Tokyo stocks opened sharply lower Monday, with the Nikkei index briefly dropping by over 2 percent, as falls in U.S. semiconductor shares and a firmer yen spurred selling of technology and export-related issues. In the first 15 minutes of trading, the 225-issue Nikkei Stock Average fell 714.44 points, or 1.80 percent, from Friday to 38,974.50. The broader Topix index was down 39.06 points, or 1.43 percent, at 2,687.74. On the top-tier Prime Market, decliners were led by mining, machinery and transportation equipment issues. At 9 a.m., the U.S. dollar fetched 146.77-78 yen compared with 147.04-…