InP Travelling-Wave HPTs for Ultra High Frequency Photonic Systems

L. Gomez-Rojas

The use of heterojunction phototransistors (HPTs) in fibre-radio communication systems has attracted the interest of researchers for many years. These devices combine the functions of photodetection and amplification and are attractive for realising low-cost distribution points. As with the case of photodetectors, the 'travelling-wave' (TW) concept has been proposed and successfully demonstrated as a mean of increasing the bandwidth and power-handling capability of HPTs. However, the development of optimum TW-HPT structures is still ongoing. The purpose of this work is to investigate the fabrication and design of ultra high speed photodetectors by using the InP travelling-wave approach. Working alongside the universities of Leeds and Sheffield our main contribution lies in the design of the optical waveguiding characteristics of the HPT structure, the high frequency characterisation of the HPT, the design and characterisation of an on-chip antenna and the modelling and characterisation of an integrated microwave photonic demonstrator.

TW-HPT devices have been fabricated and fully characterised and a small signal model has been developed with most of its elements calculated based on the physical parameters. The simulated frequency responses match the measured results up to 20 GHz. In parallel a more comprehensive three-dimensional FDTD electromagnetic modelling technique applied to the novel coplanar waveguide (CPW) InP TW-HPT structure has also been developed. The simulation has shown for the first time the velocity mismatch between the optical wave and the photogenerated electrical signal in the device. Future work comprises the improvement of exciting devices and the integration of such in a fully integrated demonstrator.

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