Observation of radiative and nonradiative tunneling in GaAs/AlxGa1−xAs heterojunction bipolar transistors with compositionally graded base-emitter heterojunctions

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Abstract
A comparative experimental study of 77‐K current gain and electroluminescent (EL) spectra in Npn GaAs/AlxGa1−xAs heterojunction bipolar transistors (HBTs) with either compositionally graded or abrupt base‐emitter heterojunctions is reported. Shifting‐peak spectra associated with radiative tunneling of carriers into the base‐emitter heterojunction space‐charge region are observed to dominate the 77‐K EL spectra of HBTs with linearly graded emitters. The 77‐K EL spectra of the abrupt HBTs are characterized by a peak whose energy position is invariant with respect to base‐emitter bias and corresponds to recombination of electrons diffusing across the HBT base. Comparison of EL peak intensity and transistor base current indicates that the graded HBT current gain at 77 K is determined by nonradiative tunneling of electrons and holes to deep levels at the base‐emitter junction. The loss of minority carriers to radiative and nonradiative tunneling current mechanisms in the graded HBT causes the 77‐K current gain of the graded HBT to be significantly lower than that of the abrupt HBT.