Analysis and modeling of silicon germanium heterojunction bipolar transistors

Since the realization of the first SiGe HBT in 1987, remarkable device RF performance, such as cutoff frequency of 110 GHz and maximum frequency of oscillation of 120 GHz have been reported. With continued shrinking in base thickness and optimal design of SiGe profile, the cutoff frequency beyond 150 GHz is expected. Because of its superior high speed performance, the SiGe HBT is an excellent candidate for low power and high bandwidth wireless communication circuit applications.; Although many experimental data on the SiGe HBT performance have been published, accurate modeling and analysis of the SiGe HBT is still missing. In this dissertation, the base transit time with different Ge profiles in the base are modeled and analyzed. The neutral-base recombination effect on the Early voltage and output conductance as a function of Ge grading is examined. The base-collector heterojunction barrier effects on the performance of SiGe HBT at high current densities are studied. Analytical equations of electron barrier height, collector current, transconductance and base-collector heterojunction capacitance including high current barrier effect have been derived. Heterojunction device simulations were used to justice the self-consistent analysis. Experimental data published in the literature are compared to verify the model utility and accuracy.; Because of the aiding field resulting from nonuniform base doping, the base transit time of a nonuniform base is smaller than that of a uniform base. The linear Ge profile has better performance in base transit time compared to uniform, exponential and trapezoidal Ge profiles. Early voltage is a strong function of Ge content and base grading. Neutral-base recombination decreases the Early voltage of the SiGe HBT. The degradation of the Early voltage is significantly enhanced when Ge grading in the base is increased. The electron barrier increases with the collector current density. This barrier effect results in collector current clipping, transconductance degradation and base-collector heterojunction capacitance variation at high base-emitter biases. The base-collector heterojunction barrier effect could be suppressed by using the retrograde doping profile and the Ge profile design in the collector.