Abstract
The photoreflectance (PR) spectroscopic technique has been used to investigate the microstructures of the graded InAlAs/InGaAs heterojunction bipolar transistor at room temperature. The energy features of the PR spectrum were fitted and identified as band-to-band transitions in the graded layers which were grown using pulsed molecular beam epitaxy and InGaAs as well as InAlAs layers. A linear variation relationship of band gaps with Al composition z was observed and approximated by Eg=0.737+0.759z eV. From the observed Franz-Keldysh oscillations, we have evaluated the built-in dc electric fields in the i-InGaAs collector and n-InAlAs emitter regions. These electric fields are in good agreement with the continuity condition of electric displacements in the interfaces.
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