Base and collector current mechanisms in bipolar NPN gallium aluminum arsenide/gallium arsenide heterojunction transistors

Described is an experimental study of the components that are found in the base and collector currents of bipolar Ga sub 1-y Al sub y As(N)-GaAs(PN) heterojunction transistors that are made with liquid phase epitaxy (LPE). The subscript y in Ga sub 1-y Al sub y As denotes the aluminum mole fraction in the emitter material. Reviews of the theories of current flow by diffusion and drift, recombination through band-gap states in the depletion regions and at the interface, tunneling, and tunneling-recombination involving band gap states are first presented. The experimental dependence of base and collector current on base-emitter voltage V sub BE and temperature are used to identify the nature of the current flow in five LPE heterotransistors built with different doping levels, emitter aluminum concentrations, and degrees of graduality in the heterojunction. A relationship is found between the natures of the base and collector currents and both are discovered to be strongly dependent on the graduality of the heterojunction.