OPTICAL SEMICONDUCTOR DEVICE AND ITS FABRICATION

PROBLEM TO BE SOLVED: To provide an optical semiconductor device which can reduce its threshold value and can improve its high-temperature operational characteristics. SOLUTION: In the optical semiconductor device, an active region 20 having a laser active layer 3 and an inactive region 21 having an inactive MFC layer are sequentially formed along a major surface of a substrate 1. The optical semiconductor device is also formed with a mesa stripe 22. At the time of forming buried layers 7 and 8 of the mesa stripe 22, the buried layers 7 and 8 are formed only on sides of the mesa stripe 22 in the active region 20, whereas, the buried layers are formed, in addition to the sides of the mesa stripe 22, even on an upper side of the stripe in the inactive region 21. Semiconductor laminated layers including the buried layers 7 and 8 form a current blocking structure of a p-n-p-n junction type, whereby, in the inactive region 21, a current can be prevented from flowing through the NFC layer. Thus a current efficiency as a laser can be improved and its threshold can be decreased.