1. 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

2. Active Matrix Circuit
Scan Line
Drain
Source
Liquid Crystal

3. Active Matrix: A Complex Device
Drain

4. 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

5. 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.

6. 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.