Physical insights into SiGe HBT operation at cryogenic temperatures: gain enhancement and early effect mechanisms

This paper systematically investigates the current gain, output characteristics, and Early effect of SiGe HBT devices under various cryogenic temperatures ranging from 300 K to 4.8 K. Through Gummel measurements, gain analysis, and output characteristic evaluations, the critical mechanisms of temperature-dependent behavior are revealed. TEM line scan results are employed to analyze the Ge composition gradient in the base region, and energy band diagrams are utilized to elucidate the performance enhancement brought by Ge incorporation. The Early effect is further examined using both Forced–IB and Forced–VBE measurement modes. By extracting Early voltages under different temperatures, the impact of base width modulation at cryogenic conditions is clarified. The results indicate that proper design of Ge profile and bias control are essential for ensuring the stability and performance of SiGe HBTs in cryogenic applications.