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SPECTRAL RESPONSE MODELLING AND ANALYSIS OF HETEROJUNCTIONBIPOLAR PHOTOTRANSISTORS
[Thesis]. Manchester, UK: The University of Manchester; 2010.
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Abstract
The optoelectronics industry continues to demand improved materials, devices and systems for the generation, transmission, detection, amplification and processing of optical signals. Heterojunction phototransistors (HPTs), in recent years, have attracted considerable interest for optical detection due to their intrinsic gain, low noise performance, high-frequency operation and process and the device layer compatibility with heterojunction bipolar transistors for high-speed optoelectronic monolithic microwave/millimetre-wave integrated circuit (OEMMIC) photoreceivers. A key performance parameter of HPTs is their spectral response (SR) which is critical in their usage in optical applications. The SR depends on several inherent factors including material absorption coefficient, refractive index, device structure, doping and temperature of operation along with the external factors such as bias voltage and the energy of incident radiation. The spectral response and optical characteristics of GaAs-based and InP-based sHPTs have been successfully predicted for the first time through an advanced absorption theoretical model. The model is based on the accurate prediction of photocarriers in the active layers of the phototransistor which, when related to the base current of the transistor in forward active mode, enables the prediction of optical characteristics. The importance of collection efficiency in accurate SR modelling is highlighted and the layer dependence of the optical flux absorption profile at near-band gap wavelengths is also investigated and its generalisation as a single-exponential has been refuted for GaAs-based HPTs. Analytical modelling of the spectral response has also been developed from the resolution of continuity equations that govern the excess optically generated minority carrier variation in the active layers of the HPT, taking into account the related physical parameters. Realistic boundary conditions have been considered for efficient device operation and a detailed optical flux absorption profile is constructed for accurate device modelling. This analytical model provides insight into the direct influence of various parameters (such as base width and carrier concentration) on the device performance, thus, providing a valuable optimization tool for the future design of HPTs in optical receivers. The measured results at 635 nm, 780 nm 808 nm and 850 nm for AlGaAs/GaAs HPTs and 980 nm, 1310 nm and 1550 nm for InP/InGaAs HPTs show good agreement with the predicted data, validating the proposed theoretical model. Finally, a detailed absorption model and photoresponse of double heterojunction phototransistors in a top/surface-illuminated orientation has been analyzed with a modified small-signal model. The effect of incident optical illumination on intrinsic small-signal parameters such as resistances and capacitances has been discussed and analyzed for photoresponse modelling.