SAN FRANCISCO Nvidia chips do not contain "backdoors" allowing remote access, the U.S. tech giant has said, after Beijing summoned company representatives to discuss "serious security issues". The California-based company is a world-leading producer of AI semiconductors, and this month became the first company to hit $4 trillion in market value. But it has become entangled in trade tensions between China and the United States, and Washington effectively restricts which chips Nvidia can export