On the Emitter Back-Injection Current in Advanced SiGe HBTs at Cryogenic Temperatures

Designing current amplifiers at cryogenic temperatures requires an accurate description of the current gain of HBTs, which is impacted by the forward base current injected back into the emitter. Its bias dependence for a given temperature so far has been modeled by a diode equation with an empirically modeled ideality factor $m_{\mathrm {{B E}}}$ for describing its experimentally observed strong temperature dependence. This work analyzes different carrier statistics and transport mechanisms and provides the most likely physical origin for the observed strong increase of $m_{\mathrm {{B E}}}$ at cryogenic temperatures.