A more complete model for InP/InGaAs Double Heterojunction Bipolar Transistors (DHBT) is obtained in this thesis by physically analyzing the transport process of the main current components. The potential distribution of the energy barrier constitutes a fundamental analytical concept and is employed for applying the diffusion, the thermionic emission, and the tunneling theories in investigating the injection mechanisms at the e-b heterojunction interface. The diffusion transport is considered first for electron injection from the emitter into the base. The thermionic emission is applied properly at the point of maximum potential energy as one of the boundary conditions at that interface. A suitable energy level is selected with respect to which the energy barrier expression is expanded for the calculation of the tunneling probability. The first "spike" at the conduction band discontinuity is described as the potential energy for the injected electrons to obtain kinetic energy to move into the base region with a substantially high Velocity. The electron blocking action of the second "spike" at the b–c junction is also analyzed by considering the transport Velocity with which electrons are swept out of that boundary. Based on the material parameters recently reported for both InP and InGaAs, computations of the nI current components are carried out to provide à characteristics in good agreement with the reported experimental results. / Master of Science