SEOUL, Sept 21 (Reuters) - South Korean exports returned to growth in the first 20 days of September, the first increase in 20-day exports in six months, data from the Korea Customs Service showed on Monday. Exports expanded 3.6% from a year earlier during Sept. 1-20, versus a 7.5% drop in the comparable period in August. Imports during the period declined 6.8% on-year. A breakdown of data showed shipments of semiconductors, South Korea’s top-selling item, surged 25.3%, while exports to China, the nation’s biggest trading partner, jumped 8.7%. Full month trade data for the month will be released on Oct. 1. (Reporting by Joori Roh; Editing by Kim Coghill)