T-Mobile is investing is Percello, a semiconductor company designing products for femtocells.

The funding is part of Percello’s $12 million second round, led by Granite Ventures and Vertex Venture Capital, and with existing investor 7-Main.

Earlier this year, T-Mobile invested in femtocell maker Ubiquisys.

The femtocell market could reach from 40 to 70 million units sold in the 2011-2012 timeframe, industry analysts said.