Method of driving diode type display unit

A method of driving a diode type display unit in which a charge regulating period is added to driving signal waveform in order to regulate amount of charge in a display picture element capacitor in the display unit wherein a two-terminal element having nonlinear current-voltage characteristics is provided two-dimensionally on a substrate, an electrooptical element capable of controlling the optical quality by means of applied voltage is disposed in response to the two-terminal element, charge is injected into the electrooptical element by utilizing the current-voltage nonlinearity in the two-terminal element for writing period, and the charge injected is held by utilizing the current-voltage nonlinearity in the two-terminalelement for holding period thereby effectig display.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of driving a diode type display unit 
wherein display is carried out by means of a combination of a two-terminal 
element and an electrooptical element. 
2. Description of the Prior Art 
The term "two-terminal element" used herein means elements, of which the 
voltage-current characteristics exhibit nonlinearity such as PN junction 
diode, metal-insulating layer-metal diode (hereinafter referred to simply 
as "MiM diode") and the like. On the other hand, the term "electrooptical 
element" means elements, of which the optical quality is controlled by 
means of impressed voltage such as liquid crystal element, electrochromic 
element, PLZT element, electroluminescent element, plasma luminescent 
element, fluorescence luminescent element and the like. 
For the sake of simplicity, a MiM diode and a liquid crystal element are 
utilized as the two-terminal element and the electrooptical element, 
respectively, in the following description. 
FIG. 1 shows a construction of a diode type display unit wherein reference 
numeral 1 designates an input signal line, i.e., input line of display 
information, reference numeral 5 designates a display panel part, and this 
display panel part is one obtained by disposing a unit picture element 
shown in two dimensional manner in FIG. 2. A scanning electrode line 
driving circuitry part 3 applies a prescribed voltage to scanning 
electrode lines of the display panel part. A signal electrode line driving 
circuitry part 4 applies a prescribed voltage to a signal electrode 
display panel part 5. A controlling part 2 supplies control signals to the 
scanning electrode line driving circuitry part 3 and the signal electrode 
line driving circuitry part 4, respectively, in order to display input 
information. 
In the unit picture element shown in FIG. 2, reference numeral 6 designates 
a scanning electrode line, 7 a signal electrode line, 8 a MiM diode being 
a two-terminal element, and 9 a display picture element capacitor composed 
of a liquid crystal layer being an electrooptical element and a display 
electrode, respectively. 
FIG. 3 illustrates a conventional driving signal waveform wherein scanning 
electrode signal waveform is represented by solid line whilst signal 
electrode signal waveform is represented by dotted line. This driving 
signal waveform consists of two types of periods, i.e., writing periods 
designated by W and holding periods designated by H in FIG. 3. A pulsing 
signal 10 or 12 is applied to the scanning electrode line during the 
writing period W whilst a holding signal 11 or 13 is applied during the 
holding period H. 
On one hand, ON signal 14 or 16 is applied to the signal electrode line in 
the case when a picture element is in ON display (voltage of display 
picture element capacitor is high) whilst OFF signal 15 or 17 is applied 
when the picture element is in OFF display (voltage of the display picture 
element capacitor is low). Problem of halftone can be solved by setting 
the voltage signal between OFF and ON signals. During the writing period 
W, charge is injected into the display picture element capacitor in 
accordance with display information, and charge of the display picture 
element capacitor is held by utilizing current-voltage nonlinearity of MiM 
diode during the holding period H. Since the voltage corresponding to the 
charge which has been held is continuously applied to the liquid crystal 
layer, high quality display is possible in comparison with voltage 
equalization driving method which exhibits remarkable deterioration in 
display quality due to increase of number of scanning electrodes. 
The problem of such conventional driving method composed of the writing and 
holding periods as mentioned above resides in that the charge of the 
display picture element capacitor immediately after the writing period 
depends upon the charge which has been written in the preceding writing 
period to the aforesaid writing period. In this connection, the problem 
will be more specifically described by referring to FIG. 4 wherein 
reference character W designates a writing period, and H.sub.1, H.sub.2 
holding periods before and after the writing period, respectively. In FIG. 
4, voltage across both ends of the display picture element capacitor 9 is 
plotted as ordinate and time as abscissa wherein reference numerals 18 and 
19 designate voltages across both the ends of the display picture element 
capacitor 9 in case of OFF display and ON display during the holding 
period H.sub.1, respectively, numeral 22 designates a voltage during the 
holding period H.sub.2 when the charge corresponding to ON display was 
written during the writing period W, and numerals 20, 21 designate 
voltages when OFF displays were written, respectively. When ON display was 
written, the voltage after writing becomes the situation 22 in either case 
that display is ON 18 or OFF 19 during the holding period H.sub.1. As a 
result, the ON display voltage 22 is obtained, which does not depend on 
the display situation prior to the writing period. On the other hand, when 
OFF display was written during the writing period W, the situation 18 
becomes the voltage 20 in case of OFF display during the holding period 
H.sub.1 whilst the situation 19 becomes the voltage 21 in case of ON 
display during the holding period H.sub.2. In other words, the voltages in 
case of OFF display during the holding period H.sub.2 depend upon the 
display situation before writing period as represented by reference 
numerals 20 and 21. Such dependence results in decrease in display quality 
such as display reliability, contrast ratio, and the like. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of driving a 
diode type display unit, which has high display quality and can eliminate 
the above-mentioned disadvantage by contriving a method for driving 
conventional display units. 
More specifically, the present invention relates to a method for driving a 
diode type display unit characterized by adding a charge regulating period 
to driving signal waveform in order to regulate amount of charge in a 
display picture element capacitor in said diode type display unit wherein 
a two-terminal element having nonlinear current-voltage characteristics is 
provided two-dimensionally on a substrate, an electrooptical element 
capable of controlling the optical quality by means of applied voltage is 
disposed in response to said two-terminal element, charge is injected into 
said electrooptical element by utilizing the current-voltage nonlinearity 
in said two-terminal element for the writing period, and the charge 
injected is held by utilizing the current-voltage nonlinearity in said 
two-terminal element for the holding period thereby effecting display. 
The above and other objects of the invention will become apparent from the 
following detailed description taken in conjunction with the accompanying 
drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The ground for adding a charge regulating period in the method of the 
present invention will be described hereinbelow by referring to FIG. 5 
wherein voltage of a display picture element capacitor during holding 
period H.sub.1 is plotted as abcissa 23, and voltage of the display 
picture element capacitor after completing the writing period W is plotted 
as an ordinate 24. The picture element has dimension of 100 .mu.m square, 
and thickness of liquid crystal cell is 10 .mu.m. 
Current-voltage characteristics of MiM diode may be expressed as follows: 
##EQU1## 
wherein K=1.times.10.sup.-14 and .beta.=4. In this case, a capacity of the 
MiM diode is 0.01 pF. 
In FIG. 5, reference numerals 25, 26, and 27 designate such a case where 
the voltage across the scanning electrode line 6 and the signal electrode 
line 7 during writing period is 10 volts, 9 volts, and 8 volts, 
respectively. It has been found in FIG. 5 that the more negative voltage 
of display picture element capacitor during the holding period H.sub.1 
causes the less voltage difference after completing the writing period. 
In this connection, more specific explanation will be made in conjunction 
with the case that a voltage is 8 volts and represented by reference 
numeral 27. If voltages of display picture element capacitor are -1 volt 
and -3 volts during the holding period H.sub.1, the voltage difference 
after completing writing period becomes 0.25 volt. On the other hand, if 
voltages are -4 volts and -6 volts during the holding period, the voltage 
difference decreases, the value of which reaches only 0.04 volt. 
The addition of a charge regulating period to driving signal waveform in 
the present invention is based on the above described phenomenon, whereby 
disadvantages of prior art methods can be eliminated. 
FIG. 6 indicates the driving signal waveform according to the present 
invention wherein scanning electrode signal waveform is represented by a 
solid line whilst signal electrode signal waveform is represented by a 
dotted line. The driving signal waveform consists of three periods, i.e., 
writing and holding periods W and H being similar to those of conventional 
examples as well as the charge regulating period R which is added in the 
present invention. Pulse signals 29, 32, and 35 are added to the scanning 
electrode line during writing period whilst holding siganls 30, and 33 are 
applied during the holding period as in conventional examples. ON signals 
35, 36, and 37 are applied to the signal electrode line when the picture 
element is in ON display whilst OFF signals 38, 39, and 40 are applied 
when the picture element is in OFF display as in conventional examples. 
During the new charge regulating period R, charge regulation pulsing 
signals 28, 31, and 34 are applied to the scanning electrode line whilst a 
charge regulating signal 41 is applied to the signal electrode line. 
During the charge regulating period R, charge having the same sign with 
that of the charge accumulated in the display picture element capacitor is 
injected thereinto to increase amount of charge. For electric potential of 
the charge regulating signal 41, any potential may be utilized so far as 
such potential increases amount of charge, but ON signal is desirable in 
view of simplicity of a driving circuit, and efficiency of charge 
injection. Namely, potential 36 is used for the charge regulation pulse 
signals 28 and 34 whilst potential 35 is utilized for the charge 
regulation pulse signal 31. 
As described above, the charge regulating period is provided for driving 
signal waveform in the present invention, whereby a diode type display 
unit having high reliability in display can be realized so that remarkable 
advantages are obtained. 
While the present invention has been described with reference to preferred 
embodiment thereof, many modifications and alterations may be made within 
the spirit and scope of the invention.