Abstruct-A new method for the evaluation of the dc base parasitic resistance, R,, of bipolar transistors is described. The method is based on impact-ionization-induced base current reversal and enables R , to be evaluated independently from the emitter parasitic resistance in a wide range of emitter current and collector-base voltage, without requiring any special device structure. The method can also extract R , in impact-ionization regime, where current crowding due to negative base current induces an increase in R , at increasing emitter current.

This work is a theoretical and experimental study of the properties of retrograde collector profiles for bipolar transistors produced using high-energy ion implantation. One-dimensional device simulations are carried out, and show that compared with a uniform profile, the retrograde profile allows transistor operation at higher current levels without incurring an unacceptable increase in the base-collector capacitance. Two-dimensional process and device simulations are also carried out to analyze the two-dimensional electrical behavior of the retrograde collector. At low collector-emitter bias, quasi saturation is observed due to the vertical drop in potential, which is associated with a lateral drop in the potential in the collector resistance, and must be minimized. At high collector-emitter bias, the injection regime, resulting from the Kirk effect, occurs. Experimental bipolar transistors are designed and fabricated based on these simulation results. The proposed self-aligned design minimizes both the extrinsic base-collector junction capacitance and the series collector resistance. A measured transit frequency (f T) of 7.8 GHz is obtained, which is ample for many analog and digital VLSI circuits which require operating frequencies up to a few gigahertz.

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