El display device

A thin-film EL display device incorporates a MOS active matrix. Each of the MOS transistor arrays is additionally provided in parallel with a Zener diode for the purpose of protecting the device from a high voltage. This Zener diode has a breakdown voltage characteristic corresponding to a difference between a luminous voltage and a non-luminous voltage of an EL display element and clamps the voltage across the MOS transistor, in the "OFF" state, to a voltage less than or equal to non-recoverable breakdown voltage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an EL display device, particularly to a new 
version of a MOS-EL integrated display device providing a protection means 
to a MOS switching element which forms an active matrix for combining the 
EL display elements. 
2. Description of the Prior Art 
Recently a device combining a thin-film electro luminescent (EL) phosphor 
and the active matrix array driver has been developed in the display 
field. For example, the thesis entitled "THIN-FILM TRANSISTOR SWITCHING OF 
THIN-FILM ELECTROLUMINESCENT DISPLAY ELEMENTS" presented on the 
Proceedings of SID, Vol. 21/2, 1980, PP. 85-90 by Z. K. Kun et al. 
introduces a display device combining a thin-film EL phosphor on the 
integrated active matrix addressing circuit substrate having a thin-film 
transistor (TFT) structure. 
FIG. 1 is an equivalent circuit of a typical display element of the 
existing EL display comprising the abovementioned TFT technology. The data 
line DL is connected to the drain terminal of the first switching element 
Q.sub.1 comprising a MOS FET, while the scanning line SL is connected to 
the gate terminal of transistor Q.sub.1. The source terminal of transistor 
Q.sub.1 is connected to the gate terminal of the second switching element 
Q.sub.2 comprising a MOS FET and is also connected to the capacitor 
C.sub.s for data accumulation. The drain terminal of transistor Q.sub.2 is 
connected to a first electrode of the display element EL. The source 
terminal of transistor Q.sub.2 is connected to ground as the reference 
voltage. The display element EL has the thin-film structure which 
sandwiches the EL phosphor layer el, such as ZnS:Mn, via an insulating 
film (not illustrated) between two electrodes. An AC voltage pulse is 
supplied to a second electrode of the display element EL from the power 
supply POW. 
When a scanning pulse signal having the specified width is supplied to the 
scanning line SL under the condition that the data line DL is set to a 
logical "1" in order to bring the display element EL into the display 
condition, the transistor Q.sub.1 becomes "ON" and the capacitor C.sub.s 
accumulates charges corresponding to the scanning signal. Thereby, the 
transistor Q.sub.2 becomes "ON" and a voltage between the drain and source 
of transistor Q.sub.2 becomes almost 0 V. Since an AC voltage .+-.V.sub.A, 
as shown in FIG. 2 (a), is supplied to the first electrode from the power 
supply POW and the second electrode of the display element EL is clamped 
to ground as shown in FIG. 2 (b) through the transistor Q.sub.2, the 
supply voltage of .+-.V.sub.A is applied across the opposing electrodes of 
display element EL as shown in FIG. 2 (c) and thereby the display element 
is brought into the display condition. On the other hand, when the 
transistor Q.sub.2 becomes "OFF" because the capacitor C.sub.s discharges, 
transistor Q.sub.2 becomes equivalent to a diode. Therefore the voltage 
.+-.V.sub.A supplied from the power supply POW as shown in FIG. 3 (a) is 
accumulated in the display element EL which acts as a capacitor. As a 
result, the drain voltage V.sub.DS of Q.sub.2 changes as much as 2V.sub.A 
as shown in FIG. 3 (b). But, the voltage V.sub.EL applied across both 
electrodes of the display element EL becomes the DC voltage of these 
voltages. Therefore, when the transistor Q.sub.2 is "OFF", the AC driven 
display element EL does not emit light. 
However, as is obvious from the above explanation, when the transistor 
Q.sub.2 is "OFF", a voltage of 2V.sub.A is applied across the source and 
drain of the transistor, requiring a very high breakdown voltage of the 
transistor Q.sub.2. Since an actual EL drive voltage V.sub.A is selected, 
as an example, to be about 160 V (320 V peak to peak), the transistor 
Q.sub.2 is required to have a breakdown voltage of about 320 V. A MOS 
transistor having such a high breakdown voltage can be obtained as a 
discrete element, but it is considerably difficult to obtain, on a 
commercial basis, an integrated MOS active matrix for combining the EL 
display. 
For example, according to J. E. Gunther's proposal indicated on page 30 of 
SID SESSION S-1 "ACTIVE MATRIX ADDRESSING TECHNIQUES" of Seminar Lecture 
Notes, Apr. 28, 1980, as a means for solving such a problem, a second 
capacitor acts as an AC voltage divider which biases the display element 
just below its threshold and is provided in parallel with the driver 
transistor Q.sub.2 providing the TFT structure. However, the addition of 
such a second capacitor to the signal accumulation capacitor C.sub.s 
requires complicated multilayer techniques for configuring the capacitor, 
resulting in a problem that the degree of integration of elements is 
restricted. 
SUMMARY OF THE INVENTION 
It is a primary object of this invention to attain technical matching 
between the EL display element, which requires a comparatively high drive 
voltage, and the active switching elements which have a low breakdown 
voltage upon incorporating the active matrix addressing circuit and the EL 
display elements, and to protect the active switching element, having a 
simple structure, from non-recoverable breakdown due to a high drive 
voltage. 
It is another object of this invention to offer a MOS-EL integrated display 
device using a silicon substrate which can be fabricated easily and with 
high reliability. 
Briefly, this invention is characterized by setting the breakdown voltage 
of the switching transistor element, which is connected to the EL display 
element, to the difference between the luminous voltage and non-luminous 
voltage of the display element. The "OFF" voltage applied to the 
transistor element is clamped to a value less than or equal to the 
non-recoverable breakdown voltage. 
In particular, the EL display device of the present invention comprises a 
semiconductor substrate, a plurality of display electrodes corresponding 
to picture elements arranged on the semiconductor substrate the opposing 
electrodes of the display electrodes are arranged across the EL layer. The 
EL display device also provides switching transistor elements on the 
semiconductor substrate for selectively driving the display electrodes 
corresponding to picture elements. The p-n junction, which is formed 
between the electrodes connected to the display electrodes of the 
switching transistors and the semiconductor substrate, breaks down at a 
voltage which is equal to the difference between the luminous voltage and 
non-luminous voltage of the EL layer. The p-n junction forms the a Zener 
diode connected in parallel with the switching transistor element and 
clamps the voltage across the transistor element in the "OFF" state to a 
voltage less than or equal to the non-recoverable breakdown voltage of the 
relevant element. It is desirable to form the p-n junction which functions 
as a Zener diode, as the junction between the drain region and substrate 
of the switching MOS transistor, but an independent diode element can be 
integrated for this purpose. In addition, the breakdown voltage of the p-n 
junction is set to a voltage greater than the difference between the 
luminous voltage and the maximum non-luminous voltage, thereby biasing the 
EL display element to a voltage lower than the maximum non-luminous 
voltage in the "OFF" condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A typical voltage-brightness characteristic of the thin-film EL display is 
shown in FIG. 4. As will be obvious from the characteristic curve of FIG. 
4, the thin-film EL display element cannot assure sufficient brightness as 
high as that detected by eyes even when the voltage applied is boosted up 
to a comparatively high voltage, V.sub.NA, but has a characteristic where 
the brightness sharply rises from B1 to B2 due to a voltage change from 
V.sub.NA to V.sub.A. The display element can be considered as being the 
non-luminous condition or "OFF" state until the brightness level reaches 
B1 which generally corresponds to about 1 fL. A voltage V.sub.NA which 
gives a brightness level of B1 can be considered as the display threshold 
voltage or the maximum non-luminous voltage and a voltage, up to V.sub.NA 
can be defined as the non-luminous voltage or "OFF" voltage V.sub.OFF. On 
the other hand, the brightness level B2 which is sufficient for the "ON" 
state is generally 20 fL or higher and a voltage V.sub.A which gives a 
brightness level of the "ON" state is defined as the luminous voltage or 
"ON" voltage V.sub.ON. 
This invention is based on the voltage-brightness characteristic of the EL 
display element. The non-luminous voltage up to V.sub.NA is always applied 
to the display element and the "ON," "OFF" status of the display element 
is controlled by switching between the luminous voltage V.sub.ON and 
non-luminous voltage V.sub.OFF with the transistor for selectively driving 
the display element. In order to attain such operation, the present 
invention provides a clamping diode, having the breakdown voltage V.sub.Z 
satisfying the relation of V.sub.Z .gtoreq.V.sub.A -V.sub.NA, in parallel 
with the transistor for selectively driving the display element and 
connected in series with the EL display element. 
FIG. 5 schematically shows the sectional view of the N channel MOS 
transistor used in the present invention in place of the TFT type 
switching transistor Q.sub.2 shown in FIG. 1. It is well known that the 
diode D.sub.Z, as shown in the figure, is formed at the junction area of 
the drain region 13 and substrate 11 when the source region 12 and drain 
region 13 are formed by diffusing an n type impurity into the p type 
silicon substrate 11. Therefore, when the N channel MOS transistor is in 
the "ON" state because the predetermined voltage is applied to the gate 
terminal G provided on the insulating film 14, the diode D.sub.Z can be 
ignored. But when the N-channel MOS transistor is in the "OFF" state, the 
diode D.sub.Z cannot be ignored. 
FIG. 6 shows an equivalent circuit of the display device considered in the 
case where the source terminal S and substrate are grounded, the drain 
terminal D is connected to the display element EL and the FET is in the 
"OFF" state. It is a characteristic of the present invention that the 
display element EL can be grounded via the backward diode D.sub.Z, and the 
clamping function of the constant voltage characteristic of this diode 
D.sub.Z can be utilized considering it as a Zener diode and not just as 
the backward diode. 
FIG. 7 shows the characteristic curve of the relation between the 
drain-source voltage V.sub.DS of the drive transistor Q.sub.2 and a 
voltage V.sub.EL which is applied across the display element EL when the 
power supply POW becomes positive. The horizontal axis represents the 
voltage V.sub.DS, while the vertical axis represents the voltage V.sub.EL. 
When Q.sub.2 is "ON" and the voltage V.sub.DS is 0V, a voltage V.sub.A, 
for example, 160 V is applied across the display element EL. As a result, 
the element EL emits light at a brightness B2 of 20 to 30 fL, resulting in 
the display being in the ON state. However, when a voltage applied to the 
diode D.sub.Z increases and the voltage V.sub.DS becomes V.sub.X, a 
voltage V.sub.EL applied to the display element EL becomes V.sub.NA, and 
the brightness decreases to the level B1, for example, about 1 fL, 
resulting in the display being in the "OFF" state which cannot be detected 
visually. Moreover, when the voltage V.sub.DS increases, the positive 
voltage V.sub.EL is applied to the display element EL at such a timing 
that V.sub.DS becomes equal to V.sub.A which is 0V. 
Here, when the relation V.sub.X =V.sub.A -V.sub.NA exists, and the 
drain-source voltage V.sub.DS ranges from 0V to V.sub.X while the 
transistor Q.sub.2 is "OFF", a voltage of V.sub.NA or higher is applied to 
the display element EL and the display element is in the "ON" state. 
However, if the voltage V.sub.DS is a value higher than V.sub.X, a voltage 
applied to the diode D.sub.Z increases and a voltage V.sub.EL applied to 
the display element becomes V.sub.NA or lower. Thus the display element is 
in the "OFF" state. Therefore, even when the breakdown voltage V.sub.Z of 
the diode D.sub.Z is not higher than the voltage 2V.sub.A, the 
non-luminous state can be obtained when the transistor Q.sub.2 is "OFF". 
Namely, the breakdown voltage V.sub.Z can be set to a value smaller than 
2V.sub.A and higher than V.sub.X within the operating voltage range. A 
smaller breakdown voltage V.sub.Z is desirable for fabrication and it is 
more desirable to set it to a value equal to V.sub.A -V.sub.NA or a little 
higher. 
Respective waveforms, when V.sub.Z is set to a value as indicated above and 
the driver transistor Q.sub.2 is in an "OFF" state, are shown in FIGS. 8 
(a), (b), and (c). FIG. 8 (a) is a waveform of the signal supplied from 
the power supply POW and FIG. 8 (b) is a waveform of the voltage V.sub.DS 
across the drain and source of transistor Q.sub.2. FIG. 8 (c) is a 
waveform of the voltage V.sub.EL applied across the display element EL in 
the "OFF" state. As an example, since V.sub.A is 160 V and V.sub.NA is 125 
V, V.sub.Z is set to about 40 V. 
Therefore, in this case, a voltage across the transistor Q.sub.2 is clamped 
to about 40 V and a voltage of 40 V or a little higher is sufficient as 
the breakdown voltage of Q.sub.2. The MOS transistor having such a 
breakdown voltage can be easily integrated by the fabrication process 
which is now explained. 
FIG. 9 and FIG. 10 are examples of the EL display element arranged in the 
form of an active matrix circuit for driving the semiconductor display 
device. FIG 9 is a plan view of the element and FIG. 10 is a sectional 
view of the element along the line X--X. 
On the silicon substrate 117, the transistors Q.sub.1, Q.sub.2, capacitor 
C.sub.s and display element EL are formed in a multilayered structure. The 
display element EL comprises a display electrode 111a which is independent 
of each element, thin-film EL phosphor el comprising ZnS:Mn sandwiched on 
both sides by an insulating film 111b like Y.sub.2 O.sub.3, and a 
transparent electrode common to all elements (ITO film) 111c. The 
conductor 114 for the data line is input to the drain terminal D of 
transistor Q.sub.1, while the conductor 115 for the scanning line is input 
to the gate terminal G of transistor Q.sub.1. The electrode 116 is used in 
common as the gate terminal G of transistor Q.sub.2 and the one electrode 
of capacitor C.sub.s, and the capacitor C.sub.s is composed of the 
electrodes 116 and 118. The conductor 113 works as the shielding 
electrode. 
Here, the clamping diode element having the breakdown voltage is considered 
since the MOS type FET provides the diode function between the drain 
region and the substrate. Therefore, it is enough to set the breakdown 
voltage V.sub.Z to that of the p-n junction as explained above. In this 
case, the MOS type FET employed for the switching function may employ 
either an N type or P type FET in the channel structure since positive and 
negative (bipolar) pulses are used as the driving source voltage. The 
voltage V.sub.Z can be controlled by adjusting the impurity concentration 
and depth when forming the drain region for the substrate. In the case of 
a P channel MOS, the direction of the diode is naturally inverted for the 
embodiment shown in FIG. 6. 
Meanwhile, it is also possible, as shown in FIG. 11 (a), to externally 
connect a diode element D.sub.Z1 between the drain terminal D and source 
terminal S without using the rectification function which the MOS type FET 
has and to set the breakdown voltage V.sub.Z of this diode D.sub.Z1 to the 
specified value in accordance with the present invention. In addition, 
when a bipolar transistor is used as in the case of FIG. 11 (b), the diode 
element D.sub.Z1 can also be connected externally between the collector 
terminal C and emitter terminal E. 
As is obvious from the above explanation, according to the present 
invention, the breakdown voltage required for the switching transistor can 
be reduced by providing a Zener diode in parallel to the switching 
transistor for selectively driving the EL display element and by setting 
such breakdown voltage V.sub.Z to be the difference between the luminous 
voltage and non-luminous voltage of the EL display element. Therefore, 
application of the present invention to the EL display device integrating 
the active matrix makes it easy to fabricate the MOS switching transistor 
through integration and to supply at a low cost a highly reliable device. 
Moreover, this invention is advantageous in the case of constructing a 
modular type display device, such as proposed by T. Unotoro et al in U.S. 
patent Application Ser. No. 236,621 assigned to the same assignee of this 
invention. Now U.S. Pat. No. 4,368,467.