Abstract:
A current-drive thin-film transistor display apparatus that simultaneously achieves a reduction in the off-current of a switching thin-film transistor and an increase in the on-current of a current thin-film transistor. 
     In an exemplary embodiment, the switching thin-film transistor is formed as a transistor of lightly doped drain structure or offset structure while the current thin-film transistor is formed as a transistor of self-alignment structure. Alternatively, each of the switching thin-film transistor and the current thin-film transistor is formed as a transistor of lightly doped drain structure or offset structure, and the lightly doped drain length or offset length of the switching thin-film transistor is increased relative to that of the current thin-film transistor.

Description:
This is a Continuation of application Ser. No. 09/077,072 filed May 18, 1998 now U.S. Pat. No. 6,542,137, which is a 371 of PCT/JP97/03424, filed Sep. 25, 1997. The entire disclosure of the prior application is hereby incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a display apparatus in which a current light-emitting element is driven with a thin-film transistor (current-drive thin-film transistor display apparatus). 
   2. Description of the Related Art 
   A number of thin-film transistor display apparatuses of various types for realizing lightness in weight, smallness in size, high image qualities and high resolution have been used. Thin-film transistor display apparatuses hitherto developed, as represented by thin-film transistor liquid crystal displays, are mainly for transmission of signal voltages or transfer of minute charges. However, it is anticipated that an element capable of current driving and having a memory function will become indispensable to self-light-emitting type panels, such as EL (electroluminescence) displays, heat-developing panels and the like which are expected to be developed in future. 
   FIGS.  10 ( a ) and  10 ( b ) are an equivalent circuit diagram and a potential relationship diagram, respectively, of a current-drive thin film transistor display apparatus, in which an organic fluorescent material is used as a light emitting material. 
   In FIG.  10 ( a ), symbol  121  represents a scanning line; symbol  122 , a signal line, symbol  123 , a common current supply line; symbol  131 , a switching thin-film transistor; symbol  132 , a current thin-film transistor; symbol  151 , a holding capacitor; symbol  152 , a pixel electrode; symbol  164 , an organic fluorescent material; and symbol  165 , an opposed electrode. In FIG.  10 ( b ), line  421  represents a scanning potential; line  422 , a signal potential; line  423 , a common potential; line  451 , a held potential; line  452 , a pixel potential; and line  465 , a counter potential. 
   The switching thin-film transistor  131  is a transistor for controlling conduction between the signal line  122  and holding capacitor  151  by a potential on the scanning line  122 . That is, signal potential  422  is transmitted to held potential  451  by scanning potential  421 . With respect to a displaying pixel, signal potential  422  becomes high and held potential  451  becomes high. With respect to a non-displaying pixel, signal potential  422  becomes low and held potential  451  becomes low. 
   On the other hand, the current thin-film transistor  132  is a transistor for controlling conduction between the common current supply line  123  and the pixel electrode  152  by the potential on the holding capacitor  151 . That is, common potential  423  is transmitted to pixel potential  452  by held potential  451 . With respect to a displaying pixel, conduction is effected between the common current supply line  123  and the pixel electrode  152 . With respect to a non-displaying pixel, the common current supply line  123  and the pixel electrode  152  are shut off from each other. 
   Consequently, a current is caused to flow between the pixel electrode  152  and the opposed electrode  165  with respect to a displaying pixel, thereby causing the organic fluorescent material  164  to emit light. With respect to a non-displaying pixel, no current flows and emission of light is not caused. 
   Thus, the current-drive thin film transistor display apparatus has the switching thin film transistor  131  and the current thin-film transistor  132 , each of which is a field effect type transistor manufactured by an ordinary semiconductor manufacturing process. As such, for the two thin-film transistors for conventional current-drive thin film transistor display apparatuses, thin-film transistors of the same structure have been used because the manufacturing cost can be reduced if the two transistors are made in accordance with the same specifications. 
   Actually, even if the structures of the two thin-film transistors are the same, the current-drive thin-film transistor display apparatus has no serious defect relating to it. However, if a high-quality product is aimed at, it is preferable to construct the above-described two thin-film transistors in such a manner that importance is attached to making the characteristics of the two transistors different from each other. 
   That is, with respect to switching thin-film transistor  131 , a reduction in off current is needed for the purpose of enabling the charge to be retained more reliably in the holding capacitor  151 . In contrast, with respect to current thin-film transistor  132 , an increase in on current is needed for the purpose of increasing the luminance of emission of light from the organic fluorescent material  164 . 
   However, no technical idea of positively making the characteristics of the above-described two thin-film transistors different from each other in a current-drive thin-film transistor display apparatus has been conceived. 
   SUMMARY OF THE INVENTION 
   The present invention has been achieved based on such a knowledge, and an object of the present invention is to provide a current-drive thin-film transistor display apparatus in which a reduction in the off current of switching thin-film transistor  131  and an increase in the on current of current thin-film transistor  132  are achieved simultaneously. 
   To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, the switching thin-film transistor is designed in such a manner that importance is attached to reducing the off current, and the current thin-film transistor is designed in such a manner that importance is attached to increasing the on current. 
   According to an aspect of the invention, each of the switching thin film transistor and the current thin-film transistor is constructed according to required performance of the transistor in such a manner that importance is attached to reducing the off current or increasing the on current, so that charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably. 
   To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a low concentration impurity region is formed between a channel region and a high concentration impurity region of the switching thin-film transistor, and a channel region and a high concentration impurity region of the current thin-film transistor are directly connected to each other. 
   That is, the switching thin-film transistor is formed as a transistor of LDD structure while the current thin-film transistor is formed as a transistor of self-alignment structure. 
   According to another aspect of the invention, the switching thin-film transistor is designed so as to reduce the off current while the current thin-film transistor is designed so as to increase the on current, so that a charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably. 
   To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin film transistor controlling conduction between the signal line and the holding capacitor by a potential on the canning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a low concentration impurity region is formed between a channel region and a high concentration impurity region of each of the switching thin-film transistor and the current thin-film transistor, and the length of the low concentration impurity region of the switching thin-film transistor is increase relative to the length of the low concentration impurity region of the current thin-film transistor. 
   That is, each of the switching thin-film transistor and the current thin-film transistor is formed as a transistor of LDD structure, and the length of the low concentration impurity region (LDD length) of the switching thin-film transistor is increase relative to the LDD length of the current thin-film transistor. 
   The invention set forth in claim  3  also makes it possible to achieve the same effect as the invention set forth in claim  2 . 
   To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, and a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the holding capacitor, a region having an impurity concentration substantially the same as that of a channel region of the stitching thin-film transistor is formed between the channel region and a high concentration impurity region of the switching thin-film transistor are directly connected to each other. 
   That is, the switching thin-film transistor is formed as a transistor of offset structure while the current thin-film transistor is formed as a transistor of self alignment structure. 
   According to an aspect of the invention, the switching thin-film transistor is designed so as to reduce the off current while the current thin-film transistor is designed so as to increase the on current, so that charge can be retained more reliably in the holding capacitor, and so that a sufficient current supply to the pixel electrode can be effected more reliably. 
   To achieve the above-described object, according to an aspect of the invention, in a display apparatus in which a plurality of scanning lines, a plurality of signal lines, and a plurality of common current supply lines are formed, a switching thin-film transistor, a current thin-film transistor, a holding capacitor and a pixel electrode being formed at each of the intersections of the scanning lines and the signal lines, the switching thin-film transistor controlling conduction between the signal line and the holding capacitor by a potential on the scanning line, the current thin-film transistor controlling conduction between the common current supply line and the pixel electrode by a potential on the hold capacitor, a region having a certain impurity concentration is formed between a channel region and a high concentration I purity region of each of the switching thin-film transistor and the current thin-film transistor, the impurity concentration of the region between the channel region and the high concentration impurity region being substantially the same as that of the channel region, and the length of the region having an impurity concentration substantially the same as that of the channel region of the switching thin-film transistor is increased relative to the length of the region having an impurity concentration substantially the same as that of the channel region of the current thin-film transistor. 
   That is, each of the switching thin-film transistor and the current thin-film transistor is formed as a transistor of offset structure, and the offset length of the switching thin-film transistor is increased relative to the offset length of the current thin-film transistor. 
   The invention set forth in claim  5  also makes it possible to achieve the same effect as the invention set forth in claim  4 . 
   According to another aspect of the invention, the holding capacitor is formed by using a gate insulating film between the scanning line and the channel region of the switching thin-film transistor or the current thin-film transistor. 
   According to another aspect of the invention, a thin gate insulating film is used for the holding capacitor to enable the holding capacitor to be formed as a small-area large-capacity capacitor. 
   In contrast, in the display apparatus according to another aspect of the invention, the holding capacitor is formed by using an interlevel insulating film between the scanning line and the signal line. 
   According to another aspect of the invention, an interlevel insulating film is used for the holding capacitor to improve the degree of freedom of designing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a circuit diagram showing a portion of a display apparatus which represents a first embodiment of the present invention. 
     FIGS.  2 ( a ) and  2 ( b ) comprise a cross-sectional view and a plan view of the display apparatus in the first embodiment. 
     FIGS.  3 ( a )- 3 ( e ) are diagrams showing the process of manufacturing the display apparatus in the first embodiment. 
       FIG. 4  is a diagram showing characteristics of thin-film transistors in the first embodiment. 
     FIGS.  5 ( a ) and  5 ( b ) comprise a cross-sectional view and a plan view of a display apparatus which represents a second embodiment of the present invention. 
     FIGS.  6 ( a ) and  6 ( b ) comprise a cross-sectional view and a plan view of a display apparatus which represents a third embodiment of the present invention. 
     FIGS.  7 ( a )- 7 ( e ) are diagrams showing the process of manufacturing the display apparatus in the third embodiment. 
       FIG. 8  is a diagram showing characteristics of thin-film transistors in the third embodiment. 
     FIGS.  9 ( a ) and  9 ( b ) comprise a cross-sectional view and a plan view of a display apparatus which represents a fourth embodiment of the present invention. 
     FIGS.  10 ( a ) and  10 ( b ) comprise an equivalent circuit diagram and a potential relationship diagram of a current-drive thin-film transistor display apparatus. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described below with reference to the drawings. 
   (1) First Embodiment 
     FIGS. 1  to  4  are diagrams showing a first embodiment of the present invention. In this embodiment, a display apparatus in accordance with the present invention is applied to an active matrix type display apparatus using EL display elements. 
     FIG. 1  is a circuit diagram showing a portion of a display apparatus  1  in this embodiment. The display apparatus  1  is constructed in such a manner that a plurality of scanning lines  121 , a plurality of signal lines  122 , and a plurality of common current supply lines  123  are formed on a transparent display substrate, the signal lines  122  extending perpendicularly to the scanning lines  121 , the common current supply lines  123  extending parallel to the signal lines  122 . A pixel region unit  1 A is provided at each of the intersections of the scanning lines  121  and the signal lines  122 . 
   A data-side drive circuit  3  having a shift register, a level shifter, video lines, and analog switches is provided in connection with the signal lines  122 . A scanning-side drive circuit  4  having a shift register and a level shifter is provided in connection with the scanning lines  121 . In each pixel region  1 A are provided a switching thin-film transistor  131  having a gate electrode to which a scanning signal is supplied via the scanning line  121 , a holding capacitor  151  for holding an image signal supplied from the signal line  132  via the switching thin-film transistor  131 , a current thin-film transistor  132  having a gate electrode to which the image signal held by the holding capacitor  151  is supplied, a pixel electrode  152  into which a drive current from the common current supply line  123  flows when the pixel electrode  152  is connected to the common current supply lines  123  by the current thin-film transistor  132 , and an organic fluorescent material  164  interposed between the pixel electrode  152  and an opposed electrode  165 . 
   FIGS.  2 ( a ) and  2 ( b ) are a cross-sectional view and a plan view, respectively, of each pixel region  1 A shown in FIG.  1 . Cross-sectional view  2 ( a ) is taken along line A—A of plan view  2 ( b ). In FIGS.  2 ( a ) and  2 ( b ), an element  141  represents a channel region; an element  142 , a high concentration impurity region; an element  143 , a low concentration impurity region; an element  146 , a relay wiring; an element  161 , a gate insulating film; an element  162 , an interlevel insulating film; and an element  163 , an uppermost insulating film. 
   FIGS.  3 ( a ),  3 ( b ),  3 ( c ),  3 ( d ), and  3 ( e ) are cross-sectional views showing the process of m manufacturing the display apparatus  1  and correspond to the A—A cross sectional view of FIG.  2 ( b ). In FIGS.  3 ( a )- 3 ( e ), an element  211  represents a resist mask, arrows  221  represent high concentration impurity doping, and arrows  222  represent low concentration impurity doping. 
   The manufacturing process is described below in detail. 
   First, as shown in FIG.  3 ( a ), a semiconductor film is formed, on which channel regions  141  and source and drain regions of switching thin-film transistor  131  and current thin-film transistor  132 , and one electrode of holding capacitor  151  are formed as described below. This semiconductor film is patterned into semiconductor film lands  140 . A gate insulating film  161  is formed so as to cover the semiconductor pattern lands  140 . 
   Next, as shown in FIG.  3 ( b ), a film for forming resist masks  211  is formed and patterned. At this time, resist mask  211  at a position where switching thin-film transistor  131  (left resist mask  211  as viewed in FIG.  3 ( b )) is formed so that its width is slightly larger than the length of the channel region. Thereafter, high concentration impurity doping  221  is performed to form high concentration impurity regions  142 . 
   Next, as shown in FIG.  3 ( c ), a metal film is formed and patterned to form scanning line  121  and relay wiring  146 . Thereafter, low concentration impurity doping  222  is performed with scanning line  121  and relay wiring  146  used as a mask. Low concentration impurity regions  143  are thereby formed below scanning line  121  and inside high concentration impurity regions  142  since the width of scanning line  121  is equal to the length of the channel region. Channel region  141  is defined inside the low concentration impurity regions  143 . 
   Thus, switching thin-film transistor  131  of LDD structure and current thin-film transistor  132  of self-alignment structure are formed. 
   Thereafter, as shown in FIG.  3 ( d ), interlevel insulating film  162  is formed, a contact hole is formed, and a metal film is formed and patterned, thereby forming signal line  122  and common current supply line  123 . 
   Next, as shown in FIG.  3 ( e ), pixel electrode  152  is formed (not shown) and uppermost insulating film  163  is formed. Thereafter, organic fluorescent material  164  and opposed electrode  165  are formed. 
     FIG. 4  is a diagram showing a characteristic of each of switching thin-film transistor  131  and current thin-film transistor  132  in the first embodiment. In  FIG. 4 , line  311  indicates a characteristic of switching thin-film transistor  131  of LDD structure, and line  321  indicates a characteristic of current thin-film transistor  132  of self-alignment structure. As can be understood from  FIG. 4 , switching thin-film transistor  131  has a smaller off current while current thin-film transistor  132  has a larger on current. 
   That is, in the display apparatus  1  of this embodiment, a reduction in the off current of switching thin-film transistor  131  and an increase in the on current of current thin-film transistor  312  are simultaneously achieved. As a result, a charge can be reliably retained in holding capacitor  151  and a sufficient current supply to pixel electrode  162  can be effected more reliably. 
   In this embodiment, holding capacitor  151  is formed by using gate insulating film  161 . In general, gate insulating film  161  is formed so as to be thinner than other insulating films. Therefore, the use of gate insulating film  161  has the advantage that small-area large-capacity holding capacitor  151  can be formed. 
   The structure of the thin-film transistor display apparatus, the method of manufacturing the apparatus and materials of the apparatus can be freely selected as long as they accord with the idea of the present invention. 
   (2) Second Embodiment 
   FIGS.  5 ( a ) and  5 ( b ) show a second embodiment of the present invention and comprise a cross-sectional view and a plan view showing the structure of a display region  1 A, which correspond tot hose in FIGS.  2 ( a ) and  2 ( b ). Cross-sectional view  5 ( a ) is taken along line B—B of plan view  5 ( b ). The same components as those in the first embodiment are indicated by the same reference numerals and the description of them will not be repeated. 
   That is, in this embodiment, each of switching thin-film transistor  131  and current thin-film transistor  132  is of LDD structure. However, the LDD length of switching thin-film transistor  131  is larger than the LDD length of current thin-film transistor  132 . 
   This construction also makes it possible to simultaneously achieve a reduction in the off current of switching thin-film transistor  131  and an increase in the on current of current thin-film transistor  132 , as in the above-described first embodiment. 
   (3) Third Embodiment 
     FIGS. 6  to  8  are diagrams showing a third embodiment of the present invention. In this embodiment, a thin-film transistor display apparatus in accordance with the present invention is also applied to an active matrix type display apparatus using EL display elements, as is that in the first embodiment. The entire construction of this embodiment is the same as that of the first embodiment shown in FIG.  1 . Therefore, the illustration and description of it will not be repeated. Also, the same components as those in the first embodiment are indicated by the same reference numerals, and the description of them will not be repeated. 
   FIGS.  6 ( a ) and  6 ( b ) comprise a cross-sectional view and a plan view showing the structure of a display region  1 A, which correspond to those in FIGS.  2 ( a ) and  2 ( b ). Cross-sectional view  6 ( a ) is taken along line C—C of plan view ( b ). Portions  144  represent regions having an impurity concentration substantially the same as that in the channel region. 
   FIGS.  7 ( a ) to  7 ( e ) are cross-sectional views showing the process of manufacturing the display apparatus  1  of this embodiment. The manufacturing process shown in these figures is substantially the same as that in the first embodiment, and differs in that low concentration impurity doping  222  for forming low impurity regions  143  is not performed. 
   That is, as shown in FIG.  7 ( c ), a metal film is formed and patterned to form scanning line  121  and relay wiring  146 , thus completing switching thin-film transistor  131  and current thin-film transistor  132 . Since regions  144  having an impurity concentration substantially the same as that of channel region  141  are formed between high concentration impurity regions  142  and channel region  141  in switching thin-film transistor  131 , the thus-formed switching thin-film transistor  131  is a transistor of offset structure. 
     FIG. 8  is a diagram showing a characteristic of each of switching thin-film transistor  131  and current thin-film transistor  132  in this embodiment. In  FIG. 8 , line  312  indicates a characteristic of switching thin-film transistor  131  of offset structure, and a line  321  indicates a characteristic of current thin-film transistor  132  of self-alignment structure. As can be understood from  FIG. 8 , switching thin-film transistor  131  has a smaller off current while current thin-film transistor  132  has a larger on current. 
   That is, also in the display apparatus  1  of this embodiment, a reduction in the off current of switching thin-film transistor  131  and an increase in the on current of current thin-film transistor  132  are simultaneously achieved. As a result, a charge can be reliably retained in holding capacitor  151  and a sufficient current supply to pixel electrode  162  can be effected more reliably. 
   In this embodiment, holding capacitor  151  is formed by using interlevel insulating film  162 . Therefore, scanning line  121  and signal line  122  can form holding capacitor  151  without high concentration impurity region  142 , thus improving the degree of freedom of designing. 
   (4) Fourth Embodiment 
   FIGS.  9 ( a ) and  9 ( b ) show a fourth embodiment of the present invention and comprise a cross-sectional view and a plan view showing the structure of a display region  1 A, which correspond tot hose in FIGS.  2 ( a ) and  2 ( b ). Cross-sectional view  9 ( a ) is taken along line D—D of plan view  9 ( b ). The same components as those in the above-described embodiments are indicated by the same reference numerals and the description of them will not be repeated. 
   That is, in this embodiment, each of switching thin-film transistor  131  and current thin-film transistor  132  is of offset structure. However, the offset length of switching thin-film transistor  131  is larger than the offset length of current thin-film transistor  132 . 
   This construction also makes it possible to simultaneously achieve a reduction in the off current of switching thin-film transistor  131  and an increase in the on current of current thin-film transistor  132 , as in the above-described third embodiment. 
   Industrial Applicability 
   As described above, according to the present invention, a reduction in the off current of the switching thin-film transistor  131  and an increase in the on current of the current thin-film transistor can be achieved simultaneously, thereby ensuring that charge can be reliable retained in the holding capacitor, and that a sufficient current supply to the pixel electrode can be effected more reliably.