The present invention generally relates to light valves having a liquid crystal layer confined between a pair of substrates, and more particularly to a light valve of the foregoing type with carrying MOS transistors having a single crystal semiconductive channel forming region provided on a semiconductor layer as switching elements for selectively applying a voltage signal to the liquid crystal layer to activate individual picture elements.
Conventionally, light valves of the active matrix type LCD are known. In the light valve of the foregoing type, thin film transistors are formed on an amorphous or polycrystal silicon layer that is deposited on a glass substrate, for individually switching liquid crystal pixels. As an amorphous silicon layer or polysilicon layer can be deposited easily on a glass substrate by a CVD process, the active matrix type LCD having the foregoing construction is suitable for realizing a light valve having a large display area.
On the other hand, the conventional light valves that use an amorphous silicon or polysilicon layer have a drawback in that miniaturization of transistor elements and increase of the integration density is difficult, although such a device is suitable for constructing a large area LCD device of the direct-view type. These problems have made it impossible to produce a high resolution transmissive type or projection type, displays, such as a light valve.
In order to overcome the foregoing problem associated with the light valves that use an amorphous silicon or polysilicon layer, a light valve was proposed recently that uses a single crystal semiconductor layer formed on a carrier layer of an insulating material. In the foregoing device, a number of MOS transistors are provided on the single crystal semiconductor layer as switching transistor elements. When a single crystal layer of silicon is used for the single crystal semiconductor layer, one can achieve an improved operational speed together with an increased integration density for the switching transistor elements.
FIG. 4 shows a cross section of a composite semiconductor substrate wherein a single crystal semiconductor layer is provided on a carrier layer. In FIG. 4, elements such as the liquid crystal layer, the substrates opposing each other across the liquid crystal layer, the polarizers provided adjacent to the substrates, and the like, are omitted from the illustration.
Referring to FIG. 4, it will be noted that a single crystal semiconductor layer 25 as well as a device isolation region 32 are formed on an insulation layer 31 that is provided on a carrier layer 29 of an insulating material, with a bonding layer 30 interposed between the layers 29 and 31. In the single crystal semiconductor layer 25, a source region 21 and a drain region 22 are formed by doping an impurity element, and a gate insulation layer 26 covers the upper surface of the layer 25. Further, a gate electrode 24 is provided on the gate insulation film 26. Thereby, the source region 21, the drain region 22, the gate insulation film 26 and the gate electrode 24 form a MOS switching transistor element 20. Further, in correspondence to the source region and the drain region, a drain electrode is provided such that the drain electrode acts as a source electrode 27 as well as a pixel electrode 28. Further, the entire structure is buried under an insulation layer 33, and a semiconductor composite substrate having a planarized surface is formed. Typically, silicon oxide is used for the insulation layer 31 while a single crystal, or monocrystalline, silicon is used for the single crystal semiconductor layer 25.
In the light valve having the foregoing construction for the switching transistor elements, however, there occurs a problem in that OFF leakage currents tend to increase in the MOS transistor made of a signal crystal semiconductor when light irradiates the transistor, which problem does not exist in a transistor made of a poly or amorphous semiconductor material.