In recent years, GaAsSb/InP double heterojunction bipolar transistors (DHBTs) have been demonstrated to be promising alternatives to InP/InGaAs HBTs, for next generation microwave/millimeter wave applications and optoelectronic integrated circuits (OEICs). However, GaAsSb-based DHBTs featuring the novel base material and type-II band alignment have not been well studied. This thesis investigated type-II GaAsSb DHBTs in the following aspects: periphery surface recombination current, Kirk effect, two dimensional (2D) simulation and device optimization. The present work provided insights into device operation, and guidances for further device development. A series of physical models and parameters was implemented in 2D device simulations using ISE TCAD. Band gap narrowing (BGN) in the bases was characterized by comparing experimental and simulated results. Excellent agreements between the measured and simulated DC and RF results were achieved. Emitter size effects associated with the surface recombination current were experimentally characterized for emitter sizes of 0.5 by 6 to 80 by 80 square micrometer. The 2D simulations by implementing surface state models revealed the mechanism for the surface recombination current. Two device structures were proposed to diminish surface recombination current. Numerical simulations for type-II GaAsSb-InP base-collector (BC) junctions showed that conventional base "push-out" does not occur at high injection levels, and instead the electric field at the BC junction is reversed and an electron barrier at the base side evolves. The electron barrier was found to play an important role in the Kirk effect, and the electron tunnelling through the barrier delays the onset of the Kirk effect. This novel mechanism was supported by the measurement for GaAsSb/InP DHBTs with two base doping levels. The study also showed that the magnitude of the electric field at the BC junction at zero collector current directly affects onset of the Kirk effect. Finally, optimizations for the emitter, base and collector were carried out through 2D simulations. A thin InAlAs emitter, an (Al)GaAsSb compositionally graded base with band gap variance of 0.1eV, and a high n-type delta doping in the collector were proposed to simultaneously achieve high frequency performance, high Kirk current density and high breakdown voltage.