In this work we investigate the optical and band structure properties of full InGaP/GaAs based heterojunction bipolar transistor (HBT) epitaxial structures grown by metalorganic chemical vapour phase epitaxy (MOVPE). In related work, full HBTs have been fabricated from the two wafers studied, which exhibit high and low common-emitter current gain (h(FE)) parameters on electrical test, The focus of this study is to investigate and compare the photoluminescence and photoreflectance spectroscopy response of these known good and bad epitaxial wafers. The results of low temperature (10-300 K) spectral and transient photoluminescence (PL) analysis are presented, revealing evidence of the nature of the InGaP ordering induced non-radiative loss mechanism. The results also demonstrate the modification to the PL lineshape arising from the InGaP/GaAs interfacial conditions. The experimental results are supported by X-ray diffraction data and finite-element device simulation, showing the effect of intermixing layers on the interfacial band potentials. The optical modulation technique of photoreflectance (PR) spectroscopy was employed to investigate the band structure and interfacial electric fields, F-s, of the HBT structures. Following the polarisation-[110] and [1(1) over bar 0]-dependence of the sub-lattice ordering PR response, it was found necessary to include an emitter/base intermixing layer in order to account for the InGaP/GaAs F-s data. It is concluded that non-optimal MOVPE growth conditions for one of the structures resulted in both sub-lattice ordering and layer intermixing effects, consistent with the low h(FE) of the HBTs fabricated from this material.