The Nikkei stock index tumbled over 2 percent Friday morning as technology shares fell sharply, tracking their U.S. counterparts, and investors moved to sell before a three-day weekend. The 225-issue Nikkei Stock Average dropped 884.72 points, or 2.26 percent, from Thursday to 38,196.53. The broader Topix index was down 38.73 points, or 1.44 percent, at 2,656.78. The U.S. dollar briefly weakened to the upper 151 yen range on yen buying amid speculation the Bank of Japan could raise interest rates at its December policy meeting after its chief signaled Thursday the BOJ is closer to a further ra…