JPH06347753A - Method and equipment to recover threshold voltage of amorphous silicon thin-film transistor device - Google Patents

Abstract

PURPOSE: To provide a method and a device capable of simply and easily recovering the shift of threshold voltage for an amorphous silicon thin film transistor(TR) liquid crystal display(LCD) device. CONSTITUTION: Gate voltage Vg is supplied to respective TRs in using out of plural thin film amorphous silicon TRs 12 deposited on a substrate 10, so that threshold voltage Vth is shifted with the lapse of time. Time when the LCD device is not used is detected, and when the LCD device is not used, voltage Vg' having polarity reversed from that of the voltage Vg is generated and sent to the gates of the TRs in the LCD device, so that the voltage Vth is shifted in the reverse direction against the shift generated by the voltage Vg . Consequently effective driving voltage for the TRs in the LCD device can be held.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to methods and apparatus for threshold voltage shift recovery associated with amorphous silicon (a-Si: H) thin film transistor (TFT) devices. More particularly, the invention is used in amorphous silicon thin film transistor devices used in liquid crystal display (LCD) devices such as data driver circuits and scan driver circuits, or in pixel switching devices. That is the goal.

[0002]

2. Description of the Related Art Amorphous silicon TFT devices are widely used in active matrix LCD display devices because of their good switching characteristics. However, due to the shift of the threshold voltage during operation,
It causes the instability of amorphous silicon. Thus, during use of an LCD display device, factors such as temperature and applied voltage may change, changing the characteristics of the device. Studies have shown that this change in threshold voltage over time is due to changes in gate bias voltage. The higher the device is used in high temperature conditions, the faster the shift of the threshold voltage.

The threshold voltage V _th is plotted in graph form by plotting the square root of the TFT drain current I _ds along the vertical axis to yield the TF along the intersecting horizontal axis.
It is defined as the voltage V _{g at} the gate of the T device. During operation of the LCD display, a gate voltage V _g greater than V _th is required to provide sufficient current to drive the TFT pixel elements of the LCD display. As _Vth shifts over time, and as it accelerates at higher temperatures, the effective drive voltage between the gate and source will decrease, and also the performance of the LCD-TFT display will degrade.

Therefore, the shift of the threshold voltage V _th is
Retaining the threshold voltage V _th during the lifetime of the LCD display is very important and desirable because it causes instability in the gate voltage V _g to properly open or close the pixel TFT. Is.

For example, as shown in FIG. 1, when a positive 40 volt DC voltage is applied to the gate of a TFT device at 25 ° C. for about 15 hours, a shift in threshold voltage V _th of 6 to 7 volts is _achieved . Can happen. As shown in FIG. 2, when a 30 volt AC pulse with a 50% duty cycle is applied to the gate of the TFT device for about 88 hours, a V _th shift of about 5 volts can occur. The direction of the threshold voltage shift depends on the sign of the voltage between the gate and source of a given TFT device. A negative threshold voltage shift occurs when a negative DC voltage is applied to the gate of the TFT device for a fixed time interval. For example, in FIG. 1, a negative shift in V _th of about 2.3 volts is shown when a negative 20 volt voltage is applied to the gate of a TFT device.

[0006] equation commonly used to describe the shift △ V _th of the threshold voltage V _th is expressed by the following equation.

[0007]

## EQU1 ## ΔV _th = A exp (-Ea / kT) (lo
g t) ^α V _g ^β ,

Where A is a constant, k is a Boltzmann constant,
T is the absolute temperature of the TFT device and t is the amount of time the bias V _g is applied to the gate of the TFT device. Activation energy E _a and parameters α and β
Is the best value experimentally obtained by the well-known method of least squares. This is because these parameters depend on the nature of the amorphous silicon sample and the nature of the insulator used in the display.

The above equations clearly show how the threshold voltage V _th depends on temperature, time and gate voltage. One theory explaining this phenomenon is that ΔV _th is due to charge trapping in the nitride. Another theory is that metastable Si dangling bonds are generated in the stack of amorphous silicon thin films.

A TFT-LCD device has a temperature of about 10,0 at 80 ° C.
After operating for 00 hours, a voltage shift of about 4 volts was observed. Usually, in order to maintain a sufficient drive current, it is necessary that ΔV _th is 2 V or less during the lifetime of the display device. In demanding applications such as projection TV, TFT devices will always operate under high temperature conditions, and _AVth will increase significantly during short time intervals. Another application where ΔV _th is expected to be large is in aircraft or automobile applications.

During operation of a TFT-LCD display, due to the low mobility of the amorphous silicon TFT device, a high gate is required to obtain sufficient current to drive the pixels or other elements of the display. Voltage is needed.
As V _th shifts, it reduces the effective drive voltage between the gate and the source, which reduces the source-drain current I _ds , resulting in poor performance. Therefore, within the life of the display device, V
It is strongly required to keep the shift of _{th at} a small value.

Several methods are known for holding the threshold voltage V _th shift to a small value and / or slowing it down. One well known method is, for example,
It is a method of high temperature annealing in which an LCD display device is baked in a high temperature furnace for a certain period of time to delay the shift of the threshold voltage V _th . However, TFT-
It is impractical and expensive to anneal an LCD display after it has been assembled.

[0013] Another method of controlling the △ _{V th} is, △ _{V th}
Is proportional to the gate voltage V _g , the applied gate voltage V
_This is a method of reducing _g . However, amorphous
Due to the low mobility of silicon TFT devices, high gate voltages are required to obtain sufficient current to drive other devices. Therefore, if the gate voltage is lowered,
The result is that the characteristics of the LCD display device are significantly degraded.

The third method is to shift V _{th in the} positive direction.
Is a method of using a negatively biased voltage in order to "drive" back. This method is a method of applying a gate voltage to a scanning line of the TFT device while the scanning line is not activated. This method requires a complicated analysis to minimize the shift of _Vth due to the practical limits on the magnitude and duration of the applied negative gate voltage. That is because for each frame, each scan line must be turned on for 1 / 60th of a second. This makes the circuit required to properly shift V _th very complex.

[0015]

Accordingly, one object of the present invention is to provide an amorphous silicon TFT-LC.
A method and a device for recovering the shift of the threshold voltage of the D device in an improved manner.

Another object of the present invention is to provide a method and apparatus that is simple and easy to implement for reducing the shift of the threshold voltage.

Still another object of the present invention is TFT-LC.
D device is to obtain a method and device for restoring the threshold voltage shift while the device is off and not in use.

According to the present invention, the threshold voltage V of a thin film amorphous silicon transistor deposited on a substrate is
A method for recovering _th is obtained. Each of the transistors constitutes a pixel element of an LCD display, and a gate voltage V _g is applied to each of these transistors during use, thereby shifting V _th over time. The method comprises detecting when the LCD display is not used and a voltage V having a polarity opposite to V _g when the LCD display is not used.
a 'and generating a _{V g} the gate of the TFT transistors of the LCD display device' _g the steps of adding, to. This shifts V _{th in the} opposite direction of the shift caused by V _g . As a result, the effective driving voltage is held for the pixel element of the LCD display device.

[0019]

The above and other objects of the invention will be more clearly understood by the following detailed description of the preferred embodiments with reference to the accompanying drawings.

FIG. 2 shows a typical threshold voltage shift for amorphous silicon after applying 30 volts AC to the gate for about 88 hours. As can be seen in FIG. 2, after this 88 hours of use, V _th has shifted by about 5 volts.

FIG. 3 is a graph of threshold voltage recovery obtained according to the present invention. FIG. 3 shows that _Vth is about 0.9 volts of the initial _Vth after a negative 20 volts is applied to the TFT gate for about 16 hours during the TFT-LCD display is off. It shows that it has recovered within the range of. If a longer and / or higher negative voltage is applied to the gate, the threshold voltage can be fully restored to the original threshold voltage.

The magnitude of the negative DC voltage applied to the gate of the TFT device and the duration of this negative voltage can be specified according to the application to the display device.

FIG. 4 is a block diagram of the recovery circuit of the present invention. As shown in FIG. 4, a thin film transistor 12 is deposited on the TFT glass substrate 10. A temperature sensor 14, a diode sensor or a resistor sensor is shown in the substrate 10. These sensors are
Can be deposited directly on the substrate 10, or
The sensor can be mounted in the display so that it is adjacent to the substrate 10. The voltage generator circuit 16 is connected to the TFT device 12. A timer, temperature sensor and detection circuit 18 is connected to the temperature sensor 14 and a timer, temperature sensor and detection circuit 18 is connected to the voltage generator circuit 16 for controlling the operation of the voltage generator circuit 16. It

A circuit 20 is provided to direct the detection circuit 18 when the TFT device 12 is used. The circuit 20 has a normally open relay 22 and an on / off switch 24. When switch 24 is turned on, normally open relay 22 is closed. This allows the detection circuit 18 to activate the timer and begin determining the use of the LCD display.
Further, the detection circuit 18 detects when the relay 22 is open. When the detection circuit 18 detects that the relay 22 is open, the circuit 16 sends the voltage V _g to the gate of the TFT device 12 only when it detects that the LCD display device is in the off state. This shifts V _th in the opposite direction of the shift caused by V _g during the period when the LCD display is on.

When using the present invention, the voltage V _g 'applied to the gate of amorphous silicon transistor 12 is a fixed time such that it always cancels the voltage threshold shift whenever the LCD is in the off state. During the interval and at a fixed size, you can simply add.

The novel method of the present invention requires detecting when the LCD display is in the off state, and
In order to recover the threshold voltage shift that occurs during the time the LCD displays are in the on state, we need to apply a negative voltage to the gates of the TFT transistors only during the time they are in the off state. The method also includes T for a predetermined time interval during the off state of the LCD display.
There is the step of applying a constant negative voltage to the gate of the FT transistor.

The method according to yet another embodiment further comprises applying a negative voltage to the gate of the TFT transistor for a predetermined time interval according to the following equation:

[0028]

## EQU2 ## ΔV _th = A exp (-Ea / kT) (lo
g t) ^α V ^β ,

Thus, amorphous silicon TF
Novel methods and devices have been disclosed for recovery of threshold voltage shifts associated with T-devices. In particular, when the device is used in an LCD display device in a data driving circuit and a scan driving circuit or in an LCD display device in a pixel switching element, the novel method and device are applied. This novel method and apparatus uses a self-generated negative voltage signal in an LCD display and only when the LCD display is turned off or not in use, the negative voltage signal is applied to an amorphous silicon TFT device. Send to the gate. This method and apparatus will restore the threshold shift of the device, which will enhance the operation of the display and prolong the useful life of the display.

However, in more sophisticated applications, in order to more precisely calculate the required magnitude and duration of the voltage V _g ′ to be applied to the transistor 12, it was connected to a temperature sensor which detects the temperature of the substrate 10. The detection circuit 18 makes it possible to calculate the appropriate voltage for the LCD display in the off state and the time required to apply that voltage. Such calculations may be necessary in applications such as HD-TV projectors where TFT devices are under relatively high temperatures.

Although the present invention has been described with reference to preferred embodiments, the foregoing description is not meant to limit the scope of the invention to the embodiments. The present invention includes all the embodiments that are changed or modified and the embodiments equivalent to the above embodiments.

[Brief description of drawings]

FIG. 1 is a graph showing a shift of a threshold voltage of a TFT with respect to an applied gate voltage.

FIG. 2 is a graph showing the shift of the threshold voltage of the TFT after 88 hours of operation.

FIG. 3 is a graphical representation of TFT threshold voltage recovery after a negative 20 volt voltage is applied for 16 hours.

FIG. 4 is a block diagram of an apparatus for controlling V _{th according} to the present invention.

[Explanation of symbols]

 16 voltage generator 18 detection circuit

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[Procedure amendment]

[Submission date] January 14, 1994

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Correction content]

[Claims]

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Di Chungfu Taiwan, Xinchushein, Pausang County, Shung-Chi Village, 153-144F

Claims (19)

[Claims] 1. In a thin film amorphous silicon transistor deposited on a substrate of a liquid crystal display (LCD): (a) generating a voltage V _g ′ having a polarity opposite to the gate voltage V _g ; B) When the LCD display device is in the off state so that the transistor of the LCD display device shifts the threshold voltage V _th in the opposite direction to the shift caused by the gate voltage V _g to hold the effective driving voltage. Only, applying the opposite polarity voltage V _g ′ to the gates of the transistors of the LCD display device, the gate voltage V _g being applied to each of the transistors when the LCD display device is in the ON state. Thereby, the threshold voltage V _th of the thin film amorphous silicon transistor of the thin film amorphous silicon transistor, which shifts over time. A method for recovering the threshold voltage V _th . 2. The method of claim 1, comprising connecting a circuit to the LCD display device to detect when the LCD display device is in an off state.
The method further comprising. 3. The method of claim 1, further comprising detecting the amount of time the LCD display device is in the on state. 4. The method of claim 1, further comprising detecting the temperature of the substrate of the LCD display device when the LCD display device is in the on state.
The method. 5. The method of claim 1, further comprising generating the voltage V _g ′ in a magnitude proportional to the amount of time the LCD display is in the on state. 6. The method of claim 4, wherein the LCD display is proportional to the amount of time that the LCD display is on and the temperature of the LCD display when the LCD display is on. The method, further comprising the step of generating the voltage V _g ′ in magnitude. 7. The method of claim 1, further comprising the step of applying a voltage V _g ′ to the LCD display transistor for a predetermined time and with a predetermined magnitude. Method. 8. The method of claim 1, further comprising the step of generating a voltage V _g ′ at a battery power source. 9. The method according to claim 1, wherein the transistor includes a data driver (column driver), a select driver (row driver), and a pixel element on a substrate of the LCD display device. A transistor. The method, further comprising the step of generating a voltage _Vg 'with a battery power source. 10. The method of claim 4, wherein the L
The step of detecting the temperature of the CD display device substrate is the LC
The method further comprising the step of providing a temperature sensor on the substrate of the D display device. 11. The method of claim 4, wherein the L
In the step of detecting the temperature of the CD display device, the L
The method, further comprising providing a temperature sensor adjacent the substrate of the CD display device. 12. The method of claim 11, wherein the temperature sensor is a sensor selected from a group of sensors including a thermocouple sensor, a diode sensor, and a resistor sensor. 13. A detection device operably connected to the LCD display device to detect when the liquid crystal display device (LCD) is not in use, and a gate voltage when the LCD display device is not in use. A voltage generator operably connected to the detection device to generate a voltage V _g ′ of a polarity opposite to V _g , and holding an effective drive voltage for a transistor of the LCD display device. In order to reduce the threshold voltage V _th only when the LCD display device is not in use, applying the voltage V _g ′ to the gate of the transistor of the LCD display device, the LCD display each of the transistors during use of the device to the gate voltage V _g was added, thereby increasing the threshold voltage V _th with time, L D
A device for recovering the threshold voltage _Vth of a thin film amorphous silicon transistor deposited on a substrate of a display device. 14. The device according to claim 13, wherein the detection device has a device for detecting an amount of time the LCD display device is used, and the LCD display device is not used for a certain period of time. The voltage V _g ′ is applied to the gate of the transistor only when the LCD display device is on and with a magnitude proportional to the amount of time the LCD display device has been used. 15. The device according to claim 14, wherein the detection device has a device for detecting a temperature of the LCD display device during a period of use of the LCD display device, and the LCD display device is used. The voltage V _g ′ is applied for a fixed amount of time during the time period and with a magnitude proportional to the temperature of the LCD display device. 16. The device of claim 15, wherein the temperature sensing device comprises a temperature sensor deposited on a substrate of the LCD display device. 17. The apparatus according to claim 15, wherein the temperature detection device comprises a temperature sensor disposed adjacent to a substrate of the LCD display device. 18. The apparatus of claim 17, wherein the temperature sensor is a thermocouple sensor, a diode sensor, and
The device being a sensor selected from a group of sensors including a resistor sensor. 19. The device of claim 13, wherein the voltage generator comprises a battery power source.