Nonconventional electron diffusion current in GaAs/AlxGa1−xAs N-p-n heterojunction bipolar transistors with heavily doped base layers

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Abstract
Measurements of collector current density in a GaAs/AlxGa1−xAs N‐p‐n heterojunction bipolar transistor with a p‐type base doping of 6×1018 cm−3 are compared with the conventional expression for electron transport by diffusion across the base layer. The experimental collector current density exceeds the conventional diffusion theory result by more than a factor of 4, even after band‐gap shrinkage and bandtailing due to the heavy p‐type base doping are taken into account in the determination of the thermal equilibrium electron density in the base. The potential necessity of retaining an additional term in the fundamental electron current density equation to account for the experimental collector current is stressed. The form of this additional current‐driving term is exhibited equivalently in both an electron activity coefficient formalism and in terms of a spatial gradient of the conduction‐band density of states. A conclusive assessment of the relative importance of this additional term will require a theoretical treatment of the influence of excess minority‐carrier electron concentrations on the density of states and electron activity coefficient for excited p‐type GaAs.