Improved VBIC model for SiGe HBTs with an unified model of heterojunction barrier effects

An improved bipolar transistor model considering heterojunction barrier effect (HBE) in SiGe double heterojunction bipolar transistors is developed. The effect of barrier formation due to high level injection, which is related to the rapid degradations of the dc current gain (β) and cutoff frequency (f_T), is carefully investigated and analyzed. As the collector current becomes high, the conduction band barrier is induced and increased. It causes the saturation of collector current (J_C) due to the blocking of carrier transport, the sharp increase of base transit time (τ_B) due to the additional charge storage, the increase of base current (J_B) due to the increased recombination, and the decrease of intrinsic base resistance (R_bi) due to the increased charge and base pushout. Those phenomena are included into a vertical bipolar intercompany model (VBIC) compact model by employing a unified model of the HBE on J_C, J_B, τ_B, and R_bi. Furthermore, portions of τ_B and R_bi from the Kirk effect itself are modeled according to the high current model description and the new formulation of widened base, respectively. A full extraction of parameters has been performed and the modified VBIC model is applied. The modeling accuracy is significantly improved at the high current region for the dc and RF characteristics.