Base currents of Si/SiGe/Si HBT in dependence on the processing conditions

The influence of processing conditions on the base current (IB) to base-emitter voltage (VBE) characteristics of Si/SiGe/Si HBT was investigated. The transistors were fabricated from epitaxial layers deposited by different techniques, like MBE, APCVD and RTCVD. The low doped Si emitter design was varied as well as the base region concerning B and Ge profiles. Device simulation was used to explain some of the measured effects. At low VBE the base currents are dominated by carrier recombination within the part of the base-emitter space-charge region (SCR) touching the SiGe layer as a result of reduced carrier lifetimes in SiGe compared to Si. The grade of lifetime reducing was found to depend on the epitaxial technique. A nearly ideal IB(VBE) slope over several current decades was measured in case that the base-emitter SCR only touches the graded SiGe layer a little or not at all.