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 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. 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. 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. 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 time Gate Voltage Source Drain time Voltage time T/N T T VG time Gate Voltage 1 2 3 4 VSD VON Source Drain Voltage time VP time Pixel Voltage 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.