Profile design issues and optimization of silicon-germanium heterojunction bipolar transistors

Current SiGe technology development is almost exclusively centered on npn SiGe HBTs. However, a complementary (npn + pnp) bipolar technology offers significant performance advantages and circuit design flexibilities over an npn-only technology for high-speed analog and mixed-signal applications. The parasitic energy band barrier induced by the SiGe/Si transition in the collector-base space-charge region of a SiGe HBT produces very different design constraints for pnp and npn transistors, and they must be optimized independently. Chapter 2 of this thesis contrasts the Ge profile design differences between pnp and npn SiGe HBTs and provides the guidelines for Ge profile design for future complementary SiGe technologies.; It is well known that the backside Ge profile shape strongly influences high-injection heterojunction barrier effect in SiGe HBTs, which produces premature roll-off of β and f_T at high current density. However, the impact of the backside Ge profile on the impact ionization and apparent neutral base recombination (NBR) of SiGe HBTs has not been previously investigated. In Chapter 3, we show that the backside Ge profile also alters the electric field distribution in the base-collector space-charge region, and therefore affects both impact ionization and apparent NBR in SiGe HBTs. Impact ionization and apparent NBR are critical to the breakdown voltage and output conductance of SiGe HBTs, and are thus key considerations for circuit designers.; Finally a new base current (I_B) degradation mechanism was identified for the first time in bipolar transistors, which is fundamentally different from the I_B degradation caused by the well-known Emitter-Base reverse-bias (EB) stress or radiation or high current forward-bias stress conditions. In Chapter 4, we show that the base leakage current becomes significantly higher after current-voltage measurements with simultaneously applied high V_BE and high V_CB. Today's SiGe HBTs are usually biased at high J_C and high V_CB to achieve the maximum performance, and therefore such I_B degradation can occur in the practical SiGe circuits.