Picture element structure of current programming method type active and driving method of data line
View Patent ↗The present invention provides a novel structure of picture elements in current programming-type semiconductor devices, and in particular, the structure of picture elements of an active matrix organic light emitting diode (OLED) display. The device makes a self-compensation for OLED current deviations due to the deterioration in threshold voltage and uneven electric characteristic in thin film transistors. The invention also provides a method for driving a data driver capable of compensating for the uneven electric characteristic of thin film transistors in the driver for driving picture elements in the current programming-type active matrix OLED display device.
1. A picture element structure in a current programming type of active matrix organic light emitting diode (OLED), comprising:
first and second transistors for selecting a driving picture element based upon a scan signal applied from an exterior, said first and second transistors being adapted to receive a data current;
a capacitor for storing electric charges applied from the first and second transistors;
a third transistor adapted to be selected by the first and second transistors, for writing the data current thereto and receiving an external power source;
a fourth transistor having a gate that is coupled to a gate of the third transistor, for receiving a voltage based upon the electric charges stored in the capacitor to supply a current to a corresponding picture element, wherein gate voltages of the third transistor and fourth transistor are always equal to each other;
an organic light emitting diode (OLED) for emitting light by a current flowing through said fourth transistor; and
a fifth transistor connected in series to the fourth transistor and formed of a conductivity type of transistors different from that of said first to fourth transistors having the same conductivity type of transistors, a gate of said fifth transistor being coupled with a scan line,
wherein during a gate selection session for writing the data current to the third transistor, the scan signal is low, the first and second transistors are turned on, the third transistor operates in a saturation area and the fourth transistor is turned off, and
wherein after the gate selection session, the scan signal is high, the first and second transistors are turned off, the third transistor operates in a linear area, the fourth transistor operates in a saturation area, and a current I OLED flows through the third transistor operating in the linear area and the fourth transistor operating in the saturation area so that the current IOLED is scaled down compared with the data current.
2. The picture element structure in a current programming type of active matrix organic light emitting diode (OLED) according to claim 1 , wherein the magnitude of the current applied to the corresponding picture element is operatively controllable depending upon a ratio of channel width/channel length (W/L) in the third transistor and channel width/channel length (W/L) in the fourth transistor.
3. A picture element structure in a current programming type of active matrix organic light emitting diode (OLED) having at least one scan line, at least one data line and a power supply source, comprising:
a capacitor connected to the power supply source, for storing electric charges supplied to said data line;
a first transistor having a P-type thin film transistor, of which gate is connected to the scan line and source/drain current path is connected to the data line;
a second transistor having a P-type thin film transistor, of which gate is connected to the scan line and source/drain current path is formed between said capacitor and said first transistor;
a third transistor having a P-type thin film transistor, of which gate is connected to the capacitor and source/drain current path is formed between said power supply source;
a fourth transistor having a P-type thin film transistor, of which gate is connected to the gate of the third transistor and source/drain current path is connected in said third transistor, wherein gate voltages of the third transistor and fourth transistor are always equal to each other; and
an organic light emitting diode (OLED) for emitting light by a current flowing through said fourth transistor,
wherein during a gate selection session for writing the data current to the third transistor, the scan signal is low, the first and second transistors are turned on, the third transistor operates in a saturation area and the fourth transistor is turned off, and
wherein after the gate selection session, the scan signal is high, the first and second transistors are turned off, the third transistor operates in a linear area, the fourth transistor operates in a saturation area, and a current L OLED flows through the third transistor operating in the linear area and the fourth transistor operating in the saturation area so that the current IOLED is scaled down compared with the data current.
4. A picture element structure in a current programming type of active matrix organic light emitting diode (OLED), comprising:
first and second transistors for selecting a driving picture element based upon a scan signal applied from an exterior, said first and second transistors being adapted to receive a data current;
a capacitor for storing electric charges applied from the first and second transistors;
a third transistor adapted to be selected by the first and second transistors, for writing the data current thereto and receiving an external power source;
a fourth driving transistor having a gate that is coupled to a gate of the third transistor, for receiving a voltage based upon the electric charges stored in the capacitor to supply a current to a corresponding picture element, wherein gate voltages of the third transistor and fourth transistor are always equal to each other;
an organic light emitting diode (OLED) for emitting light by a current flowing through said fourth transistor; and
a fifth transistor connected in series to the fourth driving transistor, for making an output resistance of the fourth driving transistor increase to eliminate a kink-effect current drop, wherein said fifth transistor having a biasing connection so that said fifth transistor is always in a turned on state,
wherein during a gate selection session for writing the data current to the third transistor, the scan signal is low, the first and second transistors are turned on, the third transistor operates in a saturation area and the fourth transistor is turned off, and
wherein after the gate selection session, the scan signal is high, the first and second transistors are turned off, the third transistor operates in a linear area, the fourth transistor operates in a saturation area, and a current IOLED flows through the third transistor operating in the linear area and the fourth transistor operating in the saturation area so that the current IOLED is scaled down compared with the data current.