1. Field of the Invention
The present invention relates to a display panel drive circuit and display panel and, more particularly, to a display panel drive circuit and display panel in which thin film transistors for the display panel drive circuit can be prevented from deteriorating.
2. Description of Related Art
In recent years, there have been proposals for LCD (liquid crystal display) panels utilizing low-temperature polysilicon TFTs (thin film transistors). Such display panels can be formed, on one common substrate, not only together with pixel transistors but also with peripheral drive circuits, such as scanning shift registers and sampling circuits. Accordingly, display can be by mere external connection with reduced number of signal lines, reducing the number of parts and improving reliability. Large display panels of an approximately 20-40 type are under considerations.
There are recently found cases where a large color liquid crystal panels is equipped on a camera-integrated video tape recorder (VTR) in order for use as monitors or finders. Of these camera-integrated VTRs, there are structures that a display panel is arranged to rotate about a horizontal axis to shift its position. In such a case, horizontal and vertical scanning directions has to be changed depending upon the panel direction so that display is properly viewed when the panel is rotated. Due to this, the scanning shift register includes a scanning direction control circuit using, for example, an analog switch circuit.
A display panel having a drive circuit for controlling the scanning direction, as mentioned above, was formed on one substrate, for conducting test. It was confirmed that deterioration is encountered in the TFTs of a signal input circuit to which scanning start pulses are externally applied, causing a problem that initial failure occurs resulting in impossibility of scanning.
The cause of such deterioration in the signal input circuit TFTs externally applied by scanning start pulses is to be presumed as follows. That is, the start pulse drive circuit is high in deriveability, and circuit board mounting is separated from the display panel with connections to the display panel through cables, flexible circuit boards or the like. During driving or upon switching the scanning direction, a high voltage occurs due to the effect of interconnection inductance, etc., resulting in deterioration or breakage of transistors. It is also to be presumed as one of reasons for the deterioration that the analog switch circuit, to which an external start pulse is first inputted, is not configured as a gate input circuit.
Also, where the panel is made larger, a problem of time delay occurs particularly for a pixel section. In such a case, there is a necessity of forming the interconnection (gate) with using a low-resistance material such as aluminum. In the above-stated display panel, however, a pixel section and its peripheral circuits are formed by a common process so that the interconnections for the peripheral circuit are formed also by the low-resistance material. Due to this, there has been a problem that the peripheral circuit elements are liable to undergo dielectric breakdown.
Further, where using a high insulation substrate such as a glass substrate, there occurs concentration of electric fields through the interconnections during a plasma process for the TFT manufacture, resulting in a problem that so-called plasma antenna effects occur, i.e., the elements connected to these interconnections undergo damage. This phenomenon is liable to occur, particularly, at end portions of an interconnection pattern, at discontinuous portions or at large electrode areas. This condition is met by a start pulse input terminal pattern.
It is an object of the present invention to solve the above-stated problems as encountered in the prior art, and to provide a display panel drive circuit and display panel which is simple in structure but is free from occurrence of initial failure leading to impossibility of scanning.
A display panel drive circuit according to the present invention is characterized in that: thin film transistors constituting a signal input circuit connected to a circuit outside the display panel are formed in a structure having a dielectric breakdown strength higher than those of thin film transistors constituting other circuits.
In the present invention, only a circuit to which signals are externally applied or thin film transistors of the same circuit is formed by a structure to withstand high voltage, whereby they operate in a manner preventing against deterioration and hence occurrence of initial failure.