Mechanically strained Si-SiGe HBTs

The current gain (β = I_C/I_B) variations of the mechanically strained Si-SiGe heterojunction bipolar transistor (HBT and Si bipolar junction transistor (BJT) devices are investigated experimentally and theoretically. The β change of HBT is found to be 4.2% and -7.8% under the biaxial compressive and tensile mechanical strain of 0.028%, respectively. For comparison, there are 4.9% and -5.0% β variations for BJT under the biaxial compressive and tensile mechanical strain of 0.028%, respectively. In HBT, the mechanical stress is-competing with the compressive strain of SiGe base, inherited from the lattice misfit between SiGe and Si. The current change due to externally mechanical stress is the combinational effects of the dependence of the mobility and the intrinsic carrier concentration on strain.