JPH0343629B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention relates to an improvement of an AC-driven EL display device, and particularly to an improvement of the device configuration including a drive circuit.

(b) Background of the technology A phenomenon in which when a voltage is applied to a material, the material itself emits light - this is called electroluminescence.
The phenomenon called ``uminescence'' is well known. And this substance is zinc sulfide (Zns)
A well-known example is one in which manganese and Mn are added, and it has recently been developed as an EL display element with a thin film structure.

By applying an alternating current voltage to such a substance, it is possible to control not only the presence or absence of light emission from the substance, but also its brightness. That is, when the voltage value of the applied AC voltage increases, the brightness also increases, and conversely, when the voltage value decreases, the brightness also decreases. Therefore,
By controlling the brightness when a high voltage is applied and the brightness when a low voltage or no voltage is applied, two states can be controlled: the former is a display state (on state), and the latter is a non-display state (off state).

By using semiconductor manufacturing technology, such an electroluminescent display body can arrange a plurality of display elements in a smaller area, and can also manufacture the display elements and display drive circuit integrally. Because of these advantages, it is attracting attention as an alternative to display devices using cathode ray tubes, liquid crystals, and gas discharges.

(c) Prior art Figure 1 shows a display device using an EL display. This embodiment has a format in which one drive circuit is provided for one capacitive display element EL, and has a data line DL and a scanning line SL. The data line DL is
First switching element Q ₁ consisting of MOS type FET
The scanning line SL is connected to the gate terminal of the transistor _Q1 . The source terminal of transistor _Q1 is a second transistor consisting of a MOS type FET.
is connected to the gate terminal of switching element _Q2 ,
On the other hand, it is connected to a capacitor C _S for data storage. The drain terminal of transistor Q ₂ is the display element
Connected to one electrode of EL. transistor Q ₂
The source terminal of is connected to ground potential as a reference potential. The display element EL has a thin film structure in which the display element EL is sandwiched between two electrodes via an insulating film (not shown), and a pulsed AC voltage is supplied to the other electrode from a power source POW.

Now, when a pulse-like scanning signal with a predetermined width is supplied to the scanning line SL with the data line DL set to "1" in order to put the display element EL into the display state, the transistor _Q1 turns "ON". The capacitor C _S stores charge in response to the scanning signal. As a result, transistor Q ₂ turns “ON” and the voltage between the drain and source of transistor Q ₂ becomes approximately 0.
It becomes [V]. In other words, as shown in Figure 2, the power supply
The AC voltage ±V _A shown in a from POW is the display element.
The voltage V _DS supplied to one electrode of the EL and between the drain and source is 0 [V], and the other electrode of the display element EL is clamped to the ground potential as shown in Figure 2b, so the display element As shown in FIG. 2c, the supply voltage itself of ±V _A is applied between the opposing electrodes of the EL.

On the other hand, the _transistor is
When Q ₂ is turned “ _OFF ”, the voltage on the drain side of Q ₂ V _DS becomes
It changes by 2V _A as shown in Figure b. Therefore, the voltage V _EL applied between both electrodes of the display element EL
The difference voltage between them becomes a DC voltage as shown in FIG. 3c.

Here, the relationship between the alternating current voltage applied between both electrodes of the display element EL and the luminance will be explained with reference to FIG. 4. In Figure 4, the horizontal axis represents the AC voltage _VEL ,
The vertical axis shows the brightness B, and when the voltage V _EL is set to V _A , the brightness is the displayable luminance brightness B ₂ , and V _NA
When this happens, light is emitted at an off-state non-display brightness of approximately 1 fL, which is indicated by _B1 . Therefore, the peak value of the AC voltage from the power supply POW is ±V _A as shown in Figures 2 and 3.
Then, when the transistor Q ₂ is "ON", the display element EL emits light at the display brightness B ₂ of the on state, and when the transistor Q ₂ is "OFF", the applied voltage is -
Since it is a direct current of V _A , it does not emit light.

(d) Problems with the Prior Art Now, what should be noted here is the displacement of the voltage V _DS between the drain and source of the selective drive transistor Q ₂ . Voltage V _DS turns transistor Q ₂ “ON”
When it is, it is 0 [V], but the transistor Q ₂
In other words, when the display element EL is set to “OFF”,
The voltage V _DS rises abnormally when it is in the non-display state. That is, as shown in FIG. 3, every time the applied AC voltage changes from 0 [V] to +V _A [V], the voltage V _DS becomes +2V _A [ which is twice the peak value of the applied AC voltage]. V]. This is the display element EL
The AC voltage +V _A
This is thought to be because [V] is added.

On the other hand, focusing on the transistor Q ₂ , one of the characteristics required of the transistor Q ₂ is that when the display element is turned "OFF" to make the display non-display, the drain current is set to 0, and the display is not displayed. Element EL
is to control the voltage applied across the .
That is, a function is required to set I _DS =0 when transistor Q ₂ is turned off. Therefore, as shown in FIG. 5, the characteristics of transistor Q ₂ must be set so that its breakdown voltage V _Z satisfies V _Z ≧2V _A. Furthermore, Figure 5 shows the general
FIG. 7 is a V _DS -ID _{characteristic} diagram showing a change in drain current _ID with respect to drain-source voltage V _DS of transistor Q ₂ when a MOS type transistor is used.

In this way, the breakdown voltage V _Z of the transistor Q ₂ must be set to a large value in order to set _ID to 0 when the transistor Q 2 is turned “OFF”. As an example, if the voltage at the time of light emission of the display element el, that is, the value of the applied AC voltage V _A is 160 [V], the breakdown voltage V _Z required of the transistor Q ₂ is 320 [V] or more.

However, it is extremely difficult to manufacture a switching element having an extremely large breakdown voltage _VZ , and the breakdown voltage VZ becomes high. Further, the breakdown voltage of a switching element depends not only on the breakdown voltage but also on the dielectric breakdown voltage of the gate oxide film, and it is extremely difficult to prevent the gate oxide film from being destroyed by such a high voltage as described above.
Especially when considering that switching elements are manufactured using semiconductor manufacturing technology like MOS FETs,
Attention must be paid to many aspects such as its structure and materials.

(e) Purpose of the Invention In view of such conventional drawbacks, the present invention provides a method for display elements.
To provide an EL type display device in which the breakdown voltage V _B of a selective drive transistor can be set to a value smaller than the applied 2V _A without deteriorating the luminance characteristics in the display state and non-display state of the EL. It is something.

(f) Structure of the Invention Examining the operation of an EL display device with a thin film structure, in the display state of the display element _EL , as shown in FIG. An AC voltage of V _A must be applied across the EL, but in order to obtain a non-display state (off state), the drain current I _D must be set to 0 [A].
It is not necessarily necessary to do so. In other words, as is clear from the brightness characteristic curve in Figure 4, even if the voltage of the EL display element is increased to a relatively high voltage _VNA , it is still insufficient to obtain the brightness that is visible to the human eye _. It has the characteristic that the brightness rises rapidly when the voltage changes from V A to V _A. Therefore, V _NA can be seen as the threshold voltage for display, and even in the non-display state, this V _NA is applied to both ends of the display element.
Application of alternating voltage up to [V] is permissible. This invention focuses on the characteristics of such an EL display element and reduces the breakdown voltage V _B of the selective drive transistor to 2V _A [V]
This device is equipped with a rectifying element for clamping to set a value smaller than V _NA on the display body el.
It can be set up to just before an AC voltage of [V] or more is applied. That is, in this invention, the breakdown voltage V _Z is V _A in parallel with the selection drive transistor of the EL display element.
The present invention is characterized in that it includes a rectifier for voltage clamping in the range of −V _NA ≦V _Z <2V _A to clamp the voltage across the transistor to below the breakdown voltage. This rectifying element may be provided by a junction between the substrate and the drain within the transistor _Q2 , or may be combined as a separate element.

(g) Embodiments of the Invention The EL display device according to the present invention will be specifically described below.

FIG. 6 is a schematic diagram showing a cross section of an N-channel MOS FET used as the driving transistor _Q2 of FIG. 1. It is well known that when N-type source and drain regions are diffused into a P-type semiconductor substrate, a diode as shown in the figure is formed at the junction surface thereof. Therefore, when a predetermined voltage is applied to the gate terminal G to turn it "ON", the existence of this diode D _Z can be ignored, but when it is turned "OFF", the diode D _Z cannot be ignored.

Now, the equivalent circuit of the display device when the source terminal S and the substrate are grounded, the drain terminal D is connected to the display element EL, and the FET is turned "OFF" is the seventh
It can be considered as shown in the figure. In other words, the display element EL
can be thought of as being grounded via the diode D _Z in the reverse direction, and the point of this is that this diode D _Z is not simply a reverse voltage diode, but is used as a Zener diode to utilize the clamping effect due to its constant voltage characteristics. This is a feature of the invention.

In the equivalent circuit of Fig. 7, when an alternating pulse voltage with a peak value V _A is supplied from the power supply POW, the transistor Q ₂ with respect to the breakdown voltage V _Z of the diode D _Z
The characteristics of the drain-source voltage V _DS are shown in Figure 8. In Figure 8, the horizontal axis shows the breakdown voltage V _Z
The vertical axis represents the drain-source voltage V _DS , and when the breakdown voltage V _Z is 0 [V], in other words, when the diode D _Z is short-circuited, the voltage V _DS is also approximately 0. It becomes [V]. On the other hand, V _Z is V _ZI [V]
Then, V _DS becomes 2V _A [A], and from then on, even if V _Z increases, unless the power supply voltage V _A increases, the voltage V _DS
maintains 2V _A [V]. Here, V _ZI corresponds to 2V _A.

Furthermore, Fig. 9 is a characteristic diagram showing the relationship between the drain-source voltage V _DS of the driving transistor Q ₂ and the voltage V _EL applied when the power supply POW becomes positive across both terminals of the display element EL. It is. Voltage on the horizontal axis
V _DS [V] is represented by the voltage V _EL on the vertical axis. When Q ₂ is “ON” and the voltage V _DS is 0 [V], for example, 160V V _A [V] is applied to both ends of the display element EL, and 20
It emits light at a display brightness _B2 of ~30fL and provides a display state. However, when the voltage divided by _the diode _DZ increases _and the voltage V _{DS becomes V} It becomes about 1fL and becomes a hidden state where it is practically invisible. Further voltage V _DS
When V _DS increases, the positive voltage V _EL applied to the display element EL becomes 0 [V] at the point where V _DS becomes equal to VA.
Here, there is a relationship of V _X = V _A − V _NA , and the drain-source voltage when transistor Q ₂ is “OFF”
In the range of V _DS from 0 to V _X [V], the display element EL
A display state occurs when a voltage of V _NA [V] or more is applied, but if the voltage V _DS is chosen to be V _X [V] or more, the voltage divided by the diode D _Z increases and the voltage applied to the display element V _EL becomes less than or equal to V _NA [V] and enters the non-display state. Therefore, in order to obtain a non-display state when the transistor Q ₂ is OFF, the breakdown voltage V _Z of the diode D _Z does not need to be larger than the voltage 2V _A [V], but should be smaller than 2V _A [V], and It can be set within the range of V _X [V] or more. Since it is convenient for manufacturing that the breakdown voltage V _Z is small, V _A −V _NA ≦
It is desirable to set V _Z <2V _A and equal to or slightly larger than V _A −V _NA .

Here, FIG. 10 shows the waveforms of each part when V _Z is set as described above and the driving transistor Q ₂ is turned "OFF". In Figure 10, a
is the waveform of the voltage supplied from the power supply POW, b is the waveform of the drain-source voltage _VDS , and c is the waveform of the voltage _VEL applied across the display element EL in the off state. As an example, since V _A is about 160V and V _NA is about 125V, V _Z is set to about 35V.

In this case, the voltage across Q ₂ is clamped to 35V, so the withstand voltage of Q ₂ is sufficient to exceed 35V.

Figures 11 and 12 are matrix array ELs.
An example is shown in which a display element and an active matrix circuit for driving the display element are integrated using semiconductor manufacturing technology. Fig. 11 is a plan view of one element, and Fig _. 12 is a cross-sectional view taken along the show.

On the silicon substrate 117 are transistors Q ₁ ,
Q ₂ , the capacitor C _S and the display element EL are formed with a multilayer structure. The display element EL includes a display electrode 111a independent for each element and an insulating film 111b such as Y ₂ O ₃ sandwiched between ZnS:Mn on both sides.
It consists of a display body el and a transparent electrode (ITO film) 111c common to all elements. The conductor 114 for the data line is input to the drain terminal D of the transistor _Q1 , and the conductor 115 for the scan line is input to the drain terminal D of the transistor _Q1.
is input to gate terminal G of. The electrode 116 serves both as the gate terminal G of the transistor Q ₂ and one electrode of the capacitor _CS, and the electrodes 116 and 118 constitute the capacitor _CS . Note that the conductor 113 functions as a shield electrode.

If we consider the rectifying element for the clamp that has a breakdown voltage V _Z , the rectifying function is provided during the formation process of the MOS FET, so as explained above, the breakdown voltage V _Z is determined by the built-in rectifying element. You can set it to . In this case, the driving power source for the switching MOS FET is a pulse source with both positive and negative polarities, so the channel configuration is N.
It may be either type or P type, and the voltage V _Z can be controlled by adjusting the depth etc. when forming the drain region relative to the base. In the case of a P-channel MOS, the direction of the built-in diode is naturally opposite to that of the embodiment shown in FIG.

On the other hand, as shown in Fig. 13a, a diode element D _ZI is externally connected between the drain terminal and the source terminal without using the rectification function built in the MOS FET, and the breakdown voltage V _Z of this diode element D _ZI is It is also conceivable to set the value to a predetermined value according to the present invention. Also, as shown in Figure 13b, when the transistor is a bipolar transistor, a diode element D _ZI is placed between its collector terminal and emitter terminal.
can also be an external connection.

As explained above, according to the present invention, by setting the breakdown voltage V _Z of the rectifying element connected in parallel with the switching transistor to V _A −V _NA ≦V _Z <2V _A , the non-display state can be displayed. Since the breakdown voltage of the switching element can be lowered without deteriorating the contrast between states, even a standard MOS type FET can be used satisfactorily. Therefore, especially when applied to an EL display device having an integrated drive circuit structure as illustrated in FIGS. 11 and 12, the MOS type FET can be manufactured easily and the display device can be manufactured at low cost.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an example of an EL display device, Fig. 2 is a voltage waveform diagram in the display state of Fig. 1, Fig. 3 is a voltage waveform diagram in the non-display state of Fig. 1,
Figure 4 is a diagram of the luminescence characteristics of the EL display element EL with respect to the applied AC voltage, and Figure 5 is a diagram of a general MOS type transistor.
Voltage-current characteristics diagram for Q ₂ , Figure 6 is for MOS type
A cross-sectional view of the FET, Figure 7 is a circuit diagram of the EL display section using a MOS type FET as transistor _Q2 , and Figure 8 is a MOS type FET.
Figure 9 is a characteristic diagram of the AC voltage applied across the EL display element with respect to the drain-source voltage in Figure 7. Voltage waveform diagram in non-display state in figure 11
The figure is a plan view of an example in which the EL display element and the drive circuit are integrated using semiconductor technology.
A cross-sectional view taken along line X-X in the figure, FIG. 13 shows a modification according to the present invention. In the figure, EL is a display element, el is a display body, Q ₁ and Q ₂ are transistors, D _Z and D _ZI are diodes, and V _Z is a breakdown voltage.

Claims (1)

[Claims] 1. A display element EL including an electroluminescent display 1 provided between electrodes with an insulating film interposed therebetween, and a wave sufficient to provide a display state to one electrode of the display element EL. It includes a power supply POW that supplies an AC voltage of a high value V _A , and a switching element Q ₂ connected between the other electrode of the display element EL and a reference potential, and the switching element Q ₂ connects the electrode of the display element EL. In a display device configuration that controls the display state and non-display state of the display element EL by controlling the voltage applied between them, a rectifying element _DZ is connected between the other electrode of the display element EL and a reference potential. And the breakdown voltage of the rectifier D _Z
By setting V _Z in the range of V _A −V _NA ≦V _Z <2V _A (where V _NA is the maximum voltage to maintain the non-display state), the voltage across the switching element _Q2 is set to An EL type display device characterized by being clamped below the breakdown voltage.