With the rapid development of frontier fields such as aerospace, deep space exploration, and quantum computing, electronic systems are required to operate reliably under extreme temperature conditions, particularly at cryogenic temperatures. Silicon-Germanium Heterojunction Bipolar Transistors (SiGe HBTs) have emerged as a promising candidate for wide-temperature-range analog/mixed-signal integrated circuits due to their high performance and compatibility with silicon-based processes. However, existing commercial compact models, such as HICUM and Mextram, exhibit insufficient accuracy at cryogenic temperatures, limiting their applicability. This paper presents an improved Mextram-based compact model for SiGe HBTs operating from 80K to 400K. Key enhancements include temperature-dependent modeling of ideality factors and saturation currents in both the main current and base current models, as well as the incorporation of a heterojunction barrier effect model. A comprehensive characterization methodology and parameter extraction strategy for DC and RF behaviors across the wide temperature range are also proposed. Experimental validation shows that the proposed model achieves good agreement with measured data, with an average relative error of less than 20%, demonstrating its accuracy and practicality for extreme-environment circuit design.