Neutral base recombination and its influence on the temperaturedependence of Early voltage and current gain-Early voltage product inUHV/CVD SiGe heterojunction bipolar transistors

We present the first comprehensive investigation of neutral base recombination (NBR) in ultra-high vacuum/chemical vapor deposited (UHV/CVD) SiGe heterojunction bipolar transistors (HBT's), and its influence on the temperature characteristics of Early voltage (V_A ) and current gain-Early voltage product (βV_A). We show that a direct consequence of NBR in SiGe HBT's is the degradation of V_A when transistors are operated with constant-current input (forced-I_B) as opposed to a constant-voltage input (forced-V_BE). In addition, experimental and theoretical evidence indicates that with cooling, V_A in SiGe HBT's degrades faster than in Si bipolar junction transistors (BJT's) for forced-I_B mode of operation. Under the forced-V_BE mode of operation, however, SiGe HBT's exhibit a thermally-activated behavior for both V_A and βV_A, in agreement with the first-order theory. The differences in V_A as a function of the input bias and temperature for SiGe HBT's are accurately modeled using a modified version of SPICE. The performance of various practical SiGe HBT circuits as a function of temperature, in the presence of NBR, is analyzed using this calibrated SPICE model