In recent years, an insulated gate semiconductor device having an active layer (also called an active region) in the form of a thin film has been investigated. Especially, an insulated-gate transistor in the form of a thin film which is known as a TFT has been earnestly investigated. Transistors of this kind are formed on a transparent insulating substrate and used either to control each pixel in a display device such as a liquid-crystal display having a matrix structure or to form a driver circuit. Depending on the material or the state of crystallization of the used semiconductor, they are classified as amorphous silicon TFTs or crystalline silicon TFTs.
Generally, amorphous semiconductors have small field mobilities and so they cannot be used in TFTs which are required to operate at high speeds. Accordingly, in recent years, crystalline silicon TFTs have been investigated and developed to fabricate circuits of higher performance.
Since crystalline semiconductors have higher field mobilities than amorphous semiconductors, the crystalline semiconductors are capable of operating at higher speeds. With respect to crystalline silicon, PMOS TFTs can be fabricated, as well as NMOS TFTs. For example, it is known that the peripheral circuit of an active-matrix liquid-crystal display is composed of CMOS crystalline TFTs similarly to the active-matrix circuit portion. That is, this has a monolithic structure.
FIG. 3 is a block diagram of a monolithic active-matrix circuit used in a liquid-crystal display. A column decoder 1 and a row decoder 2 are formed on a substrate 7 to form a peripheral driver circuit. Pixel circuits 4 each consisting of a transistor and a capacitor are formed in a matrix region 3. The matrix region is connected with the peripheral circuit by conductive interconnects 5 and 6. TFTs used in the peripheral circuit are required to operate at high speeds, while TFTs used in the pixel circuits are required to have a low-leakage current. These are conflicting characteristics in terms of physics but it is necessary that these two kinds of TFTs be formed on the same substrate at the same time.
However, all TFTs fabricated by the same process show similar characteristics. For example, TFTs using crystalline silicon fabricated by thermal annealing, TFTs used in the matrix region, and TFTs in the peripheral driver circuit all have similar characteristics. It has been difficult to obtain a low-leakage current suited for the pixel circuits and a high mobility adapted for the peripheral driver circuit at the same time. It has been possible to solve the above difficulty by using thermal annealing and crystallization using selective laser annealing at the same time. In this case, TFTs fabricated by thermal annealing can be used in the matrix region, whereas TFTs fabricated by laser annealing can be employed in the peripheral driver circuit region. However, the crystallinity of silicon crystallized by laser annealing has quite low homogeneity. Especially, it is difficult to use these TFTs in a peripheral driver circuit which is required to be defect-free.
It is also possible to use crystallization relying on laser annealing in order to obtain crystalline silicon. If a semiconductor circuit is fabricated from this silicon crystallized by laser annealing, TFTs in the matrix region and TFTs in the peripheral driver circuit all have similar characteristics. Accordingly, an alternative method of crystallizing silicon may be contemplated. In particular, TFTs in the matrix region are formed, using thermal annealing. TFTs in the peripheral driver circuit are formed, using laser annealing. However, where the thermal annealing is adopted, the silicon must be annealed at 600.degree. C. for as long as 24 hours, or the silicon must be annealed at a high temperature exceeding 1000.degree. C. In the former method, the throughput drops. In the latter method, the material of the usable substrate is limited to quartz.