Negative photoresponse in modulation doped field effect transistors (MODFET's): theory and experiment

A model for the mechanism of negative photoresponse, namely, the decrease of drain current under illumination, in AlGaAs-GaAs MODFET's is presented. Also, a comprehensive experimental study discussing the dependence of this phenomena on gate and drain to source bias voltages and optical power, as well as a comparison with devices that show the usual positive photoresponse, are reported. The negative photoresponse is attributed to trapping of photogenerated carriers in the GaAs buffer layer, causing a change in the potential profile and consequent reduction in the number of carriers in the 2-DEG channel. The above theory is supported by numerical solution of Poisson's and electron continuity equation, using the finite-elements method. Finally, the implications of the negative photoresponse on the high-speed photodetection properties of MODFET's devices are discussed.<
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