Improved minority-carrier lifetime in Si/SiGe heterojunction bipolar transistors grown by molecular beam epitaxy

Noncontact-probe photocarrier lifetime measurements have been used to give rapid feedback on minority-carrier lifetimes of epitaxial Si/SiGe layers grown by molecular beam epitaxy (MBE). In this manner, problems with impurity incorporation during the crystal growth can be rapidly diagnosed in a device-processing-independent fashion. These improvements in minority-carrier lifetime translate directly into high gains in the heterojunction bipolar transistors (HJBTs). HJBT test devices fabricated from the MBE-grown Si/SiGe layers show current gains as high as 800. The homojunction gain of this device is estimated to be ∼6, making the heterojunction gain on the order of 130.