Temperature-dependent dc characteristics of an In Ga As ∕ In Ga As P heterojunction bipolar transistor with an InGaAsP spacer and a composite-collector structure This paper presents quantitative analysis of direct current (DC) characteristics of an N/p þ /n Ga 0.5 In 0.5 P/GaAs heterojunction bipolar transistor (HBT) using thermionic-field-diffusion model including high current effects such as base push-out, parasitics like emitter resistance and base resistance. These effects play an important role in predicting DC characteristics of HBT. Software was developed to simulate the Gummel plot, current gain, collector-emitter breakdown voltage, and the offset voltage. As per their design, the authors got Ga 0.5 In 0.5 P/GaAs heterojunction bipolar transistor fabricated from IQE Inc. The simulated Gummel plot agrees with the experimental data in the complete operating range of applied bias. The current gain, b, of the HBT is found to be $100-120. A broad comparison of current gain of various devices was made and analyzed as a function of collector current density. The b of various HBTs were also analyzed in terms of base sheet resistance for three values of collector current density. The breakdown voltage and offset voltage were simulated to be of the order of 10 V and 120 mV, respectively, for the designed device. These values agree with the experimental data. The computed electric field in the base-collector region and the multiplication factor in the avalanche breakdown of the HBT are presented as a function of collector-emitter voltage.