Correlation of photoreflectance spectra with performance of GaInP/GaAs heterojunction bipolar transistors

We have measured the photoreflectance (PR) spectra at 300 K and dc current gain for GaInP/GaAs heterojunction bipolar transistors (HBTs) grown by metalorganic chemical vapor deposition with different growth conditions. From the observed Franz-Keldysh oscillations, we have evaluated the built-in dc electric fields and associated band gaps in the GaInP emitter and GaAs collector regions. The observed increase in current gain with the low GaInP band gap is in agreement with band alignment between partially ordered GaInP and GaAs, but the observed current gain is not strongly dependent on electric field. For samples with comparable GaInP band gaps we have thereby detected a limiting factor, base bulk recombination, governing the current gain of HBTs from Gummel plots.