| abstract |
In the case of an LDD-structure thin film transistor, an on-current becomes large as impurity concentration of low level impurity source and drain regions is increased. Then, when the impurity concentration is increased to a first impurity concentration, the on-current reaches to a substantially maximum point. On the other hand, a cut-off current I off becomes substantially minimum when the impurity concentration is decreased to a second impurity concentration. The cut-off current is gradually increased even if the impurity concentration becomes higher or lower than the second impurity concentration. Therefore, impurity concentration of low level impurity source and drain regions (22b) of a thin film transistor (14) for a peripheral circuit is set to a first impurity concentration, and that of low concentration impurity source and drain regions (21b) of a thin film transistor (12) for a matrix circuit is set to a second impurity concentration. |