An investigation of negative differential resistance and novel collector-current kink effects in SiGe HBTs operating at cryogenic temperatures

Abstract

In this paper, a new negative-differential-resistance (NDR) effect and a novel collector-current kink effect are investigated in the cryogenically operated SiGe heterojunction bipolar transistors (HBTs). Theory based on an enhanced positive-feedback mechanism associated with heterojunction barrier effect at deep cryogenic temperatures is proposed to explain both the observed NDR and the collector-current kink. The accumulated charge induced by the barrier effect acts at low temperatures to enhance the total collector-current, indirectly producing both phenomena. This theory is confirmed using the calibrated 2-D DESSIS simulations over temperature. These unique cryogenic effects also have significant impact on the ac performance of SiGe HBTs operating at high injection. Technology evolution plays an important role in determining the magnitude of the observed phenomena, and the scaling implications are addressed. In addition, the present NDR effect is also compared with previously reported NDR and hysteresis effects observed in highly scaled SiGe HBTs operating under forced-IB -base bias. The input drive condition of the transistor during its use in circuits, either under pure forced-current bias or under pure forced-voltage bias, or more practically, somewhere in between, determines the magnitude of the observed NDR and is of potential concern for circuit designers and must be carefully modeled. © 2007 IEEE.