On the optical response of modulation doped field-effect phototransistors

Summary form only given. The effects of illumination of modulation-doped field-effect transistors (MODFETs) and related structures have been subject to several investigations aiming to combine photodetection and amplification in a single monolithic phototransistor, potentially simplifying the optical receiver configuration. In this paper, the optical characteristics and potential applications of the two photoeffects occurring in the MODFET are discussed. These two mechanisms are characterized by distinct time constants in the frequency spectrum and are described as: (1) a photoconductive mechanism, caused by photoelectrons that reach the 2-DEG channel and are collected by the output terminal, and (2) a photovoltaic mechanism, due to the buildup of photogenerated holes that are swept towards the high resistivity semi-insulating substrate. In steady state, this positive charge accumulation is neutralized by the injection of electrons from the source into the 2-DEG channel. Our experimental results have shown that the MODFET photovoltaic response is characterized by a logarithmic dependence on light intensity and extremely high optical gain. The photovoltaic effect was further studied by comparing the photoresponse of a pseudomorphic MODFET to a high-speed p-i-n photodiode.