Electric field effects associated with the backside Ge profile in SiGe HBTs

A comprehensive investigation of electric field effects associated with the backside Ge profile in SiGe heterojunction bipolar transistors (HBTs) is conducted using calibrated simulations. We show for the first time that the backside Ge retrograde can alter the local electric field distribution in the base-collector space-charge region near the SiGe to Si heterojunction, thereby affecting the impact ionization and the apparent neutral base recombination (NBR) (I_B(V_CB)/I_B(V_CB = 0)) of SiGe HBTs. The changes in the electric field induced by the Ge-induced band offsets contributes to a decrease of the observed impact ionization between comparably doped SiGe HBTs and Si bipolar junction transistors (BJTs), as well as an improved V_CB dependence of I_B (apparent decrease in NBR). Experimental data on SiGe HBTs with various Ge profiles and a Si BJT control are used to support our claims.