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Two kinds of TFTs are fabricated by the same process with a high production yield to manufacture an active-matrix circuit and a peripheral driver circuit on the same substrate. The active-matrix circuit is required to have a high mobility and a high ON/OFF current ratio. The peripheral driver circui

Two kinds of TFTs are fabricated by the same process with a high production yield to manufacture an active-matrix circuit and a peripheral driver circuit on the same substrate. The active-matrix circuit is required to have a high mobility and a high ON/OFF current ratio. The peripheral driver circuit needs a complex interconnection structure. The active-matrix circuit and the peripheral driver circuit comprising the TFTs are fabricated monolithically. In this step, the gate electrodes of the TFTs of the active-matrix circuit is coated with an anodic oxide on their top and side surfaces. The gate electrodes of the TFTs of the peripheral driver circuit is coated with the anodic oxide on only their top surfaces; substantially no anodic oxide is present on the side surfaces.

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1. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; crystallizing the semiconductor film by a heat treatment; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decom

1. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; crystallizing the semiconductor film by a heat treatment; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decomposing TEOS; doping a first impurity element into the semiconductor layers; forming a mask over a portion of the semiconductor layers; and doping a second impurity element into at least one of the semiconductor layers over which the mask is not formed. 2. A method of manufacturing a display device according to claim 1, wherein the substrate comprises a glass substrate.3. A method of manufacturing a display device according to claim 1, wherein the mask comprises a photoresist.4. A method of manufacturing a display device according to claim 1, wherein the first impurity element comprises phosphorus, and the second impurity element comprises boron.5. A method of manufacturing a display device according to claim 1, wherein a dose amount of the second impurity element is larger than that of the first impurity element.6. A method of manufacturing a display device according to claim 1, wherein the gate insulating film is formed by plasma CVD.7. A method of manufacturing a display device according to claim 1, wherein the display device is a liquid crystal display device.8. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; crystallizing the semiconductor film by a heat treatment; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decomposing TEOS; and performing a rapid thermal annealing to the semiconductor layers after forming the gate insulating film. 9. A method of manufacturing a display device according to claim 8, wherein the substrate comprises a glass substrate.10. A method of manufacturing a display device according to claim 8, wherein the rapid thermal annealing is conducted by irradiating the semiconductor layers with an infrared light.11. A method of manufacturing a display device according to claim 8, wherein the rapid thermal annealing is conducted for several seconds to several minutes.12. A method of manufacturing a display device according, to claim 8, wherein the gate insulating film is formed by plasma CVD.13. A method of manufacturing a display device according to claim 8, wherein the display device is a liquid crystal display device.14. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; crystallizing the semiconductor film by a heat treatment; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decomposing TEOS; and irradiating the semiconductor layers with a laser light after forming the gate insulating film. 15. A method of manufacturing a display device according to claim 14, wherein the substrate comprises a glass substrate.16. A method of manufacturing a display device according to claim 14, wherein the laser comprises an excimer laser.17. A method of manufacturing a display device according to claim 14, wherein the gate insulating film is formed by plasma CVD.18. A method of manufacturing a display device according to claim 14, wherein the display device is a liquid crystal display device.19. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; irradiating the semiconductor film with a laser light to crystallize the semiconductor film; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decomposing TEOS; doping a first impurity element into the semiconductor layers; forming a mask over a portion of the semiconductor layers; and doping a second impurity element into at least one of the semiconductor layers over which the mask is not formed. 20. A method of manufacturing a display device according to claim 19, wherein the substrate comprises a glass substrate.21. A method of manufacturing a display device according to claim 19, wherein the mask comprises a photoresist.22. A method of manufacturing a display device according to claim 19, wherein the first impurity element comprises phosphorus, and the second impurity element comprises boron.23. A method of manufacturing a display device according to claim 19, wherein a dose amount of the second impurity element is larger than that of the first impurity element.24. A method of manufacturing a display device according to claim 19, wherein the laser comprises an excimer laser.25. A method of manufacturing a display device according to claim 19, wherein the gate insulating film is formed by plasma CVD.26. A method of manufacturing a display device according to claim 19, wherein the display device is a liquid crystal display device.27. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; irradiating the semiconductor film with a laser light to crystallize the semiconductor film; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decomposing TEOS; and performing a rapid thermal annealing to the semiconductor layers after forming the gate insulating film. 28. A method of manufacturing a display device according to claim 27, wherein the substrate comprises a glass substrate.29. A method of manufacturing a display device according to claim 27, wherein the rapid thermal annealing is conducted by irradiating the semiconductor layers with an infrared light.30. A method of manufacturing a display device according to claim 27, wherein the rapid thermal annealing is conducted for several seconds to several minutes.31. A method of manufacturing a display device according to claim 27, wherein the laser comprises an excimer laser.32. A method of manufacturing a display device according to claim 27, wherein the gate insulating film is formed by plasma CVD.33. A method of manufacturing a display device according to claim 27, wherein the display device is a liquid crystal display device.34. A method of manufacturing a display device comprising:forming a semiconductor film over a substrate; irradiating the semiconductor film with a laser light to crystallize the semiconductor film; etching the crystallized semiconductor film into semiconductor layers; forming a gate insulating film on the semiconductor layers by decomposing TEOS; and irradiating the semiconductor layers with a laser light after forming the gate insulating film. 35. A method of manufacturing a display device according to claim 34, wherein the substrate comprises a glass substrate.36. A method of manufacturing a display device according to claim 34, wherein the laser comprises an excimer laser.37. A method of manufacturing a display device according to claim 34, wherein the gate insulating film is formed by plasma CVD.38. A method of manufacturing a display device according to claim 34, wherein the display device is a liquid crystal display device.