The processes related to the presence of hot carriers in power AlGaAs/GaAs heterostructure field effect transistors (HFET) have been studied by means of optical (electroluminescence) and electrical techniques. Electroluminescence spectra evidence the presence of three emission bands, superimposed on the maxwellian behavior typical for the hot electron emission. Two of the previous bands are related to the bandgap transitions of cold carriers in the conduction channel and in the barrier layer, while the third one is probably related to DX centers present in the AlxGa1−xAs layers. We show that the optical measurements give more detailed information than the electrical ones; in particular the gate current seems to be a less indicative monitor of the impact ionization mechanisms than the light emitted in the conductive channel by cold carrier recombinations. No correlation between integrated light and currents is evidenced, except at low energies were an intra-band mechanism, related only to the drain current is observed. Spatial maps of the emitted light evidence that part of the cold holes produced by impact ionization move toward the source electrode and recombine with cold electrons in this region.
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