Effects of injection induced bandgap narrowing on bipolar transistors operating at low temperatures

The authors present evidence that injection-induced bandgap-narrowing plays an important role in determining the low-temperature characteristics of bipolar transistors. The transistors used for the investigation were scaled double-polysilicon self-aligned devices that have yielded sub-200 pS ECL (emitter coupled logic) gate delays at LN2 temperature. The bandgap-narrowing phenomenon is shown to occur when a high density of free carriers is injected into the neutral base region of the device under high-current conditions; the net result is a significant enhancement in the low-temperature current gain that is unaccounted for in conventional device theory. This perturbation of the low-temperature device properties must be carefully considered for accurate device and circuit modeling.