective feed through voltage AVfd_d is .