Surface recombination current and emitter compositional grading in N p n and P n p GaAs/AlxGa1−xAs heterojunction bipolar transistors

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Abstract
An experimental study of current gain in Npn and Pnp GaAs/AlxGa1−xAs heterojunction bipolar transistors (HBTs) with either compositionally graded or abrupt base‐emitter heterojunctions is reported. In both Npn and Pnp device structures, linear compositional grading increases the amount of base current and lowers the transistor current gain from that observed in devices with abrupt base‐emitter heterojunctions. Analysis of the dependence of the base current on emitter mesa size indicates that surface recombination dominates the Npn and Pnp base current in both the abrupt and graded devices. Taken together, these results suggest that the magnitude of the surface recombination current in Npn and Pnp GaAs/AlxGa1−xAs HBTs depends not only on the surface recombination velocity of the GaAs base free surface, but also on the energy barrier presented by the emitter to minority carriers attempting to enter the surface channel at the emitter mesa surface. Linear compositional grading of the emitter results in a lowering of this barrier and an increase in base current associated with carrier injection to the transistor surface.