Improved MESFET geometry for optical coupling

Summary form only given. There has been substantial interest in the physical behaviour of illuminated MESFETs and their applications to microwave photonic systems. Despite the existence of dedicated photodetectors such as PIN photodiodes which attain better optical coupling than MESFETs they are not readily amenable to monolithic integration. In contrast, using direct optical illumination obviates the need for a separate PIN, allowing one to access all the benefits of monolithic (MMIC) technology. In this paper, the profile of optical illumination impinging on a single MESFET is shown to vary from a step function, with a depth dependency. Having determined this, we then go on to show that short gate length devices will both maximise the photovoltaic effect and the photoconductive effect resulting from optical generation of carriers in the channel region. In this case, the photoconductive effect is judged to result from illumination of the inter-contact regions and the photovoltaic effect from under gate region carrier generation. A symmetrical MESFET with inter-electrode spacing=1 μm, to a depth of 1.5 μm, is described. This gives an optimum detector with dimensions compatible with the high frequency requirements (small gate length) for small device sizes. This is because the diffraction effects are enhanced with reducing slit size. Furthermore, measured results have been obtained.