Transcript Document
Active Matrix Displays • Multiplexing is limited and not adequate for high resolutions (slow response, poor viewing angle, no gray scale) • A non-linear element is build into the substate at each pixel, usually a thin-film-transistor • Being isolated from other pixels by TFT’s, the voltage remains constant while the other pixels are being addressed • Not subject to Alt-Pleshko Formalism Active Matrix Circuit Scan Line Drain Source Liquid Crystal Active Matrix: A Complex Device Drain Principle of Operation-Active Matrix • One line at a time addressing • A positive voltage pulse (duration T/N, N # rows, T frame time) is applied to the line turning on all TFT’s • The TFT’s act as switches allowing electrical changes to the LC capacitors from the columns (data or source) • When addressing subsequent rows a negative voltage is applied to the gate lines thereby turning off the transistors for one frame time T, until ready to readdress it • For AC drives schemes (LCD’s) the polarity is alternated on the data voltage 4 Basic Steps of TFT 1. At time 1, a positive voltage VG of duration T/N is applied to gate to turn on TFT. The LC pixel (ITO) is changed to VON at time 2 within T/N, due to the positive source voltage VSD=VON. 2. At time 2, the gate voltage VG becomes negative, turning off the source voltage VSD from VON to –VON. During the time period 2 and 3, of duration (N-1)/NT, the pixel voltage VP remains about >0.9 VON as the LC capacitor is now isolated from data lines. 3. At time 3 (the next addressing time), the TFT is turned on again by applying a positive gate voltage of duration T/N. The LC capacitor now sees a negative source-to-drain voltage VSD=-VON. The pixel electrode is discharged from VP=VON at time 3 to VP=-VON within the time duration T/N. 4. At time 4, the TFT is turned off by the negative gate voltage, and simultaneously the source voltage VSD changes from –VON to +VON. TFT Addressing VG T/N T T 0 1 2 VSD 0 VON time Gate Voltage time Source Drain Voltage time Pixel Voltage 3 4 VP Notice that VP is not constant during the duration (n-1)T/N because of a slight leakage current of LC cell. LC materials must have a high voltage holding ratio (VHR) to minimize this.