HSINCHU, Taiwan -- When Powerchip returns to the Taiwan Stock Exchange on Dec. 6, it will cap a remarkable comeback for a company that was once written off, even by its own backers. Once the biggest Taiwanese memory chip maker, the company delisted in 2012 as it grappled with a $4 billion debt crisis. It was forced to shutter its most advanced chip plant and sell off the facility's equipment, as well as give up its stake in a joint venture with Japan's Elpida Memory, then its most important tech partner.