TOKYO—Sony Corp., an also-ran in the smartphone market, is trying to push deeper into the business as it nears a deal to buy out Telefon AB L.M. Ericsson's stake in their mobile-phone joint venture, people familiar with the matter said.

Wresting full control of Sony Ericsson—a decade-old, 50-50 joint venture that is the world's sixth-largest cellphone manufacturer—would allow the Tokyo-based electronics maker to integrate its smartphone business with its operations in computer tablets, hand-held videogame players and personal...