HOT-ELECTRON TRANSPORT ACROSS A WIDE ALGAAS BARRIER CONTAINING QUANTUM-WELLS

An experimental and theoretical study has been made of electron transp ort over a series of wide AlxGa1-xAs barriers with graded interfaces, containing GaAs quantum wells in the central Al0.25Ga0.75As region. Ex tensive measurements of both the voltage and temperature dependence of the current were made, as well as measurements of magnetoresistance. In these structures tunnelling is expected to be negligible and hence drift-diffusion thermionic emission theory was used to interpret the d ata. Good agreement between theory and experiment was obtained using a low-field model for electric fields up to 10 kV cm-1. At applied elec tric fields greater than 10 kV cm- 1 agreement between theory and expe riment was obtained to within an order of magnitude when the model was modified to include a saturated drift velocity plus the effect of inj ected space charge. At high electric fields periodic negative differen tial resistance (NDR) oscillations were observed. The model usually gi ven to explain these oscillations involves tunnelling and is, therefor e, not obviously applicable to these structures. An alternative descri ption of their origin is discussed, based upon the NDR associated with intervalley transfer.